Retrieval-Induced Forgetting in Recall: Competitor Interference Revisited

Michael F. Verde University of Plymouth

Participants studied category–exemplar pairs (FRUIT Cherry, FRUIT Grape) and then practiced some of the items (Cherry). In Experiment 1, practice that involved retrieving the item from memory suppressed recall of related items (Grape), a finding known as the retrieval-induced forgetting (RIF) effect. In Experiment 2, practice that involved studying the item without retrieval produced no RIF effect. Both retrieval and nonretrieval practice facilitated the subsequent recall of practiced items (Cherry). The dissociation between “strengthening” of practiced items and forgetting of related items is thought to be evidence that RIF is the result of inhibition during earlier retrieval attempts rather than interference from competing memories at retrieval. However, simulations of the SAM-REM model show that competitor interference can account for this dissociation. Experiments 3–6 supported the predictions of the model by demonstrating that nonretrieval practice can produce the RIF effect under conditions that emphasize context encoding or increase the number of competitors.

Keywords: memory, recall, forgetting, retrieval-induced forgetting, memory models

We retrieve memories with incredible speed and accuracy, con- the primary cause of retrieval failure.1 Rather, memories are made sidering the breadth of stored experience at our disposal. Never- inaccessible by inhibitory mechanisms that are active during the theless, the difficulty of isolating a single memory among all retrieval process. In order to facilitate the recall of an intended accessible memories is such that our attempts at retrieval some- target, nontarget memories associated with the cue are inhibited, times end in failure. Most theories of recall assume that interfer- making them more difficult to subsequently recall. As Anderson ence from other memories is an important factor in forgetting. (2003) put it: “It is the executive control mechanism that over- However, the mechanism by which interference leads to retrieval comes interference—inhibition—that causes us to forget, not the failure remains a matter of debate. Interference theories of the last competition itself. . . . The mere storage of interfering traces is not century (see Postman, 1976, for a review) viewed forgetting as a what causes memories to grow less accessible with time” (p. 416). consequence of the associative structure of memory. Contempo- Inhibition theory takes as a premise that competition within the rary theories of recall in this tradition suggest that forgetting associative network of a cue is problematic. It departs from tradi- tional interference theories in suggesting that competition by itself results directly from competition among the associates of a cue. is normally not responsible for retrieval failure in recall. As de- When a retrieval cue activates multiple associated memories, com- tailed in the next section, competitor interference is sufficient in petition for retrieval interferes with the ability to recall the in- theory to account for forgetting in paired associate paradigms. tended target. For example, our inability to remember what we did Why is there a need for an additional mechanism? The case for Wednesday night might be due to an abundance of other memories inhibition is based in part on an appeal to plausibility. Inhibition that spring to mind when we think about the past week. In the plays a fundamental role in neuronal processing, and many theo- laboratory, interference is typically examined with paired associate rists point to its presence in a variety of higher level, cognitive paradigms. Imagine studying a series of word pairs (FRUIT domains (e.g., Dagenbach & Carr, 1994; MacLeod, 2007). Ander- Cherry, FRUIT Grape) where multiple retrieval targets are asso- son and colleagues have argued eloquently that the ability to ciated with a single cue. Later, one of the targets must be recalled inhibit memories is beneficial in many situations, from the con-

This document is copyrighted by the American Psychological Association or one of its alliedgiven publishers. the cue (FRUIT C_?). According to competitor interference, scious avoidance of traumatic reminiscences to the suppression of

This article is intended solely for the personal use ofthe the individual user and is cue not to be disseminated broadly. will activate both associates (Cherry, Grape), which then competition at a preconscious level (Anderson, 2003; Anderson & compete for retrieval. This competition reduces the likelihood of Huddleston, 2011). successfully recalling the intended target (Cherry). More important, the case for inhibition is supported by a number An influential alternative view is offered by Anderson, Bjork, of key findings, drawn mainly from investigations of retrieval- and Bjork’s (1994; Anderson & Bjork, 1994) inhibition theory of induced forgetting (RIF) in which recalling an item tends to forgetting, which suggests that competition among memories is not suppress the availability of other items in memory. This has been studied with the retrieval practice paradigm introduced by Ander-

1 Many investigators have since contributed to the development of This article was published Online First May 20, 2013. inhibition theory. Although these investigators do not espouse identical Correspondence concerning this article should be addressed to Michael views, the view expressed by Anderson and colleagues (Anderson, 2003; F. Verde, School of Psychology, University of Plymouth, Plymouth PL4 Anderson et al., 1994) and described here is the most well articulated and 8AA, England. E-mail: [email protected] represents standard thinking in the literature.

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son et al. (1994). The basic paradigm consists of three consecutive increases their association strength will have an adverse effect on phases: study, practice, and test. The study phase involves encod- memory for competitors. A straightforward way to test these ing a number of paired associates, each belonging to one of several opposing predictions is to manipulate the task used during the categories (FRUIT Cherry, FRUIT Grape, TOOL Hammer, TOOL practice phase. Many studies have found that although recalling Pliers). During the practice phase, half of the items from some of Pϩ items during practice reduces the ability to recall competitors, categories (Pϩ items) are tested with cued recall (FRUIT C_?). presenting Pϩ items for additional study does not (Anderson & Finally, during the test phase all of the studied items from both Bell, 2001; Bäuml, 2002; Campbell & Phenix, 2009; Ciranni & practiced and nonpracticed categories are tested for recall. The Shimamura, 1999; Gómez-Ariza, Fernandez, & Bajo, 2012; Hul- typical finding is that nonpracticed items from practiced categories bert, Shivde, & Anderson, 2012; Johansson, Aslan, Bäuml, Gäbel, (PϪ items; FRUIT G_?) are recalled at a lower rate than items & Mecklinger, 2007; Staudigl, Hanslmayr, & Bäuml, 2010; Wim- from nonpracticed categories (NP items; TOOL H_?). The reduc- ber, Rutschmann, Greenlee, & Bäuml, 2009).2 The finding is tion in PϪ relative to NP recall will be referred to as the RIF effect consistent with the retrieval dependence prediction of inhibition. (although, as will be seen, the effect sometimes emerges even Additional study of Pϩ items during the practice phase does when no retrieval occurs during the practice phase). According to enhance their subsequent recall in the final test, often to the same the inhibition account, recalling Cherry during the practice phase extent as retrieval practice. In other words, additional study ap- leads to inhibition of other associates of the cue FRUIT, such as parently strengthens Pϩ items while having no discernible effect Grape, making them more difficult to recall during the final test. on other items, inconsistent with the strength dependence prediction of competitor interference. The absence of RIF following Competitor Interference and RIF nonretrieval practice, combined with observations that retrieval can impair competitor recall even when no studied item is There are a variety of ways that interference theories might strengthened (Bäuml, 2002; Storm, Bjork, Bjork, & Nestojko, account for RIF (Anderson & Bjork, 1994). The present study 2006), has led to a consensus in the RIF literature summarized by focuses on the mechanism most often discussed in the RIF liter- Bäuml (2006): “Retrieval-induced forgetting studies point to re- ature: competitor blocking. There is a tradition of describing recall trieval inhibition rather than any variants of (strength-dependent) as a search through related memories (e.g., James, 1890). Modern competition as the major source of the recall impairment” (p. 563). theories often model this search as a stochastic process (Raaijmak- The conclusion that the apparent strength independence of RIF is ers & Shiffrin, 1980; Rundus, 1973; Shiffrin, 1970; Wixted & uniquely consistent with an inhibitory mechanism constitutes one Rohrer, 1994) where the probability of retrieving memory image j of the main arguments for inhibition and against competitor inter- depends on the likelihood ratio ference (Anderson, 2003). A recent review by Storm and Levy (2012) noted that nearly 200 Aj P(j Խ cue) ϭ , where k ϭ 1...n. (1) studies have investigated RIF over the last two decades. The ͚ Ak influence of inhibition theory on thinking about recall derives not only from its ability to explain findings from this large literature The numerator describes the association strength between the cue but from the seeming inability of competitor interference to do so. and image j, and the denominator describes the summed associa- It is proposed here that the theoretical treatment of competitor tion strengths between the cue and all n images in the search set. interference in the RIF literature has been inadequate. Using the The degree of competitor interference depends on the associative SAM-REM model of recall (Malmberg & Shiffrin, 2005) to illus- strengths of nontarget images in the denominator. As the value of trate the strength-dependent and number-dependent properties of the denominator increases, the likelihood of retrieving the intended competitor interference, I will show that such a model can accom- target decreases. Assuming that only one image can be retrieved modate findings in the RIF literature thought to be inconsistent per attempt and that the number of attempts is limited, increasing with strength-dependent forgetting. The discussion will turn first to the denominator effectively blocks retrieval of the target memory. how the strength-dependent properties of the model, paired with If the cue is equally associated with all items in the search set, testable assumptions about how information is encoded during the likelihood of retrieving the intended target is 1/n. In this case, practice, might account for dissociations between strengthening interference is determined solely by the number of nontarget items This document is copyrighted by the American Psychological Association or one of its allied publishers. and forgetting. It will then consider how the number-dependent in the set. If association strength varies, the strength of the target This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. properties of the model might account for RIF in the absence of relative to nontargets also becomes a factor. Interference increases competitor strengthening. as the strength of competitors increases. In short, any increase in the number or strength of competitors will have a negative impact on recall of the target. Competitor interference predicts that for- 2 Other studies, in the list-strength effect literature, do report that getting should be number dependent and strength dependent. strengthening some items in a study list via repetition, increased study Inhibition theory, on the other hand, predicts that forgetting duration, and so on has a negative effect on recall of other studied items (e.g., Malmberg & Shiffrin, 2005; Ratcliff, Clark, & Shiffrin, 1990; Tulv- should be retrieval dependent but strength independent. The pur- ing & Hastie, 1972; Wixted, Ghadisha, & Vera, 1997). However, these pose of the inhibitory mechanism is to aid retrieval by suppressing studies have primarily used free recall, a task in which output order is not competitors. Therefore, the act of retrieving Pϩ items during the controlled. Because strong items tend to be recalled earlier, a reduction in practice phase is critical to producing the RIF effect. If Pϩ items weak item recall could be due to retrieval inhibition from previously retrieved strong items rather than to increased competition from strong are practiced in a way that does not require their retrieval, there items. The conflation of memory strength and exposure to retrieval inhi- should be no need to inhibit competitors and no RIF. In contrast, bition makes it difficult to draw conclusions about strength independence competitor interference predicts that any practice of Pϩ items that from this prior work (cf. Verde, 2009). INTERFERENCE AND FORGETTING 1435

