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For inclusion in ‘Advances in Experimental Psychology’ Forgetting as a consequence of remembering: Retrieval-induced forgetting and the malleability of memory Malcolm D. MacLeod University of St. Andrews & Jo Saunders University of Swansea Total word count (including refs) = 10,065 Address for correspondence: Malcolm D. MacLeod Social & Applied Cognition Lab School of Psychology University of St. Andrews St. Andrews Fife, Scotland KY16 9JU Tel: +44 (0)1334-462064 Fax: +44 (0)1334-463042 Email: [email protected] 1 Abstract This article considers some of the most recent advances in experimental psychology regarding the role of inhibitory control in the production of misinformation effects in memory; that is, the tendency to report misleading post-event information in preference to original material. Specifically, we report our findings from a series of studies using the retrieval practice paradigm (cf. Anderson, Bjork & Bjork, 1994) that explore how retrieval-induced forgetting (i.e., the tendency to forget material as a consequence of retrieving other related material) may be involved in promoting misinformation effects. We then consider how to establish experimentally that the mechanism underlying this relationship is inhibitory. In doing so, we outline the independent probe technique (Anderson & Spellman, 1995); that is, the use of novel retrieval cues at test rather than those cues used during the initial stages of the retrieval practice paradigm. Our findings clearly indicate that misinformation effects emerged only under conditions where retrieval-induced forgetting remained active and that the retrieval-induced forgetting observed in our studies was due to inhibitory control. Implications for our understanding of how memory is updated; the design of experimental paradigms that consider the role of active forgetting in memory; and the development of police investigative techniques are also explored. 2 Forgetting as a consequence of remembering: Retrieval-induced forgetting and the malleability of memory In our quest to understand how memory works, researchers often face the problem of how to make everyday phenomena the focus of empirical study. Foremost amongst these myriad challenges is the design of appropriate experimental paradigms that permit memory to be explored in a controlled manner yet retain sufficient realism to allow extrapolation to the real world. For those who are primarily interested in solutions to applied problems, a high level of control can appear artificial and remote from the contexts and factors associated with memory in the real world. For other researchers, however, experimental control is paramount (e.g., Banaji & Crowder, 1989). While arguments can be made regarding the utility of studies that have high ecological validity, we are of the opinion that this should never be at the expense of experimental control. We wish to be clear that this preference is not due to our lack of interest in applied problems - in fact, the converse is true. Rather, we believe that good applied research is, without exception, based on well-designed and tightly controlled experimentation. Thus, in our view, highly controlled studies offer the greatest potential - both in terms of how to advance theory and the development of solutions to real world problems. Indeed, there is much to commend the sentiment expressed by Banaji and Crowder (1989) that ‘…the complexity of a phenomenon is a compelling reason to seek, not abandon the laboratory’ (p.1112). Unfortunately, all too often the complexity of the phenomena of interest is given as the raison d’etre for poorly controlled research. In order to illustrate our point of view, the present article details some of the most recent experimental studies concerning the mechanisms underlying one of the most researched of all memory phenomena - the misinformation effect. This effect refers to the bias towards 3 unwittingly recalling misinformation (i.e., information inconsistent with that originally presented) in preference to original material (Loftus, 1979a; Loftus, Miller & Burns, 1978). In considering this body of research, we hope to demonstrate how, through controlled experimentation, it is possible to enhance our understanding not only of those conditions most likely to give rise to misinformation effects in the real world but also the complex relationship that exists between forgetting and remembering. The Misinformation Effect Despite the fact that much of the pioneering work on the misinformation effect was carried out almost thirty years ago (Loftus, 1979a; Loftus, et al., 1978; Loftus & Palmer, 1974), its influence in cognitive psychology remains strong. The standard misinformation paradigm (cf. Loftus, et al., 1978) involves the presentation of a target incident such as an accident or crime scene in the form of a slide sequence, video, or piece of prose. On encoding this material, participants are presented with a series of questions about the target event. Unknown to the participants, however, a piece of misinformation (i.e., information inconsistent with that originally presented) is embedded within one of the questions. For example, participants could be asked about a car that passed by a ‘Yield’ sign whereas, in fact, the car in question had actually passed by a ‘Stop’ sign or vice versa (cf. Loftus, et al., 1978). Following a short delay, participants are asked to choose which slides they had seen previously in a two-alternative forced-choice recognition task. For non-critical