Using Inattentional Blindness As an Operational Definition of Unattended: the Case of a Response±End Effect

VISUAL COGNITION, 2004, 11 6), 705±719

Using inattentional blindness as an operational definition of unattended: The case of a response±end effect

Cathleen M. Moore, Alejandro Lleras, Marc Grosjean, and Mark T. Marrara Department of Psychology, Pennsylvania State University, USA

Inattentional blindness IB), which is the lack of awareness of stimuli that appear in unattended regions of the visual field, can be used as an operational definition of unattended. Usingseparate online measures in combination with IB can be used to probe specific aspects of processing that are engaged by unattended stimuli. Pre- vious applications of this method have provided evidence that unattended stimuli can engage processes that lead to grouping-by-similarity Moore & Egeth, 1997), as well as processes involved in the perceptual completion of partially visible surfaces Moore, Grosjean, & Lleras, 2003). Here we applied this method to address the question of whether unattended stimuli can engage response±end processes. Specifically, we used a stimulus±response compatibility effect, the Simon effect e.g., Simon & Rudell, 1967), as an online measure of response selection. As assessed through this measure, unattended stimuli failed to engage response-selection processes.

Inattentional blindness IB) is one of several recently documented illustrations of how dramatically people can fail to notice what happens around them. IB refers to the observation that salient but unexpected events, such as well above- threshold flashes of light, can go completely unnoticed if they occur in an unattended part of one's visual field e.g., Mack & Rock, 1998; see also change blindness, e.g., Rensink, O'Regan, & Clark, 1997; Simons, 2000b).

Please address all correspondence to Cathleen Moore, Department of Psychology, Penn State University, University Park, PA 16802, USA. Email: [email protected] Portions of this work were presented at the 2001 meetingof the Vision Sciences Society in Sarasota, FL. Support was provided by NSF grant SBR-9728628. Alejandro Lleras is now at the University of Britisch Columbia in Vancouver, Canada; Marc Grosjean is now at the Max Planck Institute for Psychological Research in Munich, Germany; and Mark Marrara is now at the Meta- vante Corporation in Milwaukee, WI. We wish to thank Marvin Chun and two anonymous reviewers for helpful comments on a previous version of this paper.

# 2004 Psychology Press Ltd http://www.tandf.co.uk/journals/pp/13506285.html DOI: 10.1080/13506280344000482 706 MOORE ET AL.

Operationally, IB is assessed through an experimental method that was developed by Mack and Rock and their colleagues. They wished to contrast their approach with previous methods of probingunattended processing,such as visual search e.g., Treisman & Souther, 1985) and dual-task procedures e.g., Braun, 1993; Braun & Sagi, 1990) in which observers were aware of the potential relevance of the unattended stimuli. Awareness of the relevance of the supposedly unattended stimuli, argued Mack and Rock, might have caused those stimuli to be at least partially attended. Thus, the method that they developedÐ which we will refer to as the inattention methodÐwas aimed at probingthe processingof stimuli that were in the visual field, but that observers had no reason to suspect were relevant to anythingthat they were doing,and therefore should have been truly unattended. An intuitive example of what they sought in the laboratory is the pattern of the wallpaper behind a person with whom you are speaking. It is a large, above-threshold stimulus that is in your visual field, and yet it is not relevant to what you are doingat the time. You would be able to easily describe it if someone were to draw your attention to it. The question is, assumingthat your attention was not drawn to it, would you process it and could you describe it? The inattention method engages attentional processing using a difficult primary task. Mack and Rock 1998) asked observers to report whether the vertical or horizontal bar in a large central plus sign was longer. After about three trials of this task, another stimulus was presented alongwith the plus sign, somewhere in the display. For example, a bright filled-in square might appear somewhere in the upper-left quadrant of the display. Followingthat trial, the observer is asked if he/she noticed anythingelse in the display on that last trial, and is asked to make a forced-choice response regarding it, such as which of four quadrants it appeared in. Observers often report that they saw nothingother than the plus sign, and end up guessing at chance levels as to its location or other property. Once aware that additional stimuli may appear, however, observers are typically able to see and report on them quite well. This indicates that the stimuli were sufficiently salient to be perceived as longas they were attended, and therefore that the failure to perceive them when they were unattended could be attributed to the absence of attention. This entire pattern of results is the empirical indication of IB. Although there is a large set of stimuli that fail to support explicit report when they are unattended see Mack & Rock, 1998 for a review), other research suggests that substantial amounts of unreportable information processingof unattended stimuli does occur Kimchi & Razpurker-Apfeld, in press; Mack & Rock, 1998; Moore & Egeth, 1997; Moore et al., 2003; Most et al., 2001; Simons, 2000a). This processingwithout attention presumably provides the foundational representations out of which conscious perception emerges, and is therefore of considerable importance for theories of visual perception. OPERATIONAL DEFINITION OF UNATTENDED 707