Modeling Strength and Retrieval as vectors of features, with each feature denoted by a numerical value. The stimuli in RIF studies consist of cue–target pairs. The portrayal of competitor interference in the RIF literature Typically (and in the experiments to follow), each stimulus is a commonly makes reference to a model proposed by Rundus category–exemplar pair (FRUIT Cherry) where the category name (1973). In this model, search involves sampling items with re- will eventually serve as the retrieval cue for recall of the target placement from memory. The probability of recalling a given exemplar. In the present implementation, each memory image is memory depends on the strength of its association with the cue, represented as a vector of 20 context features and 20 item features. relative to the association strengths of all other memories in the Item information represents features of the exemplar (Cherry). search set, as described by the likelihood ratio in Equation 1. The Context information represents the local context, defined as the search is terminated following a number of consecutive failures to features of the category (FRUIT).3 sample a memory that has yet to be recalled. The model can be When a probe is presented for study, its feature values are extended to situations where a cue specifies a single target (FRUIT copied into a memory vector. At each time interval, there is a of storing a value for each feature. The process is ءG_?) by assuming that a search is terminated following a number probability u of failed attempts to sample the intended target due to repeated gradual and prone to error, so that the resulting memory image is sampling of nontargets. In the Rundus model, increasing the as- an incomplete version of the original probe. It is assumed that sociation strength of competitors interferes with the sampling of a depth of processing and attention mediate the storage of informa- target. tion. For example, an elaborative encoding task might enhance What does it mean to “strengthen” a memory? In the RIF feature storage more than would a shallow task. Similarly, an literature, strength is defined operationally as the ability to recall encoding task that draws particular attention to context informa- an item. Retrieval and additional study are both said to strengthen tion might lead to the preferential storage of context features. ء -a practiced item because both forms of practice enhance subse These effects are modeled by varying the u parameter to reflect quent recall. Note that “strengthening” here means strengthening the rate of feature storage. Here, context and item features are ء ء the association between the cue and the practiced item (were this assigned separate values (u c and u i, respectively). The simula- ء not true, the Rundus model would not predict interference). The tions below will show that varying the effect of encoding on u c ء -direct correspondence between association strength and recallabil and u can produce both the presence and the absence of a RIF ity might be consistent with a relatively simple model like the one i effect. proposed by Rundus, but it is not a necessary characteristic of SAM-REM describes recall as a two-stage process of sampling competitor interference models generally. What this means is that and recovery. First, a memory image is sampled with replacement a dissociation between the ability to recall an item and the ten- from the search set. The probability of sampling the target j dency for that item to interfere with competitors is not incompat- depends on the likelihood ratio ible with competitor interference. This point will be illustrated with a more complex interference ␭ j model, the SAM-REM model described by Malmberg and Shiffrin P(j Խ cue) ϭ . (2) ␭ (2005). The model combines core elements from two parent mod- ͚ els. SAM is a general model of recall (Mensink & Raaijmakers, ␭ 1988; Raaijmakers & Shiffrin, 1980, 1981) that has been applied The variable signifies the degree to which the features of the successfully to a wide range of phenomena. REM (Shiffrin & probe match the features of an image in memory. The numerator Steyvers, 1997) models the encoding and matching of multidimen- represents the match to image j, and the denominator represents the sional memory representations. Broadly speaking, the implemen- summed matches to all images in the search set. This is the basic tation of SAM-REM described below can account for dissociations competition model of Equation 1. After an image is sampled, an between recallability and interference because the model possesses attempt is made to recover it to an extent that it can be physically two properties. First, different types of information can be stored produced. This is a function of the number of correctly stored depending on the encoding task. Second, different types of infor- feature values in the relevant portion of the image. The sampling mation contribute in different ways to retrieval and interference. It and recovery process continues until an appropriate target is pro- should be emphasized that the purpose of this discussion is not to duced or a criterion number of failed attempts leads to termination This document is copyrighted by the American Psychological Association or one of its alliedadvocate publishers. specifically for SAM-REM but instead to show that a of the search. This article is intended solely for the personal use ofcompetitor the individual user and is not to be disseminated broadly. interference model with these two basic properties can Consider a cued recall task where the category context is sup- account for the apparently strength-independent findings reported plied as a cue (FRUIT C_?). During the sampling stage, the probe in studies of RIF. consists mainly of the context features (with a few item features The Rundus model portrays memory representation in a rudi- from the initial letter of the target). Competitor interference occurs mentary fashion. Most theorists would agree, however, that rep- at this stage and is mainly a function of the degree to which context resentations are complex and multidimensional. A nuanced view features (FRUIT) have been stored and match those of the probe of memory representation is important for understanding the rela- and other memories. In the cued recall task, only the target exem- tionship between strength and interference. A common distinction plar has to be produced. Therefore, the probability of recovering is made between item and context features (Humphreys, 1978; the target (Cherry) is a function of the number of correctly stored Murnane, Phelps, & Malmberg, 1999). Item features are those of the object itself, but context features include details of the sur- 3 Other aspects of context, such as global or environmental context, were roundings, associations with other objects, and emergent proper- not modeled. This simplification does not alter the conclusions of the ties of object groupings. In SAM-REM, memories are represented simulations (see the Appendix). 1436 VERDE

item features. Recovery does not depend on the state of other given the probe FRUIT C_? The storage of item features would images in memory. In summary, successful recall requires that a improve the recovery of image B following sampling but would target is both sampled and recovered. Sampling depends mainly on have little effect on the sampling of image A, because item features context features and is prone to interference from other memories are largely absent from the probe. The differential contribution of that share the context. Recovery depends solely on item features context and item features to sampling and recovery allow the and is immune to interference. model to produce a dissociation between recallability and interfer- As an illustration of these points, a simplification of SAM-REM ence like that observed in studies of RIF. is shown in Figure 1. The probe and memory vectors each consist The idea that the encoding of item or context features might of 16 features, eight representing category context information and vary depending on the nature of the practice task will be referred eight representing item information. Imperfect encoding is as- to as the differential encoding hypothesis. Practice that selectively sumed; memory vectors have some null feature values where enhances the encoding of category context features will facilitate information has yet to be stored. A matching value (␭) is calculated recall of Pϩ items and interfere with recall of PϪ items. On the for each memory image by comparing the probe to the image, other hand, practice that selectively enhances the encoding of item feature by feature. Matching features increase ␭, and mismatching features will facilitate recall of Pϩ items but not interfere with features decrease ␭. The probe (FRUIT C_?) most closely matches recall of PϪ items. The facilitation of Pϩ items could be identical the target image A (FRUIT Cherry) but also matches image B in the two cases, although this is not necessary. All of these points (FRUIT Grape) to a moderate degree. It matches images C (TOOL are illustrated in the model simulations to follow. Hammer)andD(TOOL Pliers) to a low degree. Because the ␭ denominator of Equation 2 is based on summed , all images Model Simulations contribute to interference, meaning they all have an effect on the probability of sampling target image A. This probability would Monte Carlo simulations were run with the implementation of decrease if additional images were added to the search set. Thus, SAM-REM described in the Appendix (not the simplification in interference is number dependent. Also, because of its greater Figure 1). The model is based on one proposed by Malmberg and match to the probe, image B contributes more to the denominator Shiffrin (2005) for free recall and modified for cued recall by of Equation 2 than do images C and D. Thus, interference is Verde (2009). The simulations are qualitative: Their purpose is to strength dependent, if strength is defined in terms of the number illustrate how a competitor interference model can produce a range and value of stored features. of apparently strength-independent results like those frequently Imagine that image B was given additional encoding, as would reported in the RIF literature. The parameter values were chosen happen during practice. This would result in the storage of addi- by taking values used by Malmberg and Shiffrin and by Verde in tional category context and item features where there had previ- their simulations and making small modifications to produce rates ously been null values. The storage of context features would of recall similar to those observed in the empirical data reported increase the likelihood of image B being sampled given the probe later. Thus, the present stimulations did not require unusual pa- FRUIT. This would improve its own recallability given the probe rameter values and make their point using values similar to pre- FRUIT G_? but would also increase its interference with image A vious applications of SAM-REM. Across the four simulations described below, all parameters were fixed save for the two re- ء sponsible for the storage rates of context and item features, u c and ء u i, respectively. The simulated experiments were identical in design to the experiments reported later. Each virtual participant studied a set 48 pairs belonging to six categories, with eight pairs per category. These were analogous to category–exemplar pairs (FRUIT Cherry). Each stimulus pair was represented by a vector of 20 category context and 20 item features. Item features were generated uniquely for each stimulus. Context features were generated uniquely for each category. All stimuli within a category possessed This document is copyrighted by the American Psychological Association or one of its allied publishers. the same context features, reflecting their shared category cue. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. Following the standard retrieval practice design, three of the cat- Figure 1. In SAM-REM, each stimulus probe and memory image is egories were practiced categories, with half of the pairs within a represented by a vector of features (in this example, eight context and eight category designated Pϩ items and half of them designated PϪ item features). A nonzero feature value indicates stored information, and a items. Pairs from the remaining three nonpracticed categories were zero value indicates none has been stored. The context features of the probe designated NP items. The initial study phase consisted of a single FRUIT C_? are identical to those stored (incompletely) in images A, B, and encoding trial for each item. This was followed by a practice E. The probe also contains two item features of the word stem. The phase. In Simulations 1–3, Pϩ items were given two additional matching value (␭) of the probe to each image is a function of the number encoding trials during practice. In Simulation 4, Pϩ items were not and value of each stored feature. The description to the right of each image vector indicates relative match (high, medium, low) to the probe, based on repeated; instead, six novel stimuli belonging to the practiced the number of matching (ϩ) and mismatching (-) stored features. Image E categories (analogous to extralist items) were each given a single is a version of image B that has had additional encoding, increasing the encoding trial. Finally, all stimuli were tested with probes consist- number of stored features. The details of the model are simplified in this ing of the category context features and the initial two item example (see Appendix for full description). features, analogous to a category–word stem cue (FRUIT C_?). INTERFERENCE AND FORGETTING 1437