items, participants are required to choose between a novel slide and the original whereas, for the critical item (i.e., where misinformation had been introduced), participants have to choose between the original and the misinformation item (e.g., ‘Yield’ versus a ‘Stop’ sign). In their seminal study, Loftus and colleagues found that 59% of participants who were misled in this way chose the misinformation item in contrast to only 25% of non-misled participants. Although the magnitude of this particular effect can be modified by a range of cognitive and social factors (see Wright & 4 Davies, 1999 for a review), its robustness has never been in question (see e.g., Belli, 1989; Bekerian & Bowers, 1983; Chandler, Gargano & Holt, 2001; Christiaansen & Ochalek, 1983; Dodson & Reisberg, 1991; Lindsay & Johnson, 1989a, 1989b; Pirolli & Mitterer, 1984). There are several reasons why this work has proved to be important. Although it had been appreciated for some time that eyewitness reports could be modified by the form of questions employed to elicit information from witnesses (Binet, 1900, 1905; Harris, 1973; Loftus & Zanni, 1975; Stern, 1939), Loftus’ work has proved instrumental in prompting us to think about whether the unwitting introduction of misinformation may actually result in the distortion of memory rather than simply influencing its report. Loftus et al (1978) identified that one of the problems with the leading question paradigm was a mismatch between the stimuli used during the study phase and final test. Specifically, while the study phase generally consisted of a visual presentation, verbal tests were usually employed at final test that may have biased retrieval towards the verbal misinformation. From a theoretical viewpoint, therefore, the extent to which the leading question paradigm could reveal anything about the nature of memory was somewhat limited. By introducing a recognition test as the final phase of the misinformation paradigm, however, it became possible to determine the availability of the memorial representation for the original item following the introduction of misinformation. Arguably, it is the attention to experimental details such as this that has kept the study of the misinformation effect at the forefront of applied cognitive research for almost thirty years. Having identified the possibility that the study of misinformation effects could provide important insights into how memories are subject to destructive updating by newer, inconsistent material (Loftus, 1979ab; Loftus et al., 1978), Loftus also sought to eliminate alternative accounts for her findings. In doing so, her research indicated that motivational factors and demand characteristics appeared to have little impact on the production of such effects. Monetary incentives failed to improve recall accuracy (Loftus, 1979a) as did the provision of 5 opportunities to change initial responses (Loftus, 1979) and warnings to participants that false information may have been introduced (Greene, Flynn & Loftus, 1982). In addition to eliminating competing explanations for her findings, Loftus also sought to gather additional supportive evidence that was consistent with a destructive updating account of the misinformation effect. In particular, Loftus’ use of reaction time methodology allowed her to infer whether integration between memory for the original item and memory for the misinformation is likely to have taken place. The rationale behind this approach was that, if misled participants could be shown to be slower in choosing an item at test, this would be consistent with the extra time needed to resolve two conflicting representations. If, on the other hand, response times could be shown to be fast, it would suggest that any conflict between the original and the misinformation had already been resolved and that the misinformation is likely to have been integrated into memory for the target event shortly after the introduction of the misinformation. The fact that Loftus and colleagues found misled participants to react quickly to critical items provided support for the thesis that the misinformation effect is a product of memory change (see Cole & Loftus, 1979; Loftus, Donders, Hoffman & Schooler, 1989). Loftus also considered the possibility that the introduction of misinformation could disrupt the original memory trace in such a way that a blended representation of the original item and misinformation may be produced (Loftus, Schooler & Wagenaar, 1985; Metcalfe, 1990). This idea derives from earlier work by Loftus (1975; 1977) in which participants who had been misled about the colour of an object, tended towards a compromise option (i.e., some combination of the original and misinformation) rather than choosing the original or misinformation item. Thus, if the original target item had been ‘green’ and the misinformation suggested the target item had been ‘blue’, participants tended towards a ‘bluish-green’ compromise at test. On further detailed experimentation, however, it became apparent that the empirical evidence for such a blending mechanism was not particularly strong. Memory blending 6 is not an easy phenomenon to study given that there are relatively few instances of items that can be blended to form a single object. For example, there is no obvious blend for ‘Yield’ and ‘Stop’ signs, or for Coke and 7-Up cans. Even when exploring memory for materials that had the potential to be blended (e.g., colour), Belli (1988) found that misled participants tended to favour either the original or the misinformation