In a recent paper, we suggested that IB can be turned around and used as a tool for studyinghow unattended stimuli are processed Moore et al., 2003). Specifically, the idea is to use IB as an operational definition of `ùnattended''. Those stimuli to which observers are inattentionally blind are, by this definition, classified as `ùnattended''. From there, one can use online measures to assess in what specific ways unattended information is processed, and in what ways it is not. By online measure, we mean a measure of a targeted process that is taken while that process is occurring, and therefore, while the stimuli in question are unattended. These online measures must be indirect, because probingobservers directly about some aspect of an unattended stimulus will necessarily draw their attention to the stimulus, makingit no longerunattended. The application of IB as an operational definition of unattended is useful because defining`ùnattended'' is often the main difficulty in studyinghow unattended stimuli are processed. As noted by Mack and Rock 1998; Mack, Tang, Tuma, Kahn, & Rock, 1992; Rock, Linnet, Grant, & Mack, 1992), previous methods of assessingunattended processingare potentially problematic in that observers are often at least aware of the presence of the stimuli that were supposed to be unattended. ``Distractors'' in visual-search tasks, for example, which are often cited as beingprocessed preattentively, fall into this category e.g., Treisman, 1998; Treisman & Gormican, 1988; Wolfe, 1994). Flankers in tasks that seek to reveal the influence of unattended flankingstimuli on the processing of a central target do as well e.g., Eriksen & Eriksen, 1974; Eriksen & Hoffman, 1973). In other cases, the stimuli that are supposed to be unattended are task relevant, in that the procedure involves two tasks, the second of which concerns properties of the unattended stimuli e.g., Braun & Sagi, 1990). The possibility that the stimuli that are supposed to be unattended in these methods are at least partially attended can reduce the strength of the conclusions that can be drawn concerningthe processingof unattended stimuli. For this reason, establishingan operationally defined domain of `ùnattended'' should be liberating. We suggest here and elsewhere Moore et al., 2003) that IB is a good candidate measure for operationally constrainingthe definition of `ùnat- tended''. There are two steps involved in the approach proposed here. First an online measure of the process that one wishes to assess must be chosen or developed. A balance of criteria for this measure must be met. The measure must reflect the targeted processes, but it also must be one that will support IB. In other words, the stimuli that are required for the online measure cannot themselves be likely to capture attention. This follows because the basis of the approach is that evidence of IB to the stimuli will define the stimuli as unattended, and only then can the question of whether that process is engaged by unattended stimuli be addressed by lookingat the online measure. In Moore et al. 2003), we applied this approach to the perceptual completion of partial surfaces. We first developed an online measure of surface com- 708 MOORE ET AL. pletion. Observers reported as quickly as possible whether the central line was dashed or solid see insert in Figure 1). In some displays, pacmen in the background of the display were randomly oriented such that no particular completion pattern was supported Figure 1A). In other displays, they were aligned so that modal completion into rectangles was supported Figures 1B and 1C). When the rectangles were aligned with the gaps in a dashed line, they would in turn support the amodal completion of the dashed line behind the modally completed rectangles into a representation of a solid line Figure 1B), thereby slowingresponses to report ``dashed'' on these trials. The slow- ingwas taken as an indication that both modal completion of the rectangles and amodal completion of the line occurred. Havingestablished an online measure of surface completion, the next step was to use that measure in the context of the inattention method. The online measure indicated that both modal and amodal completion occurred. Despite this, when observers were probed regarding their awareness of the surfaces themselves, using the inattention method, they showed IB to them. On the whole, then, the results were taken to indicate that unattended stimuli, operationally defined by IB, could engage surface-completion processes.

