Source monitoring and memory distortion

MARCIA K. JOHNSON Department of Psychology, Princeton University, Princeton, NJ 08544-1010, USA ([email protected])

SUMMARY Memory distortion re£ects failures to identify the sources of mental experience (reality monitoring failures or source misattributions). For example, people sometimes confuse what they inferred or imagined and what actually happened, what they saw and what was suggested to them, one person's actions and another's, what they heard and what they previously knew, and ¢ction and fact. Source confusions arise because activated information is incomplete or ambiguous and the evaluative processes responsible for attributing information to sources are imperfect. Both accurate and inaccurate source attributions result from heuristic processes and more re£ectively complex processes that evaluate a mental experience for various qualities such as amount and type of perceptual, contextual, a¡ective, semantic and cognitive detail, that retrieve additional supporting or discon¢rming evidence, and that evaluate plausibility and consistency given general knowledge, schemes, biases and goals. Experimental and clinical evidence regarding cognitive mechanisms and underlying brain structures of source monitoring are discussed.

1. INTRODUCTION (Johnson & Raye l981; Johnson l985, 1988, 1991, 1997). Reality monitoring failures occur when people misat- The problem of verifying memories and separating true tribute something that was re£ectively generated to from false memories has received national attention in perception or vice versa. According to this view, recent years, for example in highly publicized cases of reality is not directly given in remembering, but is an children's testimony and apparently long-forgotten attribution that is the outcome of judgement processes. memories of childhood abuse that are remembered by Reality monitoring is just a particularly interesting adults, sometimes in the course of therapy (Ceci & subset of the more general cognitive activity of source Bruck 1993; Loftus 1993). These socially signi¢cant monitoring, which includes distinguishing between instances point to fundamental issues about human external sources or between internal sources, as well as memory in general. Psychologists have known for between external and internal sources. For example, some time that memory does not record experience source monitoring includes identifying who told you like a video camera (Bartlett 1932). When we read something, whether you saw an event in real life or on novels or listen to friends or see an accident, we are television, whether an event happened a week ago or a comprehending the information in light of our expecta- month ago, and whether you told a secret to your friend tions, making assumptions and inferences and ¢lling in or only thought about telling it. We have described based on prior knowledge, and perhaps failing to notice source monitoring as an attribution which is the or incorporate information that is incongruent with our outcome of judgement processes that take into account currently active cognitive schemas. Likewise, when we certain phenomenal characteristics of memories, as well think back on events and remember them, the same as consistency checks, plausibility judgements, and interpretive, inferential and motivational processes are other extended retrieval and reasoning strategies (e.g. operating. The resulting memories include not only Johnson 1988; Johnson et al. 1993; Johnson & Raye perceptual information but our inferences, imagina- 1981). Memory distortions can happen in many tions and thoughts (e.g. Bransford & Johnson 1973; see contexts and forms, but most memory distortion also Alba & Hasher 1983; Johnson & Sherman 1990, involves failures in monitoring the source of informa- for reviews). tion. To manage as well as we do, given such a construc- tive memory system, we proposed that the system must have some mechanisms or processes designed to help us 2. A SOURCE-MONITORING sort out information that was primarily derived from FRAMEWORK perception from information that was derived from our My colleagues and I proposed an integrative frame- thoughts; we called these processes reality monitoring. work for characterizing and investigating processes That is, reality monitoring refers to the processes involved in identifying the origin of information: the involved in discriminating memories and beliefs gener- source-monitoring framework (SMF) (Johnson et al. ated by re£ection from those derived from perception 1993; Johnson & Raye 1981). This approach explains