item at test rather than any compromise option. As with many lines of experimental enquiry, the paradigms employed by researchers tend to be influenced by what has gone before; that is, we tend to build upon existing knowledge and techniques. This approach has the advantage that we don’t have to keep ‘re-inventing the wheel’ but it can also have the disadvantage that some of the problems associated with earlier paradigms can be unwittingly transferred to current ones. Chandler (1989) has made the point that the misinformation paradigm is arguably no different in this regard given that parallels can be drawn between it and the A-B, A-D paired associate paradigm which was extensively employed during the classic interference era. According to Chandler’s rationale, the target incident in the misinformation paradigm can be considered the equivalent of the A stimulus, the original target item the equivalent of the B response, and the misinformation item the equivalent of the D response. Chandler and colleagues have argued (e.g., Chandler, 1989, 1993; Chandler & Gargano, 1995) that misinformation effects can best be viewed as a form of retroactive interference; that is, the learning of new information during an interpolated task (e.g., questionnaire or post-event narrative) can interfere with our ability to recall original target items. In other words, the standard misinformation paradigm fosters retroactive interference and that this interference is the cause of the misinformation effect. In a series of carefully constructed studies, Chandler demonstrated that the retrieval availability of the original item increases as retroactive interference dissipates, thereby suggesting that memory for the original item has neither been erased nor altered. Consistent with this interference interpretation, Chandler and Gargano (1995, 1998) have also demonstrated that 7 misinformation dissimilar to the original target item creates less interference and a consequent reduction in the level of misinformation reported at test. As a way of minimising such interference effects, Chandler et al (2001) emphasised the importance of reinstating retrieval cues that specifically access information contained in memory for the original event. By encouraging participants to reinstate contextual cues that are unique to the original event, memory for the original item can be accessed in preference to memory for the misinformation. Consistent with this view, Bekerian and Bowers (1983) have argued that the misinformation effect may be a function of a mismatch between encoding and retrieval cues. Specifically, thematic cues associated with the temporal order of the original presentation may provide vital cues for the subsequent retrieval of information about the target incident. They pointed out that, in the standard misinformation paradigm, the slides in the final test phase are presented in random order thereby eliminating thematic cues. Bekerian and Bowers demonstrated that when retrieval conditions at test are sufficiently strong to reinstate the retrieval conditions present at encoding (i.e., when the original temporal order is reinstated), the original item tends to be chosen at test. Conversely, when retrieval conditions at test fail to match conditions at encoding (i.e., random order of presentation at final test), participants are more likely to rely upon their most recent memory for the target event and consequently choose the misinformation item. Here then is empirical evidence to suggest that, given the provision of appropriate retrieval cues, it is possible to access memory for the original item; that is, the memorial representation for the original item still exists and appears not to have been subject to destructive updating (see also Eakins, Schreiber & Marshall, 2003; Lindsay & Johnson, 1989b) Amongst other significant challenges to Loftus’ destructive updating account, Lindsay and Johnson (1989a) demonstrated that, where participants were encouraged to attend to information concerning memory source, it was possible to eliminate misinformation effects. Although this can be interpreted as another co-existence account of the misinformation effect, it 8 differs in that it does not propose any difficulty in retrieving the original item. Rather, the source monitoring explanation suggests that misled participants believe that the misinformation had actually occurred in the original target event. Such misattributions are likely to occur because the original and misinformation items both concern the same event. As the two sources for these items are highly similar, the standard misinformation paradigm arguably fosters ideal conditions for source misattribution errors to occur. Where similarity of source is reduced (by increasing the temporal distinctiveness of the material), a concomitant reduction in misinformation effects is observed (Lindsay, 1990). Lindsay and Johnson (1989b) also showed that it was possible to produce misinformation effects under conditions in which the misinformation was presented before the target event. They argued that the demonstration of this ‘reversed suggestibility effect’ is difficult to reconcile with a destructive updating account in which original material is updated by later (i.e., newer) material. Their work also suggested that the two-alternative forced-choice recognition test employed in the standard misinformation paradigm may actually encourage participants to base their judgements on feelings of familiarity. For critical slides, misled participants may choose the misinformation