Figure 1. Illustrations of the displays used in Moore et al. 2003). The horizontal line in each of the sample displays is of the dashed type, and thus the correct response was ``dashed''. The box in the upper right of the figure illustrates all of the different line types that appeared in the experiment: Dashed, solid, and the two versions of the part-dashed/part-solid. OPERATIONAL DEFINITION OF UNATTENDED 709 CAN UNATTENDED STIMULI ENGAGE RESPONSE±END PROCESSES? In the present study, we sought to assess whether unattended stimuli can engage processes that are responsible for selectinga response to stimuli in the environment. This is an important question because accordingto one class of attentional theory the primary function of attention is the selection of information for action e.g., Allport, 1987, 1989; Tipper, Howard, & Houghton, 1998). We adopted the Simon effect e.g., Simon & Rudell, 1967) as our online measure of response selection. The Simon effect refers to the fact that responses to stimuli that appear on the same side of space as the indicated response tend to

Figure 2. A. A typical Simon task: Observers are asked to respond to the identity of the target in this case, letter name: H or S). The target can appear to the right or left of fixation and the response keys are to either side of the observer's midline. Responses are typically faster on consistent trials left panel), where the side of the stimulus and response match, than on inconsistent trials right panel), where they do not. B. An accessory-stimulus Simon task: The primary task is the same, but now responses are faster when a task-irrelevant accessory stimulus the square in the figure) appears on the same side as the correct response left panel) than when it appears on the opposite side as the correct response right panel). 710 MOORE ET AL. be faster and more accurate than responses to stimuli that appear on the opposite side see Figure 2A). Importantly, this effect occurs despite the fact that the location of the stimulus is irrelevant to the task. Although many of the specifics of what causes Simon effects are still debated, it is commonly understood to occur at least in part within response-selection processes e.g., Kornblum, Ste- vens, Whipple, & Requin, 1999; Simon, Acosta, Mewaldt, & Speidel, 1976; UmiltaÁ&Nicoletti, 1992). The idea is that a spatial code that is associated with the location of the stimulus is automatically generated and is either consistent or inconsistent with the spatial code of the signalled response, slowing or speeding the selection process, respectively. The particular version of the Simon effect that we adopted uses an accessory stimulus to induce the consistency effect see Figure 2B). Observers fixated the centre of a computer monitor, where a single letter appeared on each trial. The task was to push the left button if the letter was an H and the right button if it was an S as quickly as possible, while maintaininga criterion level of accuracy. In addition to the letter, a small square the accessory stimulus) also appeared off to the left or the right of the letter stimulus. The location of the square was uncorrelated with the identity of the letter, and it was irrelevant to the task. In a standard Simon experiment, observers would be warned of the presence of the squares and asked to ignore them. Despite these instructions, responses tend to be faster when the side on which the square appears matches the indicated response side than when they mismatch. In the present context, the question was whether the squares in the accessory Simon task would influence responses even if observers were inattentionally blind to them. Given that the locus of the Simon effect is at least partly within response-selection processes, by the logic set up in this paper, such a finding would indicate that unattended stimuli could engage response-selection processes. This findingwould be contrary to the general position of selection-for- action theories, because they hold that response selection is the primary function of attention, and therefore, that unattended stimuli would not have access to those processes. If, however, the square only gives rise to a Simon effect when observers are aware of them, then by the same logic, it would suggest that, at least in this case, unattended stimuli were not able to engage response selection. This findingwould be in keepingwith selection-for-action theories of attention.