Phil. Trans. R. Soc. Lond. B (1997) 352, 1733^1745 1733 & 1997 The Royal Society Printed in Great Britain 1734 M. K. Johnson Source monitoring and memory distortion both veridical and distorted memory with a common set A third fundamental idea is that the judgement of principles. Fundamental are several ideas. First, an processes used in any given situation and the criteria event memory consists of sets of phenomenal character- adopted will be a¡ected by such factors as the nature istics, which are the result of multiple distributed of the task, cost of mistakes, our preconceptions, cognitive processes during the event (and after if the amount of distraction and the social context. For memory is reactivated or retrieved) (e.g. Johnson 1983, example, criteria are likely to be more lax in a casual 1992). Second, memories from di¡erent sources conversation with a friend (e.g. recalling the source of di¡er in characteristic ways that can, in principle (but gossip) than in a professional meeting (e.g. recalling the only on average), be used to identify the origin of source of scienti¢c ideas), or more lax if a memory ¢ts information. with what one already believes or wants to believe than For example, perceived and imagined classes of events if it does not. di¡er in average value along a number of dimensions or attributes. Memories originating in perception typically have more perceptual detail or features (e.g. colour, (a) Sources of error and distortion in memory sound), more contextual detail such as time and place, Using this source-monitoring framework, there are more semantic detail and more a¡ective information. various ways in which source memory errors could In contrast, memories originating in re£ection typically occur (for recent reviews see Ceci & Bruck 1993; have more accessible information about cognitive opera- Johnson et al. 1993; Roediger 1996; Schacter 1995; tions, that is, about those perceptual and re£ective Wilson & Brekke 1994): The schema-based inferences processes that took place when the memory was estab- we make in simply comprehending events initially can lished. Judgement processes capitalize on these average later be misattributed to perception. For example, if di¡erences. Thus, di¡erences in average value between someone tells you that John pounded the nail and you perceptually and re£ectively derived memories along infer he used a hammer, you may later be likely to these dimensions form one basis for deciding the origin claim that the speaker told you John had a hammer of a memory. For example, one would be likely to decide (Johnson et al. 1973). Anything that disrupts the that a memory with a great deal of perceptual informa- binding of features (e.g. semantic content, spatial loca- tion and very little cognitive operations information was tion, colour) into a complex event memory, such as externally derived (Johnson et al. 1979, 1981). Similarly, stress, distraction or brain damage, will reduce source one might decide that a statement was said by person A memory (Chalfonte et al. 1997; Jacoby et al. 1989; rather than person B because the auditory information Schacter et al. 1984; Shimamura et al. 1990). Certain in the memory matches A's voice or the visual informa- types of emotional focus can disrupt feature binding as tion matches A's appearance (e.g. Ferguson et al. 1992; well (Johnson et al. 1996b). Memories that are similar Johnson et al.1996b). in perceptual features or semantic content can easily Other processes involved in source monitoring tend be confused (Johnson et al. 1988a; Lindsay et al. 1991). to be more complex. You may decide when an event Misinformation introduced through suggestion can be occurred because you can relate it to another event misattributed to a target event as shown by studies of memory that contains more de¢nitive time informa- eyewitness testimony (Belli & Loftus 1994; Zaragoza tion; or you may decide you read something in the & Lane 1994). Reactivating and retrieving memories newspaper rather than saw it on TV news because you strengthens them, but also potentially embellishes remember thinking it was consistent with the newspa- them (Suengas & Johnson 1988; Ceci et al. 1994). Any per's position, or because you cannot retrieve disruption of the ability to revive or evaluate additional perceptual information about the broadcast that you supporting or discon¢rming information may create feel you should be able to if you had seen it on TV. source monitoring errors (e.g. Craik 1982; Dodson & That is, more extended source monitoring may involve Johnson 1996; Jacoby 1991). Inappropriately lax criteria beliefs about memory and cognition as well as can produce source monitoring failures (Lindsay & retrieving additional information from memory and Johnson 1989; Dodson & Johnson 1993; Multhaup evaluating the source of the target memory given these 1995). And source memory is a¡ected by the social beliefs, other speci¢c memories or general knowledge context. Social context a¡ects when, how often and (e.g. Johnson et al. 1988b). For example, you might how we rehearse and ruminate about past events (e.g. have a vivid memory of your great-grandmother Neisser, this volume; Nelson 1993) and the criteria we holding you in her lap, but realize it cannot be accurate use for evaluating the veridicality of our memories because you know she died before you were born. (Marsh et al. 1997). For example, part of the current Source monitoring typically takes place rapidly, in a controversy over the recovery of repressed memories heuristic, non-deliberative fashion, based on the concerns the possibility that the expectations of thera- general characteristics of memories that are activated pists might set up inappropriately lax reality (e.g. amount or type of perceptual detail). Generally. monitoring criteria for patients (see Lindsay & Read slower, more deliberate retrieval of supporting 1994). memories and reasoning processes (e.g. Does this seem In short, autobiographical (i.e. episodic or explicit) plausible given other things I know?) are engaged less remembering requires source monitoring. Over and often. All source memory problems can be analysed in above the activation and use of information (as shown terms of the qualitative characteristics considered and in our everyday use of general knowledge, skills, and in the heuristic and systematic judgement processes used certain `priming' or transfer phenomena), episodic to make source attributions. remembering involves identifying the context or source

Phil. Trans. R. Soc. Lond. B (1997) Source monitoring and memory distortion M. K. Johnson 1735 of memories. Such source monitoring, in turn, depends introduced by Deese (1959) and subsequently used by on a number of processes that take place during Roediger & McDermott (1995). As found by these encoding when features of complex memories become other investigators, on a subsequent recognition test, bound together, during the retention interval when subjects were very likely to falsely recognize the unpre- memories may be rehearsed and embellished, and at sented lures. However, on a Memory Characteristics the time information is recruited (including potentially Questionnaire (MCQ, e.g. Johnson et al. 1988b) that confusable non-target information) and may be asked subjects to rate their memories on a number of subjected to relatively lax or stringent evaluation. qualitative characteristics, Mather et al. found that The next section provides examples of studies illus- memories for lures had less auditory detail and less trating some fundamental principles of source memory remembered feelings and reactions than did memories processes and the following section considers some for presented words. Norman & Schacter (1997) found evidence about brain regions underlying source similar results. Mather et al. also varied the test condi- memory. tions and found lower false recognition rates in some conditions when subjects were induced to examine their memories on several dimensions compared to (b) Cognitive studies of source memory when they were simply asked whether or not they The more similar memories from two or more remembered items. Thus, as suggested by the SMF, sources are, the more likely they will be confused. For individuals may have source information that they do example, one early study varied the number of times not, or cannot, use under some conditions. subjects saw pictures and the number of times they Studies in which the time-course of memory reacti- imagined each picture (Johnson et al. 1979). Subse- vation and judgement has been measured indicate that quently, subjects gave higher estimates of the number source information is not revived in an all-or-none of times they had seen a picture the more often they fashion but rather that memories take time to had imagined it. Furthermore, compared to poor `assemble'. Johnson et al. (1994) presented subjects with imagers, good imagers were more in£uenced by their some pictures and some words and had them imagine imaginations (see also Dobson & Markham 1993; pictures corresponding to the words (the cover task Markham & Hynes 1993). Thus one important factor asked subjects to rate the perceived or imagined in the likelihood that imagined information will be pictures for artistic merit). Subsequently subjects were misattributed to actual events is how vivid the presented with words and asked to indicate whether memory representation is. The impact on source moni- each corresponded to an item previously perceived, toring of individual di¡erences in imagery/fantasy imagined, or a new item. Also varied at test was the ability remains a relatively unexplored, but potentially time (from 300 to 1500 ms) from the memory probe important research domain. until a tone indicated the subject had to respond. The Other studies show that the more easily information time-course functions shown in ¢gure 1 indicate that is generated, the more likely it is later to be confused subjects can discriminate between old and new items with perception (Durso & Johnson 1980; Finke et al. well before they can discriminate between items from 1988; Johnson et al. 1981). For example, subjects are di¡erent sources (see also Hintzman & Curran 1994). more likely to confuse the more common instances of These time-course functions are generally consistent categories they generate with perception than the less with the idea that recognition may be based either on common instances (Johnson et al. 1981; Rabinowitz familiarity or on the retrieval of more speci¢c informa- 1989). Generating uncommon instances presumably tion (e.g. Atkinson & Juola 1973; Jacoby 1991; Mandler requires more cognitive operations, the records of 1980). Such `dual process theories' tend to characterize which can be used to identify the source of memories. remembering as consisting of two processes (familiarity Thus information that is likely to be activated sponta- and recollection) or as memory being in one of two neously (e.g. needle in response to thread; Anisfeld & states: known or remembered (e.g. Tulving 1985; Knapp 1968; Deese 1959; Roediger & McDermott Gardiner & Java 1993). Although the general contrast 1995) is likely to be falsely remembered later, especially between familiarity responses and more speci¢c recol- if there were repeated opportunities for the false lection has been (and remains) extremely fruitful (e.g. information to have been incidentally activated Jacoby 1991), as a basic theoretical distinction it is (Underwood 1965) and/or if it is the central tendency problematic (e.g. see Dodson & Johnson 1996). Based or prototype for disparate pieces of presented informa- on the SMF, mental experiences do not fall into a tion (Bransford & Franks 1971; Posner & Keele 1968). simple dichotomy, but rather are characterized as Such automatic associations, or easily inferred and potentially di¡ering along a variety of characteristics generated information, may be ¢lled in spontaneously (e.g. perceptual/contextual detail, emotional qualities, at encoding or at test and misattributed with respect to etc.), producing memories that vary in the amount of source. evidence they provide for various types of judgements Nevertheless, recent evidence suggests that even such and judgement contexts. misattributed memories may di¡er in phenomenal Not only do memories di¡er in various qualities, the characteristics from perceptually-derived memories. amount of information an individual requires for a Mather et al. (1997) presented subjects with lists of given source attribution varies as well. For example, associates to non-presented lure words (e.g. thread, we would expect anything that disrupted availability pin, eye, sewing, sharp, point, prick, thimble, haystack, of more di¡erentiated information (initial feature thorn are all associates of needle) in a procedure binding de¢cits, de¢cits in retrieval, etc.) to force