Storage parameters for encoding during initial study were fixed Simulation 1 but with no suppression of PϪ items. In summary, ϭ ء ϭ ء for all stimulations at u c .02 and u i .02 to represent equal the pattern in Simulation 1 resembles RIF following retrieval processing of context and item information. However, it is well practice where processing the category context is emphasized, and known that people can process the same stimuli differently across the patterns in Simulations 2 and 3 resemble the absence of RIF situations (perhaps reflecting differences in attention, motivation, following nonretrieval practice (e.g. extra study) where processing or strategy) and that these differences in processing lead to differ- the category context is deemphasized. Simulations 1–3 show that ences in the quality of encoding (Craik & Lockhart, 1972). This dissociations between target recallability and competitor forgetting can be modeled by varying the storage parameters. Simulations are possible and depend on how targets are strengthened (see 1–3 demonstrate the effects of strengthening, showing that differ- Malmberg & Shiffrin, 2005, for a related account of the list- ences in the processing of context and item information during the strength effect in free recall). practice phase can produce different patterns of facilitation (Pϩ Simulation 4 demonstrates the effect of varying the number of items) and suppression (PϪ items). These patterns will be related competitors. During the practice phase, Pϩ items are not pre- to real data in the next section. For the moment, the simulations sented. Instead, novel stimuli belonging to the same categories as can be viewed purely as a parametric test of the model. Pϩ and PϪ items are studied in the same fashion as the initial ϭ ء ϭ ء Simulated rates of accurate recall are shown in Figure 2. Sim- study phase (u c .02, u i .02). Obviously this does not ulation 1 demonstrates the effect of an increase in category context facilitate Pϩ items. However, encoding additional stimuli associ- ϭ Ϫ ء ϭ ء encoding during practice (u c .03, u i .02), the result of more ated with the cue suppresses recall of P items, a typical RIF focused attention to or more elaborate processing of context. The effect. Note that the degree of suppression would be even larger if result is a typical RIF effect. Practice facilitates recall of Pϩ items the novel items were encoded under the practice conditions of but suppresses recall of PϪ items relative to the NP baseline. Simulation 1 with enhanced context encoding. Despite its signif- Simulation 2 demonstrates the effect of a decrease in context icant role in forgetting, the number-dependent nature of competitor ϭ ء ϭ ء encoding during practice (u c .01, u i .02) due to reduced interference is rarely discussed in the RIF literature. attention or processing. Practice facilitates recall of Pϩ items, but In the next section, the properties of competitor interference this time there is no significant suppression; PϪ items are recalled illustrated in the SAM-REM simulations are tested in a series of at about the same rate as NP items. There is slightly less facilita- experiments. It is argued that both the simulations and the empir- ϩ ء ϩ tion of P items because u c contributes to recall of P items. ical findings require a reconsideration of several major findings in However, the difference in Pϩ facilitation between Simulations 1 the RIF literature. The focus turns first to strength-dependent and 2 is parameter dependent; under some conditions, it could be competition and a test of the differential encoding hypothesis quite small. Shifting attention from one aspect of a stimulus to demonstrated in Simulations 1–3. Experiments 1 and 2 replicate another might result in an encoding trade-off. This is demonstrated the often observed finding that retrieval practice leads to RIF but in Simulation 3, where a decrease in context encoding is accom- study practice does not. Experiments 3–5 show that the RIF effect ϭ ء ϭ ء panied by an increase in item encoding (u c .01, u i .03). can be made to appear and disappear under nonretrieval (study Practice facilitates recall of Pϩ items to the same extent as in practice) conditions by manipulating the degree to which category context and item information are processed during practice. The focus then turns to number-dependent competition. There are some situations in which RIF that is attributed to inhibition might be caused instead by interference due to an increase in the number of competitors. Experiment 6 shows that studying (without retrieving) new exemplars during practice can produce a RIF effect, as demonstrated in Simulation 4.

Strength-Dependent Competition Many studies have shown that retrieval practice suppresses recall of competitors but nonretrieval practice does not, even This document is copyrighted by the American Psychological Association or one of its allied publishers. though both types of practice “strengthen” (i.e., facilitate recall) of This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. the practiced item. This finding is typically interpreted as being inconsistent with the mechanism of competitor interference. As the SAM-REM simulations clearly show, such an interpretation is not quite correct. A dissociation between recallability and interference may pose a problem for simple models such as the one described Figure 2. Simulations of the SAM-REM model. Simulations 1–3 show .by Rundus (1973). It is not a problem for more complex models ء the effect of varying the u parameters during practice as follows: Sim 1 Note that the model used to illustrate this point was not constructed ϭ ء ϭ ء ϭ ء ϭ ء ϭ ء (u c .03, u i .02), Sim 2 (u c .01, u i .02), Sim 3 (u c .01, ϭ specifically for this purpose. SAM-REM and its parent models are ء u i .03). Sim 4 shows the effect of increasing the number of competitors. Pϩ items were not practiced; instead, novel items were presented during general models of memory that have been applied to a wide range .ϭ of phenomena in recall, recognition, and priming ء ء practice. The u parameters were identical in study and practice (u c .02, ϭ The simulations make the important point that tests of strength ء u i .02). A retrieval-induced forgetting (RIF) effect is evident in Sim- ulations 1 and 4. It is absent in Simulations 2 and 3. Pϩ,PϪϭitems from dependence often employed in the RIF literature do not, in theory, practiced categories; NP ϭ items from nonpracticed categories. discriminate between mechanisms of inhibition and competitor 1438 VERDE