METHOD Participants Twenty-five individuals from the Pennsylvania State University undergraduate subject pool participated in this experiment age range: 18±24 years). All were naõÈveastothe purpose of the experiment prior to beingtested. All reported normal or corrected-to-normal visual acuity and colour vision. They received extra credit in a psychology course for their participation. OPERATIONAL DEFINITION OF UNATTENDED 711 Stimuli All stimuli were presented on a black background at a viewing distance of approximately 60 cm. A fixation dot with a diameter of 0.198 was presented in the centre of the monitor. The letter stimuli were H and S, and were also presented centrally, replacingthe fixation cross. They were formed by removing segments of a squared-off figure-of-eight, like those used in digital clock displays. They each subtended 0.458 width) 6 0.608 height). Complete figure-of- eights were also used as masks. The accessory square was 0.38 on each side, and was presented vertically centred 0.158 edge-to-edge) to the left or the right of the letter.

Task The task was to respond to the identity of the letter by pressingthe ``z'' key on the keyboard when the letter was an H and the ``/'' key on the keyboard when the letter was an S. Observers held their left and right index fingers on the ``z'' key and ``/'' key, respectively, throughout the experiment. Instructions emphasized that responses should be made as quickly as possible while main- tainingat least 95% accuracy.

Design A2Consistency: consistent, inconsistent) 6 2Attention condition: unattended, attended) within-subjects design was used to assess the Simon effect. Consistency was mixed within blocks of trials, whereas the attention condition was manipulated between blocks. Consistent trials were those on which the square appeared on the same side as the indicated response; inconsistent trials were those on which the square appeared on the opposite side as the indicated response. For half of the trials, the target was an H; for the other half the target was an S. Squares appeared equally often on the left and the right, and were uncorrelated with the identity of the target. One block of 64 trials was completed for each attention condition, yielding32 observations in each of the four conditions. The unattended data were collected in a block of trials prior to applyingthe inattention method. The attended data were then collected in a block of trials followingthe application of the inattention method.

Procedure Participants completed a single session that lasted approximately 30 min. The session began with a set of written instructions that described the letter- identification task. These were followed by a block of 20 practice trials, in which there were no squares, only the central letter. Followingpractice, 712 MOORE ET AL. observers completed one block of 64 trials, from which the measure of the Simon effect for unattended stimuli was obtained. Every trial in this block included a square to the left or right of the central letter. Consistent and inconsistent trials were presented in a pseudorandom manner. Followingthis first block of trials, observers completed what we will refer to as the inattention block. This block was nine trials long, and was similar to those described in Mack and Rock 1998). The stimulus letter was chosen randomly for each trial. The fourth, eighth, and ninth trials were the inattention, divided- attention, and full-attention trials, respectively, and a square was presented randomly to the left or right of the central letter on these trials. For the remaining trials, no square was presented. Followingthe inattention and divided-attention trials, a small question mark appeared at fixation and the experimenter, who remained in the room throughout the session, asked the observer the following two questions: 1) ``Did you see any additional items besides the letter in the previous display?'' 2) ``If I told you that a square appeared in the previous display, on which side would you guess that it appeared, left or right?'' After recordingthe answers to these questions, the experimenter initiated the next trial when the observer was ready. After the eighth trial, and prior to initiatingthe ninth and final trial, which was the full-attention trial, the experimenter asked the observer to ignore the letter task and to simply report whether they saw a square, and if so, which side it was on. Finally, followingthe inattention block, observers completed a last block of 64 trials, of the same form as the block of trials prior to the inattention block. Trial events are illustrated in Figure 3. Each trial began with a fixation dot at the centre of the screen. 1000 ms later a placeholder figure-of-eight target and the accessory stimulus appeared together for 75 ms, the figure-of-eight then changed into the letter H or S for 100 ms, after which point the accessory stimulus offset, and the target letter changed back into the figure-of-eight placeholder until a response was made or until 2500 ms passed, whichever came first. The placeholders served as masks to make the letter-identification task sufficiently demanding.