slide because it seems more familiar to them and mistakenly believe that the item had occurred in the original presentation. Related work by Zaragoza and colleagues (Zaragoza & Lane, 1994; Zaragoza & Koshmider, 1989), however, has indicated that misled participants can choose the misinformation item at test despite being aware that they could not remember seeing it in the original event. Thus, it would seem that participants can be aware of the source of memories but still favour the misinformation item, thereby indicating that source confusion cannot always provide an adequate account of the misinformation effect. Some researchers have taken the view that the standard misinformation paradigm (cf. Loftus, et al., 1978) tells us little about memory updating because of the problems of demand 9 characteristics (i.e., implicit biases within the paradigm itself). McCloskey and Zaragoza (1985) demonstrated that where the two-alternative forced choice recognition test comprised a choice between an original item (e.g., a spanner) and a completely novel item (e.g., a screwdriver) rather than the standard procedure where the choice would be between an original item (e.g., a spanner) and a misinformation item (e.g., a hammer), memory performance for misled participants was little different from that of non-misled controls. The rationale behind McCloskey and Zaragoza’s modified test is that, if the introduction of misinformation results in the destructive updating of memory for original material (cf. Loftus, et al., 1978), misled participants could be expected to choose the original item less often than would participants who had not been misled. The fact that there was no difference between misled participants and controls led McCloskey and Zaragoza to surmise that Loftus’ misinformation paradigm provides no conclusive evidence that the introduction of misinformation modifies memory for original material. See also Zaragoza, McCloskey & Jamis (1987) and Weinberg, Wadsworth & Baron (1983) for further discussion of demand characteristics and how they affect the production of misinformation effects. There can be little doubt that social factors have the potential to influence report. The effect of others on social judgements is well-documented in the social literature (e.g., Asch, 1951). Great care, therefore, needs to be taken to minimise the effects of demand characteristics in misinformation studies. In our view, however, it is unlikely that misinformation effects can be attributed entirely to demand characteristics. On the one hand, we have a number of compelling studies that indicate the co-existence of memories for both the original and the misinformation item. In other words, one’s ability to access original material from memory is largely seen as a function of employing the most appropriate retrieval cues. On the other hand, we have equally compelling reaction time data which indicate that some form of integration is likely to have 10 taken place shortly after the introduction of the misinformation item (Loftus, Donders, et al., 1989). Of course, this body of research may simply suggest that the misinformation effect can be multiply determined - a possibility that has already been acknowledged by Loftus (e.g., Loftus & Hoffman, 1989; Loftus, Korf & Schooler, 1989). In our view, however, it is also likely that we have revealed only part of the picture. Traditionally, memory has been characterised by passive processes such as interference and decay – the same processes that have been implicated in the production of the misinformation effect. We are currently engaged in a programme of research, however, which indicates that more active forgetting mechanisms may provide a credible alternative explanation as to why memorial representations for original material appear lost under certain conditions but available to conscious inspection under others. In the remainder of this article, therefore, we detail an active forgetting mechanism that we believe may contribute to the production of misinformation effects and help to explain this complex pattern of findings. Retrieval-Induced Forgetting Our current programme of research has led us to consider the possible role of retrieval-induced forgetting in the production of misinformation effects. Retrieval-induced forgetting (Anderson, Bjork & Bjork, 1994; Anderson & Spellman, 1995) refers to a form of forgetting that occurs as a consequence of remembering other related material. The basic rationale for this phenomenon stems from the observation that the cues we typically employ during memory retrieval are insufficiently specified. This means that there is a tendency to access not only the material we wish to remember but also unwanted related material that, in turn, provides unwelcome competition for recall. The problem for memory, therefore, is how to deal with unwanted competition from related memories during retrieval. Anderson and colleagues have suggested that memory accomplishes this feat via a form of suppression or active inhibition that renders unwanted competitors unavailable to conscious inspection. In much 11 the same way that inhibition is thought to be responsible for resolving competition at other levels of information processing, inhibition may be responsible for resolving competition during memory retrieval (see Anderson & Bjork, 1994; Anderson & Neely, 1996). Retrieval-induced forgetting is typically demonstrated using the retrieval practice paradigm (Anderson et al., 1994) which comprises four phases: study, retrieval practice, distractor, and