Phil. Trans. R. Soc. Lond. B (1997) 1736 M. K. Johnson Source monitoring and memory distortion

1.0 after getting a ¢nger caught in a mousetrap. Further- more, with repeated questioning, the children's 0.9 accounts became increasingly coherent and detailed. 0.8 Similarly, Hyman et al. (1995) showed that some adults 0.7 can be induced to falsely recall childhood events (e.g. a 0.6 hospitalization for an ear infection) if encouraged to think about the events between a ¢rst and second inter- 0.5 view (see also Loftus & Coan 1997). Garry et al. (1996) 0.4 showed that asking adults to imagine a childhood event 0.3 increased their con¢dence that the event occurred. estimated accuracy D (Old-New) 0.2 d2 (Imagined) 0.1 d1 (Perceived) 3. BRAI N M ECHANISMS OF SOURCE MONI TORI NG 0 300 500 900 1500 The last section outlined source memory processes processing time (ms) and the types of failures in source memory processes that lead to memory distortions. These ideas can Figure 1. Time-course functions for old/new recognition provide a framework for exploring the brain mechan- and reality-monitoring performance (from Johnson et al. isms of source monitoring. Especially exciting are the 1994). possibilities for connecting cognitive concepts and neurobiology through advances in brain imaging and recording of electrophysiological brain activity. We do subjects to rely more heavily on undi¡erentiated, not have to start from scratch in identifying relevant global, familiarity information (e.g. as do amnesics, brain areas, of course. Observations of patients with elderly adults, individuals under conditions of distrac- brain damage provide many insights. tion, etc., see below). There are a number of reasons to expect that frontal One of the potentially most interesting factors deter- regions of the brain are critical for source monitoring mining the veridicality of memories is the rumination and veridical remembering. First, frontal regions are we do via reactivating and retrieving. One study likely to be involved in circuits with the hippocampus (Suengas & Johnson 1988) had subjects participate in to promote feature binding (Goldman-Rakic et al. or imagine participating in various simulated autobio- 1984; Johnson 1997; Shallice et al. 1994), and episodic graphical events such as wrapping a package, meeting or event memories are those that have cohesively someone and having co¡ee and cookies. Subjects bound source-specifying information. In addition, engaged in some activities and were asked to imagine frontal regions are believed to have a number of `execu- engaging in others, guided by a script. Later, subjects tive' functions (Baddeley 1986; Baddeley & Wilson ¢lled out an MCQ assessing various qualitative charac- 1986; Goldman-Rakic 1987; Norman & Shallice 1986; teristics of their memories (How well do you remember Stuss & Benson 1986): they play a critical role in stra- the spatial arrangement of objects? How well do you tegic retrieval, maintain the activity of representations remember how you felt at the time?). The impact on and underlie controlled, evaluative processes. As MCQ ratings of having subjects think about events described earlier, all these are necessary for adequate after they happened was also investigated. After the source monitoring in many situations. initial MCQ ratings, subjects returned to the lab and In addition to expecting frontal regions to be critical were cued to think about each event either 0, 8 or 16 for source memory on the grounds of the overlap times. Then they returned again and ¢lled out the between global frontal functions (e.g. Stuss & Benson MCQ. Initially, ratings on visual clarity were higher 1986; Stuss et al. 1994) and factors outlined in the SMF for perceived than for imagined events (see also Hash- (e.g. Johnson 1988), direct evidence regarding frontal troudi et al. 1990). For both perceived and imagined involvement in source monitoring comes from studies events, perceptual details tended to be less accessible of source memory in patients with frontal lesions. over time unless the event was thought about (reacti- vated), in which case these details were maintained. This ¢nding suggests that if people selectively reacti- (a) Tests of source monitoring in patients with vate imaginations, those memories could begin to rival frontal damage in vividness memories for perceived events from the On speci¢c tests of source monitoring, frontal same time frame. At the same time, rehearsal might patients have de¢cits in temporal order information: make it easier to generate the memory, reducing the for example, they have di¤culty saying which of two cognitive operations cues that signal that a mental items presented in the lab occurred most recently, or experience was initially imagined rather than di¤culty ordering historical events (Milner 1971; perceived, further increasing confusion. Milner et al. 1985, 1991; Shimamura et al. 1990; Butters Ceci et al. (1994) recently presented dramatic et al. 1994; McAndrews & Milner 1991). Schacter and evidence of the potential impact of rehearsal and others (Schacter et al. 1984; Janowsky et al. 1989; Craik elaboration. Through repeated questioning, they et al. 1990; Shimamura & Squire 1987) tested the induced some young children to report that ¢ctitious source monitoring capabilities of frontal patients or events had happened, for example, going to hospital individuals with presumed frontal damage by telling