interference. This has implications for the case for inhibition that category name given only the exemplar. The absence of a category will be explored later. A more immediate question is whether the stem cue (FR_) made it necessary to think about the category– model simulations offer not just a theoretical but a plausible exemplar association in order to produce an answer. Processing account of RIF. The best way to answer this is by empirically this association would lead to binding of context and item infor- testing the hypothesis that the form of encoding during practice mation. mediates the emergence of a RIF effect. Before doing so, it is The issue with the practice tasks described above is that when worth considering what the differential encoding hypothesis might the cue and target have a strong preexisting association, there may have to say about previous findings. be little incentive to focus on the stimulus features that cause Investigations of strength dependence in RIF have tended to use interference unless the practice task explicitly requires that focus. category–exemplar pairs like FRUIT Cherry. One can assume that When the cue–target association has to be learned during the the association between the category name and the exemplar is so experiment, on the other hand, people might be expected to focus well learned that, once the relevant categories have been estab- on the association even during study practice. Shivde and Ander- lished during the study phase, the focus of the task becomes son (2001) found that with mildly associated pairs (Arm–Missile), learning the exemplars. If pairs are presented again during the retrieval and study practice produced similar RIF effects. Ander- practice phase for additional study, participants might be expected son and Bell (2001) found the same to be true with unrelated to pay little attention to the category name and focus instead on the person–object pairs (the Actor is looking at the Violin).4 However, exemplar. The category name is no longer helpful to learning (or Ciranni and Shimamura (1999, Experiment 5) failed to find a RIF to put it another way, little is gained by attending to the category effect following study practice using stimuli consisting of shapes name). On the other hand, practice involving cued recall (FRUIT with unique locations but shared colors (three colors, four shapes C_?) forces people to process the category name much more per color). During retrieval practice, shapes were recalled given deeply; it is key to producing an answer, especially if the word the color and location cues. During study practice, colored shapes stem cues are not unique. With category–exemplar pairs, the were shown in their locations. The final test required recalling possibility that people process the category context more during shapes given color and location cues. A notable aspect of the recall than during additional study could explain the absence of the design is that there were two cues, one that led to competition with RIF effect in the latter condition (Hulbert et al., 2012; Johansson other items (color) and another that did not (location). Because et al., 2007; Staudigl et al., 2010; Wimber et al., 2009). location was a perfectly discriminative cue, a reasonable strategy Rather than using additional study, some investigators have used during study practice would be to focus on the location–shape a noncompetitive practice task requiring recall of the category association rather than the imperfectly discriminative color–shape name, not the exemplar (FR_? Cherry). The inhibition account association. Retrieval practice, on the other hand, may have often predicts no RIF effect because the task does not require the called for the supplementary use of color as a cue on trials when activation (and suppression) of other exemplars. On the other location did not immediately cue shape. Focus on different aspects hand, the task does require attention to the cue. If the category of context (some of which would not produce interference) may name is encoded as context, the competitor interference account predicts a RIF effect. The findings have been mixed, with RIF explain the different effects of retrieval and study practice in observed in some studies (Jonker & MacLeod, 2012; Raaijmakers Ciranni and Shimamura’s study. & Jakab, 2012) but not others (Anderson, Bjork, & Bjork, 2000; Applying the differential encoding hypothesis to past findings is Hanslmayr, Staudigl, Aslan, & Bäuml, 2010; Saunders, Fernandes, necessarily speculative. It also does not exclude the possibility that & Kosnes, 2009). These discrepancies invite speculation about other factors contribute to forgetting, including inhibition or other possible differences in encoding during competitive (FRUIT C_?) aspects of competitor interference. For example, Raaijmakers and and noncompetitive (FR_? Cherry) retrieval practice. Jakab (2012) proposed another competition-based explanation for ϩ Ϫ Consider that producing an answer to FRUIT C_? is difficult. In the dissociation between P recallability and P suppression. a typical design, the target Cherry is one among dozens of studied They noted that the typical absence of feedback during retrieval ϩ exemplars and previously appeared only once. Focusing on the practice means that some P items are recalled and others are not. category–exemplar association offers an efficient retrieval path to Therefore, not all Pϩ items benefit from practice. During study the target; doing so binds context and item information so that they practice, on the other hand, all Pϩ items benefit. If retrieval is This document is copyrighted by the American Psychological Association or one of its allied publishers. are stored within a single representation, as was assumed in the assumed to be more effective than study at strengthening, then, This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. SAM-REM simulations. Contrast this with the ease of producing following retrieval practice, successfully recalled Pϩ items will be the answer to FR_ Cherry? In a typical design, the target FRUIT at high strength and nonrecalled Pϩ items will be at low strength. is one of only a handful of category names, all of which have been Following study practice, all Pϩ items will be at medium strength. repeated many times. The answer may be so readily apparent from Average recall of Pϩ items may turn out to be equivalent in the the word stem (FR_) alone that there is little need to process the two conditions, but the high-strength Pϩ items in the retrieval category–exemplar association, resulting in the category and ex- condition may produce more interference than the more numerous emplar being processed separately. Failure to bind category con- medium-strength Pϩ items in the study condition. The potential text and item information within a single representation would problem created by lack of feedback is plausible and may have produce exemplar representations that have less contextual overlap with competitors. Interestingly, the two studies that did observe 4 Anderson and Bell (2001) reported that only participants who claimed RIF following noncompetitive retrieval practice (Jonker & Ma- to engage in “covert retrieval” during study practice showed a significant cLeod, 2012; Raaijmakers & Jakab, 2012) used a slightly more RIF effect. In terms of differential encoding, these participants may have difficult form of the task. Participants were asked to produce the been the ones most likely to focus on the cue–target association. INTERFERENCE AND FORGETTING 1439

played a role in some previous findings. It was avoided in the Experiments 1 and 2 experiments to follow by providing feedback during retrieval ϩ practice, so that most Pϩ were successfully recalled. Many studies have shown that though retrieving P items during the practice phase suppresses recall of PϪ items, merely studying Pϩ items during practice does not. The purpose of General Method (Experiments 1–5) Experiment 1 (retrieval practice) and Experiment 2 (study practice) was to replicate both of these findings. Materials and Design The critical stimuli consisted of eight exemplars from each of Method six semantic categories (animals, body parts, clothing, fruits, pro- fessions, instruments). Exemplar rank ranged from 3 to 9.1 (Van Participants. Fifty-five University of Plymouth undergradu- ϭ Overschelde, Rawson, & Dunlosky, 2004). No two exemplars ates participated for course credit (Experiment 1 27, Experiment ϭ within a category shared the same initial letter. All exemplars were 2 28). presented during the study phase. For the three practiced catego- Materials and procedure. See General Method. The two ries, four exemplars (PϪ items) appeared in Block 1 of the final experiments were identical in all respects save for the task used test. The remaining four exemplars (Pϩ items) appeared in the during practice. ϩ practice phase and Block 2 of the final test. Pϩ items were tested In Experiment 1, during the practice phase P items were in the last block to ensure that any effects observed on PϪ items presented for cued recall. On each trial, a single retrieval cue could be attributed to practice manipulations rather than to output consisting of the category name and initial two letters of the target interference from Pϩ items. For the three nonpracticed categories, exemplar (FRUIT Ch_?) was presented in the center of the screen. four exemplars (NPϪ items) appeared in Block 1 of the final test. Participants were instructed to press the space bar as soon as they The remaining four exemplars (NPϩ items) appeared in Block 2 of remembered the target and then type their answer on the keyboard. the final test. NPϩ and NPϪ items served as the controls for Pϩ If they did not remember studying an exemplar beginning with and PϪ items, respectively. In addition to the critical stimuli, a those letters, they were to type no. The retrieval cue remained number of filler stimuli were drawn from two other categories onscreen until a response was made. After every response, the (countries, vehicles). The assignment of categories and exemplars correct answer (FRUIT Cherry) was displayed for 1,000 ms to ϩ to conditions, and the position of items in study, practice, and test ensure encoding of the P item. Trials were separated by a lists, were randomized uniquely for each participant. 2,000-ms ISI. In Experiment 2, during the practice phase Pϩ items were presented for additional study. Participants were told that they Procedure would have another chance to see some of the pairs and, as before, Participants were seated at individual computers. The experi- should try to remember them for later. On each trial, a single ment progressed in three phases: study, practice, and final test. At category–exemplar pair (FRUIT Cherry) was shown in the center the beginning of the study phase, participants were told that they of the screen for 3,000 ms followed by a 500-ms ISI. The display would be shown a list of category–exemplar pairs that they should procedure was identical to that used during the study phase. try to remember. Individual pairs (FRUIT Cherry) were then presented on the computer screen, each for 3,000 ms followed by Results and Discussion a 500-ms interstimulus interval (ISI). The study list consisted of 56 items (48 critical items, with four filler items at the beginning and Rates of accurate recall are shown in Table 1. The critical four filler items at the end of the list as primacy/recency buffers). comparisons were between PϪ and NPϪ items, which appeared in During the practice phase, individual items were presented for the first half of the test, and between Pϩ and NPϩ items, which encoding in a manner unique to each experiment. Each Pϩ item appeared in the second half of the test. was presented twice, once in the first half and once in the second Experiment 1. During retrieval practice, participants success- half of the practice list. The practice list consisted of 28 items (24 fully recalled Pϩ items 90% of the time upon their second pre- critical items, with two filler items at the beginning and two filler sentation. The encoding that occurred during the practice phase This document is copyrighted by the American Psychological Association or one of its allied publishers. items at the end of the list). During the final test phase, each cued enhanced the memorability of the Pϩ items, which were recalled This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. recall trial began with the presentation of a retrieval cue consisting at a higher rate than NPϩ items, t(26) ϭ 11.87, p Ͻ .001. of the category name and initial letter of the target exemplar Enhanced recall of Pϩ items was accompanied by suppressed (FRUIT C_?). Participants were instructed to press the space bar as recall of PϪ items, which were recalled at a lower rate than NPϪ soon as they remembered the target and then type their answer on items, t(26) ϭ 2.48, p ϭ .02. the keyboard. If they did not remember studying an exemplar Experiment 2. The encoding that occurred during the practice beginning with that letter, they were instructed to not guess but phase enhanced the memorability of the Pϩ items, which were instead type no. The retrieval cue remained onscreen until a recalled at a higher rate than NPϩ items, t(27) ϭ 5.73, p Ͻ .001. response was made. A 2,000-ms ISI followed each response. The Enhanced recall of Pϩ items was not accompanied by suppressed final test began with four filler trials followed by two 24-trial recall of PϪ items, which were recalled at a higher rate than NPϪ blocks. PϪ and NPϪ items were tested in Block 1. Pϩ and NPϩ items, t(27) ϭ 2.16, p ϭ .04. The latter trend was unexpected but items were tested in Block 2. The two-block structure of the test must be interpreted with caution, given the slightly depressed list was not revealed to participants, who experienced only a recall rate for NPϪ items in this experiment. Table 1 shows that continuous 52-trial test list. recall rates of NPϪ items were very similar across the six exper- 1440 VERDE

Table 1 Rates of Correct Recall

Item type Pϩ NPϩ PϪ NPϪ Experiment Practice task MSDMSDMSDMSD

1 Cued recall 0.72 0.17 0.30 0.16 0.32 0.14 0.41 0.19 2 Study 0.65 0.18 0.37 0.21 0.46 0.20 0.37 0.19 3 Best category 0.70 0.15 0.37 0.16 0.33 0.19 0.42 0.16 4 Feelings (context) 0.69 0.20 0.34 0.17 0.32 0.18 0.46 0.17 5 Feelings (no context) 0.58 0.18 0.40 0.18 0.42 0.17 0.44 0.13 6 Study (extralist) 0.29 0.16 0.33 0.13 0.32 0.17 0.44 0.21 Note. The retrieval-induced forgetting (RIF) effect is measured by the comparison of PϪ and NPϪ items, which were tested in the first half of the test. The facilitation of practiced items is measured by the comparison of Pϩ and NPϩ items, which were tested in the second half of the test. In Experiment 6, Pϩ items were not practiced and appeared only in the study phase. Pϩ,PϪϭitems from practiced categories; NPϩ,NPϪϭitems from nonpracticed categories.