final test. In the study phase, participants are presented with a series of categoryexemplar pairs and instructed to memorize the items (e.g., fish-bream, fish-trout, fish-flounder, fish-salmon, flower-tulip, flower-anemone, flower-rose, flower-daffodil). Participants are then instructed to perform retrieval practice on half the studied exemplars from half the categories using cued stem tests (e.g., fish-br____, fish-tr____). Each of these cued exemplars is typically presented three times. Following a distractor task, participants are then cued with each category name (e.g., fish, flower) and asked to recall all the exemplars originally presented. In these kinds of retrieval practice studies, recall performance for three types of exemplars is assessed: Rp+ items (i.e., practiced exemplars from practiced categories, bream); Rp- items (i.e., unpracticed items from practiced categories, salmon); and Nrp items (i.e., unpracticed items from unpracticed categories, tulip). The basic idea here is that if remembering some items from a category inhibits memory for other related material, then we should see poorer recall performance for unpracticed items from the same categories as practiced items than for unpracticed items from previously unpracticed categories. In other words, despite Rp- items and Nrp items being treated in the same way (i.e., both are unpracticed), Rp- items should be recalled more poorly than Nrp items by virtue of sharing the same retrieval cue as practiced items. See Figure 1 for an example of retrieval-induced forgetting (Macrae & MacLeod, 1999, Study 1). Importantly, the mere presentation of items without retrieval practice is insufficient for retrieval-induced forgetting to emerge (Anderson, Bjork & Bjork, 2000). Rather, the act of retrieval is a necessary prerequisite to set up the inhibition of related unpracticed items. 12 ______________________________________________ Figure 1 about here ______________________________________________ Applicability to the real world On reviewing the retrieval-induced forgetting literature, however, it would be easy to gain the impression that these effects might be limited to the rather esoteric world of learning category-exemplar pairs. With this in mind, Macrae and MacLeod (1999) set out to explore whether retrieval-induced forgetting also applied to the social world. They argued that it is one thing to forget previously encountered items of fruit or drink but that it may be an entirely different matter to inhibit the retrieval of information encountered in meaningful social contexts. Also, given that participants in retrieval-induced forgetting studies are typically instructed to remember target material prior to retrieval practice, the encoding conditions could be critical for inhibition to occur; that is, the initial instruction to remember the presented material could prove central for inhibition to take place. This point is of particular relevance to our understanding of eyewitnessing given that the inherent ambiguity of many criminal episodes means that we are seldom aware that what has been witnessed is of any importance until after the event. Using a set of traits describing two hypothetical individuals (Bill and John), Macrae and MacLeod showed that selective retrieval practice on traits about one of the individuals (i.e, the target) resulted in poorer recall performance for unpracticed traits about that target person relative to unpracticed traits about the non-target person, despite the fact that no explicit instructions had been given to remember the presented material. They also showed that retrievalinduced forgetting can be elicited even when participants are motivated to remember all the presented material (e.g., under mock examination conditions). Macrae and MacLeod presented 13 participants with facts about two fictitious islands, Tok and Bilu (e.g., the main cash crop on Bilu is maize). Despite the fact that the participants were all university students and well-accustomed to studying and sitting examinations, participants still showed the typical retrieval-induced forgetting effect. Thus, it would appear that even when people are well-motivated – as honest eyewitnesses are likely to be – retrieval-induced forgetting still emerges. Finally, Macrae and MacLeod demonstrated that the amount of retrieval practice had little effect on the magnitude of the retrieval-induced forgetting effect. From an applied point of view, this is a particularly important point given that its relevance to understanding forgetting in the real world would have been severely limited if large amounts of retrieval practice had been deemed necessary for the effect to emerge. Given the apparent robustness and ease with which retrieval-induced forgetting can be produced, it is easy to appreciate its possible impact on everyday life. Arguably, nowhere is this more apparent than in the field of eyewitnessing where naturally occurring conditions (e.g., repeated questioning by police, lawyers, family members and colleagues about a witnessed event) may give rise to exactly those conditions that promote the inhibition of related memories. In order to explore this possibility, Shaw, Bjork and Handal (1995) examined memory for visually presented stimuli under conditions analogous to those experienced by eyewitnesses. Specifically, participants were told to imagine that they had attended a party and that, on leaving the party, they noticed that their wallet was missing. Participants were instructed to watch a series of slides of a student’s flat and to pay