Phil. Trans. R. Soc. Lond. B (1997) Source monitoring and memory distortion M. K. Johnson 1737 them various trivia facts (e.g. Bob Hope's father was a Table 1. Scores on source monitoring test for patient WL: ¢reman) and then later asking them where they session 1 and session 2, one year later learned these facts. Frontal patients are likely to claim to have learned the fact before the experiment, from a response magazine or newspaper. As discussed above, another manifestation of source- picture word new monitoring failure common in neurologically intact individuals is false recognition of items that were not session 1 presented (e.g. Deese 1959; Johnson et al. 1973; Under- source wood 1965), and when the probability of such errors is picture 21 0 13 manipulated in controlled experiments they provide word 13 3 18 new 12 0 20 evidence about the information and processes ordina- session 2 rily used in source attributions (e.g. Anisfeld & Knapp source 1968; Mather et al. 1997). Neurologically based memory picture 30 1 2 problems do not invariably lead to false recognitions. word 5 25 4 Patients with medial-temporal lesions, who may show new 2 3 27 profound amnesia, often make few false positives; they are likely to say `no' both to old and new items because very little seems familiar to them (e.g. Hirst et al. 1986). In contrast, some frontal patients show extremely high old, a very high rate of false recognition. Second, rates of false recognition (Schacter et al. 1997). Schacter almost all of these she claimed to have seen as pictures, et al. reported a patient, BG, who had an infarction of regardless of whether they had been pictures, words or the right frontal lobe. On recognition tests involving new items. At the second session, a year later, WL was either words, sounds, pronounceable non-words or given a similar test (table 1b) and she showed a response pictures, BG falsely recognized an unusually large pattern typical of normal subjects (Durso & Johnson number of distractors. Furthermore, his false recogni- 1980). That is, her old/new and source memory were tions were not disproportionately increased for quite good and she showed the normal asymmetry in associatively related compared to unrelated distractors. source monitoring errors on old items (more likely to However, when study items were drawn from speci¢c claim to have seen a word as a picture than vice versa) categories and the distractors were drawn from a that re£ects confusion of previously imagined pictures di¡erent semantic category than the studied items, with perceived pictures, a reality monitoring failure BG's false recognition rates were normal. Evidently, he (Durso & Johnson 1980). remembered that he had been presented with items of a What might account for the unusual response pattern of general type, but was unable to discriminate along any WL at session 1? Clearly her overall memory was quite other features that might have di¡erentiated speci¢c poor: old/new recognition was at chance (57%). If her old and new items. In short, BG seemed to need a memory was so poor, one might think that her phenom- single, unambiguous cue to di¡erentiate old from new enal experience would be a vague sense of familiarity for items. Also interesting was that BG claimed to some items, which should have been more likely to `remember'many of the items that he falsely recognized produce `word' than `picture' responses (or equal word when asked to make remember/know judgements about and picture responses if she were simply guessing). items he identi¢ed as old. Apparently, he felt that the Instead, she claimed to have seen pictures of almost all cue he could remember (general class) was su¤cient the items she called old. One possibility is that, similar to evidence for assigning a `remember' response. Patholo- patient BG, she remembered only that she had seen some gically high false recognition rates are not always pictures and so called every item that seemed familiar a associated with remember responses. A patient (who picture. Another hypothesis is that whenever a picture/ had had an aneurysm of the anterior communicating image came to mind readily at test for an item,WL attrib- artery, ACoA) reported by Parkin (1995) with uted it to recent perceptual experience. In both cases, like unusually high rates of false alarms made primarily Schacter et al.'s (1997a) BG, she would have used a simple `know' responses to falsely recognized items. heuristic to make recognition decisions: BG evidently Another ACoA patient (M. Johnson, L. Grande, & decided if an item was old on the basis of whether test W. Milberg, unpublished data) showed another intri- items ¢t the general class of items he was shown; WL guing false recognition phenomenon in the context of evidently decided that a test item had been shown as a a source monitoring task. The patient, WL, saw a picture if a picture came to mind or a concept seemed series of pictures and words referring to common familiar. Interestingly, WL's heuristic is just the sort that objects. The cover tasks used at study were patterned could, in principle, lead to confabulation, claiming events after Durso & Johnson (1980; see also Johnson et al. as real that are only imagined. And, in fact,WL did show 1994) to induce subjects to make images for the words. clinical evidence of confabulation, which had cleared by A subsequent source memory test required the patient the second testing session a year later. to identify test words as previously presented pictures, words or new. WL was tested twice, shortly after her aneurysm and a year later (table 1). For the ¢rst (b) Confabulation in frontal patients session, there were two notable aspects of her data Some of the most dramatic evidence of source-moni- (table 1a). First, she identi¢ed 12 of 32 new items as toring de¢cits from frontal damage comes from clinical