iments, as would be expected given that NPϪ items were encoded interference-relevant context should produce competitor interfer- under identical conditions. The rate of PϪ recall in Experiment 2 ence. This could include retrieval practice, but it could also include was similar to that of NPϪ items in the other experiments, con- various forms of study practice. For example, additional study of sistent with a null RIF effect. The outlier is the rate of NPϪ recall Pϩ items that actively encourages processing of the category– in Experiment 2. exemplar association should have a negative impact on the recall The findings match those of other studies using similar materials of PϪ items. The inhibition account, on the other hand, predicts a and procedures (Hulbert et al., 2012; Johansson et al., 2007; RIF effect only when the practice task involves retrieval. Staudigl et al., 2010; Wimber et al., 2009). Inhibition theory Experiment 3 tested the prediction of the differential encoding predicts that RIF should be retrieval dependent, occurring only hypothesis that a RIF effect will emerge during some forms of when the practice task involves retrieval of Pϩ items. Consistent study practice. During the practice phase, each Pϩ category– with this prediction, the RIF effect was present in Experiment 1 exemplar pair was presented, and participants were asked decide where practice involved recalling Pϩ items but was absent in whether the category was the first category they would choose for Experiment 2 where practice involved additional study of Pϩ the given exemplar. For example, with the pair VEHICLE rowboat, items. The pattern of results in Experiments 1 and 2 has been if asked “what category does rowboat belong to?” participants consistently interpreted in the literature as strong support for the were to consider whether they would choose VEHICLE or some inhibition account of RIF because it seems to be inconsistent with other category, perhaps WOODEN THINGS. The task required competitor interference. semantic processing of the item in relation to the category cue, but As the SAM-REM simulations have shown, the pattern is not it did not require that the item be retrieved from memory. necessarily inconsistent with competitor interference. The differential encoding hypothesis suggests that a task like category–word Method stem cued recall (FRUIT C_?) requires a focus on the cue–target association and will lead to enhanced storage of the context fea- Participants. Thirty-one University of Plymouth undergradu- tures responsible for producing competitive interference. A task ates participated for course credit. that requires only the study of category–exemplar pairs (FRUIT Materials and procedure. See General Method. During the Cherry) does not require a focus on the cue–target association practice phase, Pϩ items were presented for category judgments. because with materials such as these, the association is already An individual pair (FRUIT Cherry) was shown on the computer screen for 3,000 ms, after which the question “Is this the first This document is copyrighted by the American Psychological Association or one of its alliedwell publishers. learned. The challenge in the task is to learn the exemplars, category you would choose for this item?” appeared below the This article is intended solely for the personal use ofand the individual user and is notparticipants to be disseminated broadly. likely attend mainly to the exemplar when the pair is repeated during study practice. Encoding the features of the pair. Participants were instructed to consider the target word target item will contribute to its subsequent recall but will have (Cherry); if they were asked to choose a category for the word, little effect on the recall of other items. The differential encoding would the presented category (FRUIT) be their first choice or account of the dissociation between Pϩ recall and PϪ suppression would they choose a different category as their first choice? observed in Experiments 1 and 2 was tested in Experiments 3–5. Participants responded by pressing the 1 or 2 key to indicate “yes” or “no,” respectively. Trials were separated by a 2,000-ms ISI. Experiment 3 Results and Discussion The ability of SAM-REM to produce the two different patterns of performance observed in Experiments 1 and 2 makes the model Rates of recall are shown in Table 1. The encoding that occurred seem quite flexible. However, the factors that produce these dif- during the practice phase enhanced the memorability of the Pϩ ferent patterns have been made explicit and can be tested. The items, which were recalled at a higher rate than NPϩ items, model predicts that encoding that encourages the processing of t(30) ϭ 11.29, p Ͻ .001. Enhanced recall of Pϩ items was INTERFERENCE AND FORGETTING 1441

accompanied by suppressed recall of PϪ items, which were re- Method called at a lower rate than NPϪ items, t(30) ϭ 2.96, p ϭ .006. The RIF effect was similar to that observed in Experiment 1, which Participants. Fifty-two University of Plymouth undergradu- ϭ used retrieval practice. The results are consistent with the predic- ates participated for course credit (Experiment 4 27, Experiment ϭ tion that a focus on the processing of the local context (FRUIT)in 5 25). relation to the target item (Cherry) should increase competitor Materials and procedure. See General Method. In Experi- ϩ interference, as shown in Simulation 1. According to the inhibition ment 4, during the practice phase P items were presented for account, on the other hand, RIF should have been absent because feelings judgments. An individual pair (FRUIT Cherry) was shown retrieval of the target item is necessary to trigger the inhibition of on the computer screen for 3,000 ms, after which the question competitors and the practice task did not require retrieval. “How do you feel about this item?” appeared below the pair. People might sometimes adopt a covert retrieval strategy even Participants were instructed to make a subjective judgment about when retrieval is not explicitly required by the task (Anderson & their feelings. Participants were given a 7-point rating scale (with ϭ ϭ ϭ Bell, 2001). For example, while making category judgments in the the labels 1 not like, 4 neutral, and 7 like), and they pressed practice phase, participants may have attempted to recall prior a number key to indicate their feelings. Trials were separated by a encounters with the pairs during the study phase. Although this is 2,000-ms ISI. In Experiment 5, the practice task was identical unnecessary and irrelevant to the category judgments, participants except that each exemplar (Cherry) was presented by itself without may have been motivated by a desire to rehearse the exemplars in the category name. preparation for an expected memory test. If this were the case, the use of a covert retrieval strategy would seem even more likely Results and Discussion during the study practice of Experiment 2, where the focus was explicitly to rehearse for a memory test. It is hard to explain the use Rates of recall are shown in Table 1. of covert retrieval in Experiment 3 but not in Experiment 2. Experiment 4. The encoding that occurred during the practice Moreover, the many failures to observe a RIF effect following phase enhanced the memorability of the Pϩ items, which were study practice suggest that covert retrieval is not common (see recalled at a higher rate than NPϩ items, t(26) ϭ 8.41, p Ͻ .001. Verde, 2009). Finally, it might be noted that the encoding instruc- Enhanced recall of Pϩ items was accompanied by suppressed tions during practice did require the retrieval of semantic knowl- recall of PϪ items, which were recalled at a lower rate than NPϪ edge about the exemplar. However, this retrieval is noncompeti- items, t(26) ϭ 3.62, p ϭ .001. tive; it might activate information about the exemplar itself, but Experiment 5. The encoding that occurred during the practice there is no obvious reason for it to activate memories of other phase enhanced the memorability of the Pϩ items, which were studied items. recalled at a higher rate than NPϩ items, t(24) ϭ 4.93, p Ͻ .001. Enhanced recall of Pϩ items had no effect on PϪ items, which were recalled at the same rate as NPϪ items, t(24) ϭ 0.42, ns. Experiments 4 and 5 In Experiment 4, a RIF effect was again observed with a practice The intent in Experiments 4 and 5 was to manipulate the task that did not require retrieval of the Pϩ items. The category processing of context in a more controlled manner. As in Exper- name was presented alongside the exemplar during the feelings iment 3, the practice task was one that plausibly encouraged the judgment. In Experiment 5, when the category name was removed encoding of the category context. The task did not require retrieval and the exemplar was presented alone during the feelings judg- of Pϩ items and so should not, according to the inhibition account, ment, the RIF effect disappeared. The presence and absence of a produce RIF. In Experiment 4, during the practice phase, each Pϩ RIF effect tied to the presence and absence of the interfering category–exemplar pair was presented and participants were asked context is consistent with the differential encoding hypothesis. to decide how they felt about the item. Participants responded on Because the encoding instructions were identical in the two ex- a 7-point rating scale where 1 ϭ not like and 7 ϭ like. Attitudes are periments, the results also argue against the possibility raised by context dependent (Fazio, 2007), and for these judgments partic- Saunders et al. (2009) that any form of semantic retrieval related ipants would likely use the category as a way to contextualize their to the exemplar will produce RIF (see also Jonker & MacLeod, feelings. For example, with the pair VEHICLE rowboat, partici- 2012). Two things might be noted about the effect of the practice This document is copyrighted by the American Psychological Association or one of its allied publishers. pants might rate their feelings for rowboat given its properties as task on Pϩ items in Experiment 5. First, there was noticeably less This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. a vehicle. facilitation than in Experiment 4, where the category cues were In Experiment 5, the practice task was identical but the exemplar provided during practice. This is not surprising. The cue–target was presented alone without the category name. The absence of the association is helpful for later recall of the target, and practice that interfering context during practice is a strong version of the con- does not promote the encoding of this information will be less dition depicted in Simulations 2 and 3. Of course, participants beneficial. Second, practice in the absence of the cue still facili- might sometimes spontaneously think of the categories during tated recall relative to the NPϩ baseline, consistent with the role practice, given their recent exposure during the study phase. How- described by SAM-REM of item information in the recovery ever, this would certainly occur far less often than in Experiment process. 4, where the categories were explicitly presented on every trial. The differential encoding hypothesis predicts that competitor in- Number-Dependent Competition terference should, at minimum, be greater in Experiment 4 than in Experiment 5. More likely, the RIF effect should be absent in Discussion in the RIF literature has focused exclusively on the Experiment 5 as it was with the study practice of Experiment 2. strength dependence of competitor interference and has had little to 1442 VERDE