RESULTS AND DISCUSSION Observers were inattentionally blind to the squares see Figure 4A). Only 16% of participants reported havingseen any stimulus other than the central letter followingthe inattention trial, which was reliably fewer than that expected by chance, z = ±3.4, N =25.Wherever a z statistic is reported, a binomial test was used. In addition, a was set at .05 for all statistical tests.) Consistent with those reports, only 48% of the participants correctly reported on which side of fixation the square occurred, which is not reliably different from chance, z = ±0.2, N = 25. By the divided-attention trials, more observers were aware of the squares. Seventy-six per cent reported havingseen it, z = 2.6, N = 25, and eighty-eight OPERATIONAL DEFINITION OF UNATTENDED 713

Figure 3. Trial events from Experiment 1. Each trial began with a fixation dot at the centre of the screen. 1000 ms later a placeholder figure-of-eight target and the accessory stimulus appeared together for 75 ms, the figure-of-eight then changed into the letter H or S for 100 ms, after which point the accessory stimulus offset, and the target letter changed back into the figure-of-eight placeholder until a response was made or until 2500 ms passed, whichever came first. per cent correctly reported the side on which it appeared, z = 3.8, N =25, both of which were reliably greater than chance. Finally, performance following the full-attention trial indicated that the squares were well above threshold, and could be easily perceived when they were attended. Ninety-two per cent of the observers reported havingseen the square followingthis trial, z = 4.2, N = 25, and ninety-six per cent correctly reported the side on which it appeared, z = 4.6, N = 25, both of which were also reliably greater than chance. Thus, the IB results indicated that observers were inattentionally blind to the squares, thereby qua- lifyingthe squares as unattended, by the definition adopted here. The extent to which they gave rise to a Simon effect could then be used as an assessment of whether unattended stimuli can access response-selection processes. The data from the Simon-task blocks suggest that response-selection processes were not engaged by the unattended squares see Figure 4B). These data were submitted to a 2 unattended vs. attended) 6 2 consistent vs. inconsistent) analysis of variance. There was no main effect of the attention condition, F1, 24) = 2.09, MSE = 3877. The main effect of Simon consistency, however, was significant, F1, 24) = 8.08, MSE = 673, as was the interaction, F1, 24) = 5.47, MSE = 644. Follow-up comparisons revealed that there was no reliable Simon effect i.e., difference between consistent and inconsistent trials) prior to observers havingbeen made aware of the squares, mean difference = 3 ms, t24) = 0.51. In contrast, after observers had been made aware of the presence of the 714 MOORE ET AL.

Figure 4. A. Data from the inattention block in Experiment 1. The data are the percentage of observers who correctly answered each of questions followingeach of the three critical trials: Inattention, divided attention, and full attention. The ``anything'' question was ``Did you see any additional items besides the letter in the previous displays?'' Although it is a bit of a misnomer for this question, for uniformity within the figure, ``no'' was considered incorrect and ``yes'' was considered as correct, given that there was always a square present. B. Mean response time data from correct trials in the first unattended) and third attended) blocks of the experiment. squares by participatingin the inattention block, a reliable Simon effect did occur, mean difference = 27 ms, t24) = 3.12. This is important because it indicates that the stimuli were sufficient to drive a normal Simon effect. The failure of findingone in the first Simon-task block, therefore, can be attributed to the squares havingbeen unattended, rather than to insufficient stimulation. All of the same analyses were conducted on the arcsine transformations of the error rates. The mean error rates were 5.1% and 5.0% for consistent and OPERATIONAL DEFINITION OF UNATTENDED 715 inconsistent trials, respectively, duringthe unattended block. They were 3.8% and 5.3% for the consistent and inconsistent trials, respectively, duringthe attended block. No reliable effects that were in different directions than those in the response times were observed. Thus, there were no concerns about the patterns in the response time data havingbeen obscured by a speed±accuracy tradeoff. Overall, the results suggest that, in keeping with selection-for-action models of attention, unattended stimuli do not engage response-selection processes.