Phil. Trans. R. Soc. Lond. B (1997) 1738 M. K. Johnson Source monitoring and memory distortion observations of confabulation (Johnson 1991). State- aneurysm ruptured while he was in an upstairs apart- ments clinically classi¢ed as confabulations range from ment ¢xing the bathtub when he suddenly collapsed. relatively minor ¢lling in of memory gaps and confu- We compared GS to three age-matched normal sions in where or when actual events occurred to controls and to three non-confabulating frontal patients bizarre and fantastic stories. Clinical descriptions of who were matched with him on several standard confabulating patients are generally consistent with neuropsychological tests of memory, attention and so- the idea that reality monitoring involves two types of called `executive' or frontal functions. Of interest was judgement processes (Johnson 1991). Patients who have subjects' performance on several tasks that have been had a cingulectomy (to control obsessive^compulsive used to study source monitoring in normal populations. behaviour) experience reality monitoring failures that In one set of tasks, the subjects saw and heard a man appear to be the consequence of heuristic errors either and a woman say various words or sentences. Later, produced by unusually vivid mental experiences, or lax subjects had to identify whether test items were said by criteria. For example,Whitty & Lewin (1957) described the man, the woman or were new. The old/new recog- a patient who, when asked what he had been doing, nition and source identi¢cation results were both quite reported a visit from his wife. The patient then realized typical and the data are shown in rows 1 and 2 respec- that she had not visited that day, but commented on tively of table 2. Frontal patients and GS showed no how real the event seemed. Thus vivid ideas initially de¢cit relative to controls on old/new recognition, but may pass the heuristic reality-monitoring check based did show a marked de¢cit in source accuracy. Row 3 of on their qualitative characteristics, but may be table 2 re£ects scores on an autobiographical memory correctly evaluated by more systematic processes that test in which the subjects were asked to describe check for consistency, plausibility, and coherence. several events from their lives, for example, a time with Source-monitoring errors that appear to result from a friend and a vacation. Frontal patients showed a marked disruption in systematic processes are likely to be seen de¢cit compared to controls in the amount of autobio- in patients with more extensive frontal damage. Such graphical detail they could give, another typical patients may make bizarre or implausible statements, ¢nding. But GS's recall was extremely impoverished, for example, claiming to have been returned to life even in comparison to other frontal patients. Finally, after being shot and killed (Stuss et al. 1978), or in another task subjects participated in some events claiming to have been a space pirate (Damasio et al. and imagined participating in others, e.g. they actually 1985). With de¢cits in systematic processes, vivid ideas hammered a nail in a board and imagined making a that pass a heuristic reality monitoring check are not paper snow£ake with scissors. Twenty-four hours later, caught. Baddeley & Wilson (1986) and Moscovitch they were asked to describe the events, and their (1995) also identify strategic processes with frontal descriptions were rated for various types of detail. The functioning and emphasize the importance of disrupted frontal patients and GS recalled less than normals, but frontal function in producing confabulation. in contrast to both normals and the other frontal Fischer et al. (1995) considered nine patients with patients, GS recalled somewhat more detail for aneurysm of the anterior communicating artery imagined events than he did for actual events. (ACoA) and concluded that degree of confabulation In short, GS showed profound de¢cits in the was related to the extensiveness of the frontal lesions. systematic retrieval of autobiographical memories, Similarly, DeLuca & Diamond (1995) reviewed the source-monitoring de¢cits on the speaker identi¢cation literature concerning ACoA patients and suggested task, and a propensity towards detailed imaginations. that both frontal lobe and basal forebrain lesions are Any one factor alone might not produce a clinically necessary to produce fantastic confabulation (that signi¢cant pattern of confabulation, but together they persists beyond the initial confusional state). ACoA could produce confusion between fact and ¢ction. patients have also been shown to have source de¢cits in Against a background of impoverished autobiogra- temporal order (Parkin et al. 1988; Johnson et al. 1997b), phical memories, any highly detailed `memory' spatial memory (Shoqeirat & Mayes 1991) and (whether real or invented) would stand out. With poor modality (visual vs. auditory) (Shoqeirat 1989, cited in source information, and with markedly impaired DeLuca & Diamond 1995). ability to retrieve additional con¢rming or discon- Overall then, there is no question that the frontal ¢rming autobiographical memories, GS would not be lobes are critically involved in reality monitoring and able to re£ect on other evidence in determining the source monitoring in general. In addition, we have a authenticity of such memories. Consequently, he would relatively good understanding of the cognitive processes be predisposed to accept them as real events. involved in accurate source monitoring and the factors GS and the other frontal patients were matched on that result in memory distortion (Johnson et al. 1993; neuropsychological tests, not on lesion site. In fact, GS Johnson 1997). Nevertheless, we still have some way to had bilateral damage whereas the three frontal controls go to match brain structures with what has been all had evidence of left frontal damage. Burgess (1992, learned about memory and cognition. cited in Shallice et al. 1994), suggested that confabula- Some of the complexity of these issues is illustrated tion is more likely to occur after right than after left- by an ACoA patient, GS, who su¡ered frontal lobe side damage to the prefrontal cortex. However, there damage (Johnson et al. 1997b). GS's confabulations are some reports of confabulation in left unilateral were generally realistic. For example, he believed that patients, so confabulation is not restricted to right or to he had fallen and hit his head while standing outside bilateral damage. Nevertheless, it is tempting to talking to a friend whereas, according to his wife, his speculate that GS's profound de¢cit in retrieving

Phil. Trans. R. Soc. Lond. B (1997) Source monitoring and memory distortion M. K. Johnson 1739