say about its number dependence. This may be due to a perception before, should try to remember them for later. Each of the that the case against strength-dependent forgetting is sufficiently category–exemplar pairs was shown once, so that the practice list strong that other aspects of competitor interference models need not consisted of 22 items (18 critical items, with two filler items at the be considered. However, the previous section established that a beginning and two filler items at the end of the list). Individual strength-dependent model offers a viable alternative to inhibition for pairs (FRUIT Apricot) were shown on the computer screen for some cases of RIF. This section will consider whether other cases of 3,000 ms with a 5,00-ms ISI. The display procedure was identical RIF previously attributed to inhibition might instead be explained in to that used during the study phase. The novel exemplars presented terms of number-dependent forgetting. A clear example is the use of during practice were not included in the final test. extralist retrieval to produce RIF. A number of studies have shown that generating new exemplars of a category during the practice phase Results and Discussion reduces the ability to recall studied exemplars (Bäuml, 2002; Storm, Bjork, & Bjork, 2007, 2008; Storm et al., 2006; Storm & Nestojko, Rates of recall are shown in Table 1. Increasing the number of 2010). The retrieval of novel category exemplars is thought to pro- competitors by practicing novel items from the same category duce inhibition in the same way as the retrieval of studied exemplars. should interfere with recall of both PϪ and Pϩ items. This was The claim is that attempting to retrieve from a category, regardless of clearly evident for PϪ items, which were recalled at a lower rate what is retrieved, will activate other exemplars and lead to their than NPϪ items, t(28) ϭ 3.02, p ϭ .005. Pϩ items were recalled inhibition. at a slightly lower rate than NPϩ items, although the difference A study by Bäuml (2002) is of particular interest because it was not statistically significant, t(28) ϭ 1.43, p ϭ .163. A reduc- compared extralist retrieval practice to nonretrieval practice. Par- tion in the apparent effect on Pϩ items is not surprising, given that ticipants were shown exemplars with category labels and asked to presentation of these items was delayed until the latter half of the relate each exemplar to its category while rehearsing it for a test. By this time, other factors would have contributed to the memory test. During the practice phase, a new set of exemplars forgetting of both Pϩ and NPϩ items, including output interfer- from the studied categories was presented. In the retrieval practice ence and other nonspecific forms of interference (Verde, 2009). condition, participants generated the exemplars from category Thus, competition from the novel practiced items would have been labels and word stems. In the study practice condition, participants responsible for a smaller proportion of the forgetting suffered by were shown exemplars with category labels and had to name the Pϩ items, and its effect was likely overshadowed by the other exemplar. A RIF effect was observed only in the retrieval practice factors. The less adulterated measure of number-dependent forget- condition. The absence of a RIF effect in the nonretrieval practice ting lies with the PϪ items presented in the first half of the test, condition is at first glance inconsistent with the number-dependent and the effect was clear. forgetting. However, in the naming task, attention to the category– Save for the materials, the study practice task used in this experi- exemplar association is not only unnecessary but of negative ment was identical to that used in Experiment 2. The inhibition benefit, as it would slow down naming the exemplar. Failure to account has difficulty explaining why the RIF effect was observed attend to the category context could explain the lack of RIF in this here but not in Experiment 2. In competitor interference models, the particular practice task. strength and number of competitors contribute independently to in- This interpretation of Bäuml’s (2002) results is speculative, and it terference. It was suggested earlier that when the cue–target relation is is therefore useful to demonstrate number-dependent forgetting em- already well learned, as with category–exemplar pairs, people may pirically. Experiment 6 was identical in most respects to Experiment process the cue–target association upon first encounter but are likely 2, in which additional study of Pϩ items failed to produce a RIF to give it much less attention on subsequent encounters unless the effect. However, in place of the Pϩ items, a set of novel (extralist) encoding task requires it. This can explain why simply presenting Pϩ exemplars belonging to the same categories as Pϩ items was studied items again for study would produce little additional strength-based during the practice phase. As shown in Simulation 4, increasing the interference. When novel exemplars are presented for study during the number of competitors associated with the category cue can produce practice phase, the category context would be attended to in a manner a RIF effect even in the absence of retrieval. similar to that of exemplars from the earlier study phase. The result would be an increase in the number of competitors associated with the category cue, thereby producing number-based interference. Experi-

This document is copyrighted by the American Psychological Association or one of its allied publishers. Experiment 6 ment 6 demonstrates that the study of additional items causes forget- This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. Method ting of competitors that is not easily attributed to retrieval inhibition. The implication for studies that have used the retrieval of novel items Participants. Twenty-nine University of Plymouth under- to produce RIF is clear. Although these studies have assumed inhi- graduates participated for course credit. bition to be the source of forgetting, forgetting in fact can occur Materials and procedure. The experiment followed the Gen- simply as a result of increasing the number of competitors associated eral Method but with some differences in the design of the practice with a cue. phase. For each of the critical categories, an additional set of six Number-dependent interference may offer insight into one of the novel exemplars was obtained for use during the practice phase. more enigmatic findings to be offered as evidence against strength- None of these shared their initial letter with studied exemplars dependent forgetting. Storm et al. (2006; Storm & Nestojko, 2010) from the category. During the practice phase, the novel exemplars had participants study category–exemplar pairs and then asked them from each of the practiced categories were studied (Pϩ items were to generate exemplars from category–word stem cues. The word not presented during practice). At the beginning of the phase, stems did not match the studied exemplars, nor could they be legally participants were told that they would view more pairs and, as completed with any exemplar belonging to the category. This “im- INTERFERENCE AND FORGETTING 1443

possible” retrieval task produced RIF, leading Storm et al. to conclude covert retrieval even when the task does not explicitly require it that merely attempting retrieval is sufficient to produce inhibition. The (e.g., Anderson & Bell, 2001). For example, seeing the exemplar RIF effect could not have been due to the strengthening of studied during practice might prompt participants to recall its previous items, because none were produced during practice. However, appearance in the study list, triggering the inhibition of other number-dependent interference offers an alternative explanation. De- exemplars. It is not obvious why this would be desirable in the spite the nature of the cues, Storm et al. reported that participants often category and feelings judgment tasks of Experiments 3 and 4. It is produced answers. The search for potential answers may have also also not clear why participants would do this in Experiment 6 but brought to mind illegal examples of the category that were not not in Experiment 2, which used an identical practice task. The verbalized. These generated items, verbalized or not, would have findings are entirely consistent with a competitor interference become associated with the category cue, increasing the number of account. SAM-REM Simulations 1–3 show that a dissociation competitors. between facilitation of Pϩ items and forgetting of PϪ items is not inconsistent with strength dependence. Experiments 3 and 4 dem- General Discussion onstrate that a RIF effect can be obtained through strengthening in the absence of retrieval. Experiments 4 and 5 demonstrate that the Theories of recall generally agree that the ability to retrieve a manner of strengthening is important: A RIF effect depends on the specific memory is made more difficult by the tendency for a encoding of the interfering context. retrieval cue to activate competing memories. However, the mech- Competitor interference suggests that forgetting is not only strength anism by which this leads to forgetting remains a matter of debate. dependent but also number dependent. As shown in Simulation 4, the Models based on competitor interference suggest that competition SAM-REM model predicts that increasing the number of competitors for retrieval among the associates of a cue interferes directly with should exacerbate competitor interference. This prediction is consis- the ability to recall an intended target. In contrast, Anderson et al.’s tent with the results of Experiment 6, in which the study of novel (1994; Anderson & Bjork, 1994) inhibition theory proposes that (extralist) exemplars during practice produced a RIF effect. The competition among associates is normally not the primary cause of finding is difficult to explain in terms of retrieval inhibition. Study forgetting. Rather, forgetting occurs because competing associates practice does not require retrieval and was shown in Experiment 2, as are actively inhibited to facilitate retrieval of the intended target. well as in previous studies, to produce no RIF effect when practice Inhibition theory can in principle be applied to a range of phe- involved only items that had been studied earlier. nomena traditionally within the domain of competitor interference models, such as retroactive interference, output interference, part– Conclusion set cuing, and the list-strength effect (Anderson et al., 1994). Anderson et al. (1994; Anderson & Bjork, 1994) introduced inhi- However, the most compelling support for inhibition theory has bition theory as an alternative to theories of forgetting based on been its seemingly unique ability to account for RIF. competitor interference. Their initial studies of RIF suggested that A primary argument against a competitor interference mecha- inhibition alone could account for the findings. A sizable literature nism is the apparent strength independence of RIF. A dissociation devoted to investigating RIF over the last two decades has generally between the facilitation of Pϩ items and forgetting of PϪ items supported this conclusion, in the process greatly elaborating inhibition was demonstrated in Experiments 1 and 2. Practice involving cued theory and establishing it as one of the major theoretical approaches recall and additional study both benefited later recall of Pϩ items, to forgetting (Anderson, 2003; Storm & Levy, 2012). The case for but only recall practice suppressed recall of PϪ items. This rep- inhibition has relied to a great extent on its favorable comparison with licates the pattern observed in a number of previous studies. competitor interference. However, recent reviews of RIF have high- However, subsequent experiments demonstrated that it is possible lighted inadequacies in the theoretical treatment of competitor inter- to obtain a RIF effect without retrieval practice. This was shown ference models (Raaijmakers & Jakab, 2013; Verde, 2012). with three different practice tasks. With two tasks, category judg- In the RIF literature, competitor interference is discussed largely ment (Experiment 3) and feelings judgment (Experiment 4), Pϩ in terms of a model like the one outlined by Rundus (1973),in items were encoded in ways meant to encourage the processing of which an item’s strength is synonymous with its recallability. This the category–exemplar association. The SAM-REM model pre- operational definition of strengthening is implicit in the notion that dicts that strengthening should exacerbate competitor interference This document is copyrighted by the American Psychological Association or one of its allied publishers. practice that facilitates Pϩ recall but has little effect on PϪ recall to the extent that it involves encoding of the interfering context. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. is inconsistent with strength-dependent forgetting. It is also im- Consistent with this prediction, the feelings judgment produced no plicit in the notion that a failure to find a correlation between Pϩ RIF effect when the interfering context was absent during practice facilitation and the magnitude of RIF is inconsistent with strength- (Experiment 5). The third task simply involved additional study. It dependent forgetting (Hulbert et al., 2012). When strength is was hypothesized that with category–exemplar pairs, the associa- defined in a more nuanced way, as in the SAM-REM model, tion is so well learned that people attend to the category upon their neither of these holds true. A competitor interference model of this first encounter with each pair but tend to ignore it thereafter. This kind offers a viable account of the RIF effect observed in the would explain why study practice with instructions to simply present set of experiments, and this account extends to many “remember the items for later” produces no RIF effect (Experi- previous results thought to be uniquely compatible with inhibition. ment 2) unless the studied items are novel (Experiment 6). In the latter case, the SAM-REM model predicts that increasing the RIF Reconsidered number of competitors should exacerbate competitor interference. These findings are not readily explained in terms of retrieval Does inhibition offer a better account of RIF when one consid- inhibition. It has been suggested that people sometimes engage in ers the literature as a whole? The case for inhibition has centered 1444 VERDE