GENERAL DISCUSSION This study combined inattentional blindness with an online measure of response selectionÐthe Simon effect e.g., Simon & Rudell, 1967)Ðto assess whether unattended stimuli can engage response-selection processes. The results suggest that they cannot, or at least did not under this particular assessment of response- selection processes. In particular, a Simon effect occurred in response to stimuli that subjects were aware of, but did not occur to those same stimuli when the same subjects were unaware of them. To the extent that the presence or absence of a Simon effect in response to a stimulus is an index of whether or not a representation of that stimulus reached response-selection levels of processing, these results indicate that the unattended stimuli did not reach those processes. Implications of these results, as well as limitations to the conclusions that can be drawn from them, are considered in the remainder of this discussion. At a methodological level, the results of this experiment serve to validate the application of IB in combination with online measures of specific processes to the study of inattentional processing. Prior to this study, this approach had only revealed a positive answer to the question of whether a given process is engaged by unattended stimuli. Using an online measure of grouping-by-contrast- similarity, for example, Moore and Egeth 1997) concluded that at least some forms of Gestalt grouping could be engaged by unattended stimuli. Similarly, usingan online measure of modal and amodal surface completion, Moore et al. 2003) concluded that surface completion could be engaged by unattended stimuli. In contrast to these earlier studies, the present study provides an example of a process that does not seem to be engaged by unattended stimuli, response selection. Thus, this study shows that the method can reveal not only those processes that are engaged by unattended stimuli, but also can reveal those processes that are not engaged by unattended stimuli. At a substantive level, the results provide convergent support for selection- for-action conceptualizations of attention. The term selection-for-action was offered by Allport and his colleagues e.g., Allport, 1987, 1989; Tipper et al., 1998) as an alternative to the dominant conceptualization of attention as a processinglimitation that characterized the early-versus-late era of attention research e.g., Broadbent, 1958; Deutsch & Deutsch, 1963; see Lavie & Tsal, 1994 for a critical review). The general idea was to note that at the point at 716 MOORE ET AL. which a visually guided action is to be executed, a specific and severe selection limitation arises. Parameters have to be set such that this particular action occurs rather than that particular action. An example offered in several of their papers is to imagine that you are going to pick an apple from a tree. That action is based on a visual representation of where the apples are in the tree and, at some point, movement parameters must be set such that your hand and arm engage in a trajectory toward one of the alternative apples. That set of parameters is exclusive from a set that would lead to a different trajectory, and therefore direct the hand to a different or no) apple. In the context of more controlled laboratory tasks, the selection of response a over response b in a two-alternative-forced- choice task embodies this processing necessity. Allport and colleagues suggested that the function of the attention system is to subserve this need for selection for action. The findings of our experiment are in keeping with this claim. To the extent that the selection of a response requires attention, it is to be expected that unattended stimuli will not engage response-selection processes. This is exactly the implication of the results. This is not to say that attention serves no other purpose, but rather, that at least one important function is the selection of visual information for the guidance of motor responses. Some caution must accompany the conclusion that response selection is not engaged by unattended stimuli based only on the application of the Simon effect and IB, however. The logic of this approach is strongest in the direction of positive findings. That is, had a Simon effect occurred even for unattended stimuli, then it could have been concluded strongly that unattended stimuli can engage response selection. Given that no Simon effect occurred for unattended stimuli, however, there are a number of possible explanations. First, there is the general concern of statistical power and null effects. It remains a possibility that with more power, a Simon effect to unattended stimuli might emerge. This is a possibility that we cannot rule out. We can, however, offer two observations regarding this limitation. First, the null effect is not considered in isolation. Specifically, the lack of a reliable Simon effect to unattended stimuli was observed in the same subjects and the exact same stimuli, as was the reliable Simon effect to attended stimuli was. Therefore, the design, stimuli, and subject group was sufficient to reveal a reliable Simon effect when it was there. The second observation is that that we have replicated this basic finding, with some methodological differences, with two other groups of subjects. Assumingthat power is not a problem, there are two substantive possibilities that must be considered with regard to why no Simon effect occurred for unattended stimuli. One is that no spatial code was established for stimuli that are unattended. Without a spatial code, there would be no inconsistent spatial representation to compete with the correct response at the level of response selection.1 The second is a variation of the first; without a directional shift of