Table 2. Memory scores on several tasks for patient GS (d) Brain imaging (data from Johnson et al. 1997) Positron emission tomography (PET; Roland et al. frontal normal 1995) uses cerebral blood £ow as an index of brain GS controls controls activity (functional magnetic resonance imaging, fMRI, is promising but has been used less to date). The old/new recognition 0.70 0.68 0.67 PET scan is recorded while a subject performs the speaker identi¢cation 0.26 0.27 0.53 target task and, in a separate scan, a reference (or base- autobiographical recall 2.50 16.83 30.58 line) task. For example, subjects might be presented rated details with category-exemplar word pairs (e.g. poet- actual 4.20 3.93 6.32 Browning) and asked to learn them (target task) or imagined 5.47 2.61 6.34 just asked to repeat the words (reference task) and later asked to recall. Typically, brain activity during the reference task is subtracted from activity in the autobiographical memories was related to his right target task to reveal activity that is speci¢c to the frontal damage, consistent with recent proposals (see target task (e.g. Buckner et al. 1995; Fletcher et al. 1995; ½3a and ½3b) that right frontal regions are particularly Kapur et al. 1994; Shallice et al. 1994; Tulving et al. important for episodic retrieval (Tulving et al. 1994a; 1994b; see Buckner 1996, for a discussion of the Shallice et al. 1994). subtraction procedure). Imaging studies of episodic It should be noted, however, that not all frontal encoding and retrieval tasks have been reviewed by patients confabulate and not all confabulations are Buckner & Tulving (1995) and Nyberg et al. (1996a). accompanied by signs of frontal dysfunction. To go Episodic memories by de¢nition include information beyond currently available evidence, we need case about their source (Tulving 1983), thus attempts to studies with a more complete characterization of the identify brain correlates speci¢c to processing episodic nature of patients' confabulations (e.g. Dalla Barba memories are relevant to understanding source 1993), more extensive testing on a variety of source monitoring. memory tasks (e.g. Johnson et al. 1997b), and clearer Episodic encoding reliably results in greater activa- speci¢cation of the areas of brain damage (e.g. Fischer tion in the left prefrontal cortex, whereas episodic et al. 1995). retrieval reliably results in greater activation of the right prefrontal cortex (Tulving et al. 1994a). Buckner (1996) concludes that an area in the right prefrontal cortex near BA 10 appears common to retrieval of (c) Ageing and source monitoring episodic information independent of stimulus materials, Older adults show impairments in a variety of tasks orienting task, modality, etc. that assess memory for source or contextual informa- Researchers have attempted a ¢ner analysis of tion (Johnson et. al. 1993). This impairment may be episodic retrieval processes by manipulating the likeli- related to a loss of frontal functions associated with hood that subjects will be successful at remembering normal aging (e.g. Craik et al. 1990; Janowsky et al. during the block of trials within a scan (e.g. by 1989). Sometimes source accuracy is positively corre- varying encoding task or the proportion of old items lated with high functioning scores on neuro- tested) and found that high performance tends to be psychological tests of frontal function such as the associated with medial temporal hippocampal or para- Wisconsin Card Sorting Test or the verbal £uency hippocampal activity (Kapur et al. 1995; Schacter et al. (FAS) test (e.g. Craik et al. 1990) and sometimes it is 1996a; Nyberg et al. 1996b). Nyberg et al. suggest that not (e.g. Johnson et al. 1995; Spencer & Raz 1994). medial temporal activation may re£ect the actual acti- Johnson et al. (1995) suggested that such inconsistencies vation of the sought-for stored information. Kapur et al. in outcomes may not be surprising given the variety of propose that the prefrontal area drives the retrieval qualitative characteristics and processes potentially search and that successful retrieval and the experience involved in source memory and the lack of speci¢city of remembering is related to activity of the posterior of neuropsychological tests of frontal function. Glisky et multimodal associative cortex. al. (1995) recently reported that better prediction of Investigators have also begun to use PET to identify performance can be achieved when elderly subjects are brain correlates of relatively subtle di¡erences in given composite scores derived from several standard phenomenal experience. As noted previously, Mather neuropsychological tests assessing frontal or medial et al. (1997) and Norman & Schacter (1997) found that temporal functions. Using this approach, Glisky et al. subjects gave higher ratings of auditory information for found a dissociation between item memory and source memories of words they had actually heard than for memory: subjects' old/new recognition for sentences associated words that they falsely recognized. Consis- was associated with high scores on the medial temporal tent with this, Schacter et al. (1996b) recently found composite whereas subjects' ability to identify the that, compared to lures, old items resulted in greater speaker of the sentences was associated with high blood £ow in the temporo-parietal region, especially scores on the frontal composite. This composite score on the left, an area that has been linked in other approach should be particularly promising in combina- studies to phonological processing. Schacter et al. also tion with a broader range of source tasks administered noted that, compared to new items (and to old words), to each subject. the related lures produced more activation in anterior