on four predictions thought to differentiate inhibition from com- mary above suggests, inhibition does not readily explain many petitor interference (Anderson, 2003). The inhibition account pre- findings in the literature. More important, the application of more dicts that RIF should be retrieval dependent, strength independent, sophisticated alternative models to RIF data is making it increas- cue independent, and interference dependent. The first two predic- ingly clear that competitor interference is difficult to rule out as a tions are related and were the subject of the present article. First, mechanism of forgetting (see also Raaijmakers & Jakab, 2012; if RIF is a product of the retrieval process, then it will be observed Verde, 2009). This could be taken as an argument that competitor only when the practice task involves retrieval. A task that does not interference models may ultimately prove sufficient to account for require retrieving the target from memory, such as study practice, RIF without the need for postulating inhibitory mechanisms. Al- should not produce RIF. Second, if RIF depends only on the act of ternatively, it could be taken as encouragement to refine inhibition retrieval, then a task that strengthens items in the absence of theory. In framing inhibition as an alternative to competitor inter- retrieval, such as study practice, should not produce RIF. A model ference, studies of RIF have typically focused on attempting to rule like SAM-REM can produce patterns of forgetting that are con- out the latter as a source of forgetting. Yet, competition among the sistent with both of these predictions. Therefore, retrieval depen- associates of a cue is fundamental to inhibition theory in that dence and strength independence do not distinguish inhibition competition is what makes inhibition necessary. The tension be- from competitor interference in general, although they might be tween these two things must be resolved, and doing so is critical useful in ruling out certain classes of models. for continued development of the theory (Storm & Levy, 2012; The ability of cue independence to differentiate inhibition from Verde, 2012). A first step is to define precisely how the activation competitor interference has also been disputed. In a competitor of associates creates difficulty during retrieval. interference model, forgetting is tied intrinsically to the retrieval If the cost of competition is not retrieval failure, then perhaps it cue. The cue determines via its associations which competitors are takes the form of slowed retrieval. Bäuml, Zellner, and Vilimek activated, and this determines the degree of interference. Inhibition (2005) examined recall latency in the RIF paradigm from the per- acts not on the associative links but on the representation itself. An spective of two-stage models of recall. If search involves a sampling- inhibited memory becomes inaccessible in any context. Therefore, with-replacement process, as in SAM-REM, free recall latency can be RIF should be observed even when the retrieval task uses novel used as an index of interference during the sampling stage. Mean cues. The prediction of cue independence has been supported by a recall latency, measured from the initiation of retrieval, is known to be range of findings showing that RIF persists when the cue used influenced by the number and relative strength of competitors during recall, or the retrieval task itself, differs from the cue or task (Wixted, Ghadisha, & Vera, 1997; Wixted & Rohrer, 1994). Bäuml et used during retrieval practice. However, it is problematic for the al.’s materials consisted of exemplars that belonged to a common inhibition account that cue-independent RIF is often not obtained category but one of two different subcategories (INSTRUMENT (Jonker & MacLeod, 2012; Verde, 2012). This inconsistency has STRING Guitar, INSTRUMENT WIND Oboe). In Experiment 1, led some to argue that RIF is cue or context dependent, as pre- during the study phase, each exemplar was shown with its category dicted by competitor interference. A problem with evaluating the and subcategory label. During the practice phase, exemplars from one results from independent cue experiments is that people may of the subcategories (Guitar;Pϩ items) were recalled with the help of sometimes reinstate earlier cues as an aid to retrieval. These cues, category–word stem cues. During the final test, each category and rather than the novel cues, may be responsible for RIF. Two types subcategory label was presented and participants freely recalled the of evidence support this possibility. First, the presence or absence items from the given subcategory. Items from the nonpracticed sub- of RIF has been tied to thinking back to an earlier encoding category (Oboe;PϪ) were recalled at a lower rate than NP items, but episode (Camp, Pecher, & Schmidt, 2005; Perfect et al., 2004; mean recall latency of PϪ and NP items did not differ. Bäuml et al. Racsmány & Conway, 2006; Verde & Perfect, 2011). Second, cue concluded that inhibition from retrieval practice does not affect the reinstatement has been observed in recall under some conditions sampling stage but does affect the recovery stage by decreasing the (Camp, Pecher, Schmidt, & Zeelenberg, 2009; Huddleston & activation of Pϩ representations to below the threshold of recovery. Anderson, 2012).5 Retrieval failure at the recovery stage points to a mechanism Of the four predictions, interference dependence seems most other than competitor interference, perhaps inhibition. However, able to differentiate between the accounts. If the purpose of inhi- the recall latency data have to be understood in the context of a bition is to preempt interference from competitors, then the like- control experiment Bäuml et al. (2005, p. 1227) conducted to This document is copyrighted by the American Psychological Association or one of its allied publishers. lihood of an item being inhibited should depend on its potential to compare the efficacy of category and subcategory cues. The study This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. interfere with the retrieval target. RIF should therefore affect phase was identical to the RIF experiment, but there was no strong competitors but not weak competitors. With competitor practice phase. For the final test, the cues were either the category interference, there is no obvious reason for this to occur. Relatively labels alone or the category and subcategory labels together. The few studies have investigated interference dependence. Their re- addition of subcategory cues reduced mean recall latency, which sults have been mixed, with some finding that strong competitors suggests that the search set was restricted to items from the are more likely to suffer from RIF (Anderson et al., 1994; Bäuml, subcategory. In Bäuml et al.’s Experiment 1, subcategory cues 1998; Storm et al., 2007) and others finding no such effect (Garcia- were used in the final test. Because Pϩ items did not belong to the Bajos, Migueles, & Anderson, 2009; Jakab & Raaijmakers, 2009; Williams & Zacks, 2001). At present, the evidence does not 5 strongly favor one account over the other. Huddleston and Anderson (2012) found evidence of spontaneous context reinstatement only when previous contexts were highly related to the The investigation of RIF has been motivated largely by the test cues. They ruled out high cue relatedness in the stimulus sets from two desire to test and develop inhibition theory, and inhibition remains studies using the think/no-think paradigm, but whether it is an issue in the prevalent account of the phenomenon. However, as the sum- studies of RIF remains an open question. INTERFERENCE AND FORGETTING 1445