1 We are grateful to Yvonne Lippa for pointing out this possibility. OPERATIONAL DEFINITION OF UNATTENDED 717 attention to the irrelevant item which in our case was never attended), it may be that no inconsistent spatial code was established to compete with the correct response at the level of response selection Rubichi, Nicoletti, Iani, & UmiltaÁ, 1997). Thus, although the failure to find a Simon effect for unattended stimuli is consistent with the conclusion that unattended stimuli fail to engage response selection processes in general, assuming that the failure was not due to insufficient power, the more specific conclusion that unattended stimuli fail to engage response-selection processes via the spatial codes of irrelevant stimuli in particular may hold. A different online measure of response selection may reveal evidence of inattentional access to those processes. Finally, the results of Experiment 1 contrast interestingly with a growing literature demonstratingthat masked stimuli, of which observers have no phenomenal awareness, can nonetheless trigger manual responses e.g., Eimer & Schlaghecken, 1998; Fehrer & Raab, 1962; Klotz & Neumann, 1999). One possible reason for the apparent difference in results is that the masked stimuli in those studies were not strictly unattended as were the flankingsquares in the present study. That is observers were aware of their possibility but did not phenomenally experience them because of brief masked conditions. The squares in our experiment were well-above threshold, were unmasked, and once attended were easy to detect. Thus, the contrast between these masked-priming studies and the present one may point to an intermediate level of representation, such that it is insufficient to support phenomenal awareness, but nonetheless, because the stimulus from which it arose was at least partially attended, it does have access to response-end processes. In any case, the contrast warrants further investigation.

Anote regarding the application of IB as an operational definition of unattended We wish to close with a note that the application of IB as an operational definition of unattended is in keepingwith the originalthinkingbehind the development of the inattention method. Rock and Mack and their colleagues sought to reveal what processing occurred for stimuli that were truly unattended. Their discovery of IB as a phenomenon was striking, and it led to a large body of work investigating the conditions under which IB does and does not occur see Rock & Mack, 1998; Simons, 2000a). Our suggestion is that because IB occurs under conditions that are arduously designed to be truly unattended, and because so many different studies have converged to confirm that fact, the field is in a position to use the existence of IB to a particular stimulus as an indicator that that stimulus was unattended. Then one can ask, given that it was unattended, in what specific ways was it processed? One might argue that IB is not an ideal operational definition of unattended see Moore et al., 2003 for a discussion of this issue). To the extent that it is 718 MOORE ET AL. adopted, however, it sets objective boundaries for drawingconclusions about what processes are engaged by unattended stimuli. In a field that is often driven to citingJames' 1890/1983, p. 381) elegantbut ultimately evasive definition of attention, ``Everyone knows what attention is . . .'', such boundaries should be both liberating and grounding.

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Original manuscript received April 2003 Revised manuscript received November 2003