Phil. Trans. R. Soc. Lond. B (1997) 1740 M. K. Johnson Source monitoring and memory distortion prefrontal cortex (BA 10), orbitofrontal cortex (BA 11) for items that did not show a marked frontal positivity. and the cerebellum. This greater frontal activation for Conversely, older adults showed large positivity for critical lures is consistent with the idea that lures items for which they made many source errors. Based should present a particularly di¤cult source-moni- on their ¢ndings, Dywan et al. suggested that a di¡er- toring problem as subjects attempt to identify the ence between old and new items in positivity is not origin of items that seem familiar but for which clear necessarily an index of recollection but may, instead, source information is not available. The next section represent the amount of attention paid to a stimulus. presents converging evidence from event-related poten- For example, if positivity re£ects attempted retrieval tials that source identi¢cation and simple old/new or evaluation/monitoring, Dywan et al.'s results recognition (both episodic tasks but requiring di¡erent suggest that older adults are engaging such processes degrees of source monitoring) di¡er in brain activity but are less successful than younger adults in doing especially in frontal regions. so. The factors determining the size and temporal and topographical distribution of old-new di¡erences in ERPs in source monitoring paradigms remain to be (e) Event-related potentials sorted out. Although PETand fMRI provide excellent informa- The results of two recent experiments (Johnson et al. tion about spatial location of regions of activation in 1996a; 1997a) suggest that ERP components are sensi- cognitive tasks they are less useful for identifying tive to direct manipulations of source monitoring temporal properties of cognitive processing. Character- demands as predicted by the SMF. In one experiment izing the duration and sequencing of component (Johnson et al. 1996a) subjects saw pictures and words processes is central to specifying the transactions at study. In the Artist Di¤culty condition, on picture among brain regions that underlie cognitive activities trials, subjects rated how di¤cult the object would be (e.g. Goldman-Rakic et al. 1984; Johnson & Reeder for an artist to draw; on word trials, subjects imagined 1997). Excellent temporal resolution, along with coarse a line drawing of the referent of the word and rated location information, is obtained from recording elec- how di¤cult the imaged picture would be for an artist trical activity from the scalp using the event-related to draw (an explicit imagery condition). In the Func- potential (ERP) technique. ERPs are derived from tion condition, subjects rated the number of functions segments of the ongoing electroencephalogram that they could think of for each item. The Function group are averaged with respect to a discrete event, for was expected to engage in some spontaneous imagery example, the presentation of a test word. Because on word trials (Durso & Johnson 1980), but, in ERPs provide information about brain activity as it general, the perceptual information associated with develops within trials, a combination of imaging and memories for pictures and words should be more ERP techniques is likely to provide an especially similar in the case of the Artist than the Function productive approach for the future. To date, not condition because the Artist subjects should be gener- enough ERP studies have been done with variations of ating more pictorial information on word trials. the same paradigm to yield clear temporal patterns of After a brief rest, subjects were tested while ERPs source monitoring; however, ERP studies are providing were recorded. Some subjects received a surprise old/ data consistent with spatial localization ¢ndings from new recognition test; for each item (a visually presented PET and some suggestive temporal ¢ndings are in as word), they responded old if it corresponded to either a well. picture or a word from phase one, and new if it did not. There have been a number of ERP studies of episodic Other subjects received a surprise source monitoring memory (see Rugg 1995 for a review) and a few that test; for each item, they responded picture, word or speci¢cally address issues of source memory. One new. Di¡erences in information and processes required hypothesis is that the magnitude of ERP amplitude between old/new and source decisions should be di¡erences between correct responses to old and new re£ected as di¡erences in brain activity. As noted items re£ects whether or not contextual detail has been above, both old/new recognition and source identi¢ca- recovered (`recollection') or whether the subject is tion require source information, but source attempting to recover contextual detail. For example, identi¢cation typically requires more speci¢c informa- Wilding & Rugg (1996; see also Wilding et al. 1995) tion (e.g. voice, colour, etc.) whereas old/new presented subjects with words in either a male or recognition can often be made on the basis of undi¡er- female voice and subsequently presented these same entiated familiarity. Likewise, di¡erences in the items visually, mixed with new items, and asked for information that is most discriminative for source deci- old/new decisions with each followed by a source judge- sions in the Function and Artist conditions should be ment. They found a greater ERP positivity for correct re£ected in corresponding di¡erences in brain activity. old responses compared to correct news (the `old-new Figure 2 shows wave forms for correct responses e¡ect'), especially in left parietal (500^800 ms post recorded at selected frontal and occipital sites, stimulus) and right frontal (1100^1400 ms post collapsed across acquisition condition for the two stimulus) sites. Furthermore, these old-new e¡ects types of test: old/new vs. source monitoring. These were greater for items recognized as old that were also ERP waves are plotted with time (in 100 ms. units) correctly identi¢ed as to source than for items that were going from left to right, with positive voltages plotted incorrectly identi¢ed as to source. up and negative voltages plotted down. As shown in Dywan et al. (1996) reported an experiment in ¢gure 2, the main di¡erence was recorded at frontal which young adults showed good source identi¢cation sites and was long-lasting, and the two tasks did not

Phil. Trans. R. Soc. Lond. B (1997) Source monitoring and memory distortion M. K. Johnson 1741