same subcategory, they would not be expected to interfere with PϪ long-term memory: A new taxonomy. In D. Dagenbach & T. H. Carr items during the sampling stage. The null effect of Pϩ practice on (Eds.), Inhibitory processes in attention, memory and language (pp. PϪ recall latency is therefore not inconsistent with the predictions 265–326). New York, NY: Academic Press. of a competitor interference model, and it reveals little about the Anderson, M. C., Bjork, R. A., & Bjork, E. L. (1994). Remembering can role played by competition in RIF. cause forgetting: Retrieval dynamics in long-term memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20, 1063– Norman, Newman, and Detre (2007) proposed a neural network 1087. doi:10.1037/0278-7393.20.5.1063 model of RIF that incorporates both competition and inhibition. The Anderson, M. C., Bjork, R. A., & Bjork, E. L. (2000). Retrieval-induced model differs conceptually from the inhibition theory described by forgetting: Evidence for a recall-specific mechanism. Psychonomic Bul- Anderson and others. For example, inhibition leads to the permanent letin & Review, 7, 522–530. doi:10.3758/BF03214366 unlearning of associative connections, not to the temporary reduction Anderson, M. C., & Huddleston, E. (2011). Towards a cognitive and in access often described in the RIF literature. Nevertheless, Norman neurobiological model of motivated forgetting. In R. F. Belli (Ed.), et al.’s model is notable because it does make concrete its assumptions Nebraska Symposium on Motivation: Vol. 58. True and false recovered about the relation between competition and inhibition. In the model, memories: Toward a reconciliation of the debate (pp. 53–120). New an episodic representation consists of a pattern of associatively bound York, NY: Springer. episodic and semantic units. Because a retrieval cue potentially acti- Bäuml, K.-H. (1998). Strong items get suppressed, weak items do not: The vates a number of competing units, the system attempts to settle on a role of item strength in output interference. Psychonomic Bulletin & single pattern by strengthening links between units within the target Review, 5, 459–463. doi:10.3758/BF03208822 Bäuml, K.-H. (2002). Semantic generation can cause episodic forgetting. representation and inhibiting links to outside units. Importantly, inhi- Psychological Science, 13, 356–360. doi:10.1111/j.0956-7976.2002 bition occurs only when competing units become activated. This can .00464.x occur when retrieval cues are incomplete, as happens with retrieval Bäuml, K.-H. (2006). Retrieval inhibition in episodic recall: Effects on practice (FRUIT C_). When retrieval cues are complete, as happens feature binding. In H. Zimmer, A. Mecklinger, & U. Lindenberger with study practice (FRUIT Cherry), there is little inhibition because (Eds.), Binding in human memory: A neurocognitive approach (pp. nontarget units tend not to be activated. Because of this dependency 545–568). Oxford, England: Oxford University Press. between the completeness of the cue and the amount of RIF, it is Bäuml, K.-H., Zellner, M., & Vilimek, R. (2005). When remembering unclear how the model can account for the present findings in which causes forgetting: Retrieval-induced forgetting as recovery failure. Jour- study and retrieval practice produced RIF effects of similar magni- nal of Experimental Psychology: Learning, Memory, and Cognition, 31, tude. 1221–1234. doi:10.1037/0278-7393.31.6.1221 The works above illustrate two ways forward for inhibition Camp, G., Pecher, D., & Schmidt, H. G. (2005). Retrieval-induced forget- theory. The first is to explore inhibition within existing models of ting in implicit memory tests: The role of test awareness. Psychonomic Bulletin & Review, 12, 490–494. doi:10.3758/BF03193793 competitor interference. Although this has been the typical ap- Camp, G., Pecher, D., Schmidt, H. G., & Zeelenberg, R. (2009). Are proach, there is clearly a need to look beyond the simplest com- independent probes truly independent? Journal of Experimental Psy- petition models. The second way forward is to propose new chology: Learning, Memory, and Cognition, 35, 934–942. doi:10.1037/ models that describe in detail how competition and inhibition work a0015536 together to produce forgetting. A third possibility is to argue that Campbell, J. I. D., & Phenix, T. L. (2009). Target strength and retrieval- competitor interference and inhibition cause forgetting under induced forgetting in semantic recall. Memory & Cognition, 37, 65–72. wholly separate circumstances. One could take the position, for doi:10.3758/MC.37.1.65 example, that the RIF effect produced by retrieval practice is Ciranni, M. A., & Shimamura, A. P. (1999). Retrieval-induced forgetting caused by inhibition, but the RIF effect produced by study practice in episodic memory. Journal of Experimental Psychology: Learning, is caused by competition. There are several objections to this Memory, and Cognition, 25, 1403–1414. doi:10.1037/0278-7393.25.6 position beyond its lack of parsimony. There is little precedent in .1403 the RIF literature for separating the two mechanisms in this way. Craik, F. I. M., & Lockhart, R. S. (1972). Levels of processing: A framework for memory research. Journal of Verbal Learning and Verbal On the contrary, competition is assumed to be fundamentally Behavior, 11, 671–684. doi:10.1016/S0022-5371(72)80001-X intertwined with inhibition. Moreover, the significance of inhibi- Dagenbach, D., & Carr, T. H. (1994). Inhibitory processes in attention, tion theory has been its potential to alter our understanding of memory, and language. 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Appendix SAM-REM Model of Cued Recall: Simulations 1–4

Recall requires matching a probe to the contents of memory in simulation and are listed below. The stimuli in the simulations order to retrieve the specific memory that best matches the probe. were analogous to category–exemplar pairs (FRUIT Cherry). The

The SAM-REM model proposed by Malmberg and Shiffrin (2005) exemplar was defined as the item and represented by wi features. takes the matching algorithm of REM (retrieving effectively from The category name was defined as the local context and repre-

memory; Shiffrin & Steyvers, 1997) and embeds it within the sented by wc features. Each memory vector consisted of 40 fea- ϭ ϭ retrieval framework of SAM (search of associative memory; Raai- tures in all (wi 20, wc 20). Note that no provision was made jmakers & Shiffrin, 1980, 1981). These parent models have been to model other aspects of the temporal or environmental context. applied to a wide range of memory phenomena, and SAM-REM Because these features would have been the same for items being incorporates a number of theoretical principles beyond competitor compared, their inclusion would have little effect on the patterns of interference. Readers are referred to the original sources for a recall beyond adding noise. wider discussion of these issues. The description below presents A recall attempt involves an initial matching stage in which the only the details necessary to understand the simulations. retrieval cue is compared to all of the memory images in the search In REM, stimuli and their corresponding memory images are set. As a standard simplifying assumption, the search set is limited represented as vectors of features with each feature assigned a to the k items previously studied in the experiment. The retrieval value, V. Feature values are drawn from a geometric distribution, cue vector is compared to a given memory vector, feature by emulating real-world variability: feature. The numbers of matches and mismatches, as well as the environmental base rates of features as defined by the geometric vϪ1 P[␯ϭv] ϭ (1 Ϫ g) g, where v Ϫ 1,...ϱ. (A1) distribution, are important to calculating match. The comparison ␭ In the present implementation, each memory vector consists of yields a likelihood ratio, j, which represents the degree to which the retrieval cue matches the memory image j relative to other wi features corresponding to the stimulus item and wc features corresponding to the context in which the item is studied. These images: Ϫ values are initially set to zero. During encoding, the feature values c ϩ (1 Ϫ c)g(1 Ϫ g)i 1 nijm n ϱ of the stimulus item and context are copied to the memory vector. ␭ ϭ (1 Ϫ c) ij͟ ϭ ͫ ͬ . (A2) j i 1 g(1 Ϫ g)iϪ1 This process is gradual and error prone, and it depends on three parameters. For each unit of time, there are t storage attempts. On where c defines the probability of correct storage, i is a feature ء each attempt, for each feature, there is probability u that some- value ranging from 1 to infinity, nij is the number of nonzero thing will be stored. The correct value is stored with probability c. features in the memory image that incorrectly match the cue

An incorrect value, drawn randomly from the geometric distribu- (regardless of value), and nijm is the number of nonzero features tion, is stored with probability 1-c. Once a value has been stored, with value i that correctly match the cue. no further storage attempts will be made for that feature. The retrieval process of SAM has two stages, sampling and In the simulations, a unit of time was defined as a single recovery. Memory images are sampled serially with replacement encoding presentation. The encoding parameters (g ϭ .40, t ϭ 10, from the search set. The probability of sampling memory image j given the retrieval cue on a given sampling attempt is ءand c ϭ .80) were fixed across all simulations. Only the u , ءand u ءparameter was allowed to vary. Separate values, u i c ␭␥

This document is copyrighted by the American Psychological Association or one of its alliedrepresented publishers. the likelihood of storing information for item and P͑j | cue͒ ϭ . (A3) ␥␭ ء ء This article is intended solely for the personal use ofcontext the individual user and is not to be disseminated broadly. features, respectively. The values of u i and u c varied by ͚ ␬

(Appendix continues) 1448 VERDE

ϭ ء ϭ ء ϭ␥ ␥ where is a scaling parameter (set here to 0.2) that corrects once for encoding during the study phase (u c .02, u i .02). In for the skew of the likelihood ratio distributions (Shiffrin & Simulations 1–3, half of the stimuli from three categories (Pϩ Steyvers, 1998). The sampling probability depends on the degree items) were presented again twice for encoding during the practice to which the cue matches the target image relative to all other phase, allowing additional attempts to store features into their ء ϭ ء ϭ ء ϭ ء images in the search set. Interference increases as the match to memory images (Sim1: u c .03, u i .02; Sim2: u c .01, u i ϭ ء ϭ ء ϭ nontarget images increases. When an image is sampled, the infor- .02; Sim3: u c .01, u i .03). In Simulation 4, no studied mation contained in it must be recovered to a degree sufficient to stimuli were presented; instead, a new set of six stimuli from three ϭ ء support a response. The probability of successful recovery is a studied categories was presented once for encoding (u c .02, ϭ ء monotonic function of the proportion of correctly stored item u i .02). In all the simulations, a test phased followed the study features in the image, and practice phases in which each stimuli from the study phase was tested with a probe consisting of 20 context features and two P(R) ϭ␳␶. (A4) item features (the latter representing the exemplar word stem). Performance in Figure 2 represents the likelihood of sampling and where ␳ is the proportion of correctly stored features and ␶ is a recovering the target of a probe. Testing order was not modeled; scaling parameter (set here to ␶ϭ0.4). Sampling continues until this level of complexity, which may include factors such as con- a suitable item is recovered or the process terminates following a textual drift, output interference, and specific and general interfer- ϭ criterion number of sampling attempts (set here to Kmax 16). ence (e.g., Verde, 2009), is beyond the scope of the present study. In each simulation, a unique set of stimuli was created for each virtual participant (n ϭ 300). There were six categories. All stimuli Received August 29, 2012 within a category possessed the same context features but had Revision received December 17, 2012 unique item features. Eight stimuli per category were presented Accepted December 18, 2012 Ⅲ This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.