Left Right Left Right

FP1 FP2 FP1 FP2

O1 O2 O1 O2 4.00 uV 4.00 uV Ð100 ms 1948 ms Ð100 ms 1948 ms

Figure 2. Event-related potentials recorded at prefrontal Figure 3. Event-related potentials recorded at prefrontal (FP1/FP2) and occipital (O1/O2) sites for source moni- (FP1/FP2) and occipital (O1/O2) sites for source moni- toring task (bold) and old/new task (thin), collapsed across toring test task. Subjects had made either Function (bold) acquisition group. or Artist di¤culty (thin) judgements at acquisition. Test stimuli and task are identical for the two groups. look very di¡erent at more posterior sites. This is what one would expect if source monitoring is more depen- dent on frontal functions than is old/new recognition, Function groups, respectively, occurred not only for as suggested by evidence from brain damaged patients old items, but for new items as well. Presumably, new (e.g. Schacter et al. 1984; Janowsky et al. 1989). items do not give rise to the same types of information Now consider the waveforms for the source moni- (at least to the same degree) as do old items but must be toring task broken down separately for the two subjected to the same operations as old items (e.g. conditions (¢gure 3). Both conditions showed a distinct noting the amount of perceptual information they negative de£ection at approximately 400 ms, but this activate). negativity was focused at frontal sites in the Artist Our ¢ndings, along with those of Wilding & Rugg condition and at the occipital sites in the Function (1996) and Dywan et al. (1996) indicate that ERPs condition. The test list and test task were identical for may provide important converging evidence about these two conditions, thus these results indicate that brain processes central for source monitoring. Our the distribution of ERP activity depends on what was results support the idea that what the individual is initially encoded and/or what information subjects are trying to doötheir agendaöa¡ects the processes consulting (attempting to access or weighting most engaged and/or features that are activated or evalu- heavily) at test. ated. For example, our data are consistent with One hypothesis is that for Function subjects, amount evidence from brain lesion ¢ndings that frontal areas of perceptual detail is a good cue about the origin of are often more critical for source monitoring than for information in memory and, hence, the Function old/new recognition. In addition, how information group consults or relies on information represented in was encoded initially may have a greater impact on occipital sites. In contrast, for Artist subjects, amount source monitoring wave forms than on old/new recog- of perceptual detail is not as good a cue about origin nition waveforms. This is consistent with behavioural because images generated for the words would include ¢ndings from cognitive studies showing that source perceptual detail that would make them di¤cult to monitoring requires more or more di¡erentiated infor- discriminate from pictures. Artist subjects, therefore, mation than does old/new recognition (Johnson et al. are more likely to evaluate cognitive operations infor- 1994). Furthermore, like Wilding & Rugg (1996) we mation in order to make source discriminations. found greater left^right frontal asymmetry (see ¢gure Assessing records of such self-initiated operations 2) in later portions of the wave forms suggesting that should involve more frontally distributed processes. left^right asymmetries in episodic memory tasks Thus the results shown in ¢gures 2 and 3 suggest a found with PET (e.g. Tulving et al. 1994a) re£ect di¡er- general, relatively long-lasting frontal component that ences in processing that emerge over time as memories re£ects the di¡erence between old/new and source are revived and evaluated (e.g. Johnson et al. 1994). monitoring tasks and briefer speci¢c de£ections that Finally, consider another study suggesting that represent the distributed types of information likely to subjects' agendas a¡ect brain activity as they identify be playing a role in source monitoring in particular the origin of memories, and another illustration of the situations. potential value of obtaining converging evidence from Our data suggest that topographic distribution of ERP and imaging techniques. PET studies typically prominent ERP components may be associated with require that test trials occur in blocks of similar items which component processes are engaged and not neces- (e.g. a scan in which subjects are presented with all old sarily with what information is activated or found. items, followed by a scan in which subjects are First, the prominent negative de£ections over frontal presented with all lure items) because scans measure and occipital sites that di¡erentiated the Artist and average activity over a ¢xed time period in the order

Phil. Trans. R. Soc. Lond. B (1997) 1742 M. K. Johnson Source monitoring and memory distortion of several seconds. In contrast, ERP waves can be tions of patients with brain lesions, it appears that the obtained with designs in which items of various types frontal cortex is involved in all of these functions. New are randomly intermixed because individual waves are evidence for frontal lobe involvement in source memory recorded for each stimulus presentation. From the SMF, is beginning to accumulate from PETand ERP studies we would expect that this procedural di¡erence in test of normal subjects. Furthermore, initial results are might result in subjects considering di¡erent character- generally consistent with the SMF, which proposes istics of memories to distinguish between items. Given that distributed memory characteristics (e.g. stored in trials of randomly intermixed old and lure items, occipital, frontal, temporo-parietal areas) are selec- subjects may respond more on the basis of familiarity tively activated and weighted, depending on the than when sequences of test trials are all olds and all subject's agenda. The challenge for the future is to lures. In a sequence of similar items, familiarity more closely associate speci¢c frontal brain regions (or will not vary as much between items, and subjects transactions between speci¢c frontal brain regions and should be more likely to consider more speci¢c di¡er- other brain areas) with component cognitive processes entiating information in an attempt to make implicated in source monitoring (e.g. Johnson 1997; distinctions among items within a block that are not Johnson & Reeder 1997). That is, a primary goal is to easily discriminable. further specify the transactions between particular To test this prediction, Johnson et al. (1997a) looked at frontal regions and, for example, particular occipital ERP waves for falsely recognized items that were or temporo-parietal regions that we hypothesize consti- associatively related to studied items under conditions tute reactivation or evaluation of qualitative where old and lure items were either blocked or characteristics of memories such as visual or auditory randomly presented at test. In the blocked test condi- detail, and to specify the role of the hippocampal tion, there were di¡erences in the ERP waves to `old' region in source memory (e.g. binding attributes, reac- responses to presented items (hits) and `old' responses tivating memories). In this endeavour, PET and fMRI to lures (false recognitions), consistent with the PET techniques provide the best opportunity for identifying ¢ndings (Schacter et al. 1996b). In contrast, in the brain regions that are generally active in given tasks random test condition, the ERP waves for false recog- and in isolating the brain regions that discriminate nitions and hits were strikingly similar, suggesting that between tasks. ERPs appear especially promising for subjects were making old/new decisions largely on the exploring the temporal dynamics of how particular basis of overall familiarity of the semantic concept. In attributes of source are accessed and used, particularly the blocked condition, we suggest subjects were consid- if it turns out that di¡erences in wave form patterns and ering each item more completely, attempting to topography are reliably associated with various attri- discriminate among essentially similar items within a butes of source. We may be able to use PET, fMRI and block. This closer examination resulted in di¡erences ERPs in combination to map out a dynamic picture of between ERP waves for hits and false recognitions, on-line processes and brain structures underlying presumably re£ecting di¡erences in underlying repre- source monitoring in di¡erent situations. This would sentations of old and lure items (Mather et al. 1997; move us closer to understanding the brain mechanisms Norman & Schacter 1997). In behavioural studies, underlying the cognitive mechanisms that give rise to whether perceived and inferred memories appear to both veridical and distorted memory. have similar or di¡erent qualitative characteristics is in£uenced by the information subjects are induced to Preparation of this paper was supported by NIA grant consider at the time of test (Mather et al. 1997). 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