close attention to the details contained therein in order to assist police with their enquiries. The slides contained a number of household items plus two categories of target items (i.e., college sweatshirts and college schoolbooks). The retrieval practice phase of the study comprised questions about the target items which, in turn, produced evidence of retrieval-induced forgetting. In other words, guided retrieval practice on one of the classes of items (e.g., college sweatshirts) resulted in a significant decrease in recall performance 14 for unpracticed items from the same category relative to recall performance for unpracticed items from the unpracticed category (e.g., college schoolbooks). The relevance of this form of active forgetting to the real world was further explored in a recent set of studies by MacLeod (2002). In the first study, participants were asked to imagine that they were police officers attending the scene of a crime and to pay close attention to a series of slides depicting a number of items ostensibly stolen from two houses (House A and House B). Guided retrieval practice resulted in retrieval-induced forgetting. Thus, it would appear that for retrieval-induced forgetting to occur, well-established semantic links are not required as in the case of exemplars of fruit, plants, or animals. Rather, it would appear that retrieval-induced forgetting can occur for recently learned semantic information. This study also provided clear evidence that the observed effects could not be accounted for by output interference; that is, the tendency for items recalled first to interfere with the recall of subsequent items (see also MacLeod & Macrae, 2001; Macrae & MacLeod, 1999). The second study by MacLeod (2002) explored the relevance of retrieval-induced forgetting to eyewitnessing by considering the effects of guided retrieval practice on memory for details about a fraudulent event (i.e., a bogus charity collection). In this study, however, participants were neither provided with information that drew attention to particular items for encoding nor any explicit category information with which to organise the witnessed material. In doing so, this study set out to mimic the kind of visual information typically available to real-life witnesses. If inhibitory mechanisms in memory have importance beyond the confines of the laboratory, then it should be possible to demonstrate retrieval-induced forgetting for information that can be mapped on to either pre-existing schemata or where the method of organising the material is implicit within the witnessed episode. Pilot work demonstrated that participants who viewed the bogus charity incident organised their recollection via the two women portrayed in the sequence of slides (i.e., a blonde 15 and a brunette). This pilot work also offered the opportunity to identify the most common descriptive items recalled about each woman. These, in turn, formed the subject of the guided retrieval practice procedure. Despite the absence of any explicit organising principal around which the material could be organised, participants clearly organised material around the most salient perceptual dimensions (i.e., the two women). Guided retrieval practice again resulted in retrieval-induced forgetting effects. Thus, this study indicates that retrieval-induced forgetting is possible for recently learned semantic information even in the absence of explicit organising or grouping information. It would appear from these studies that, at the very least, there is a need to be as comprehensive as possible in the questioning of witnesses because of the risk that selective retrieval can result in the forgetting of other items that may ultimately prove important to the successful investigation or prosecution of a case (see Shaw et al., 1995). MacLeod has also raised the possibility that the kind of forgetting produced by guided retrieval practice may also be implicated in the production of misinformation effects. Retrieval-induced forgetting and the misinformation effect In the standard misinformation paradigm (cf. Loftus et al., 1978), participants are presented with a post-event questionnaire that probes memory for the previously witnessed target event. Inevitably, however, such questions do not constitute an exhaustive retrieval of all information known about that target event but rather focuses on a subset of items. The selective retrieval of information in the standard misinformation paradigm, therefore, may constitute exactly those conditions likely to promote retrieval-induced forgetting for items that were not the subject of the initial enquiry. If we consider this possibility further it is possible that, should misinformation be inadvertently introduced about an inhibited item, subsequent tests of memory for that item might result in a preference for the post-event misinformation given that memory for the original 16 material may no longer be available for retrieval. Thus, if we return to our earlier example from Loftus et al (1978): ‘Did another car pass the red Datsun while it was stopped at the stop sign?’, the request for information concerns a second possible car. In addressing this question, there is no requirement to retrieve information about the type of sign at which the Datsun was stopped. Assuming that no other question focuses on memory for the type of sign, memory for a ‘Yield’ sign may subsequently become inhibited to the extent that only post-event information (i.e., that it was a ‘Stop’ sign) remains available for retrieval. In the real world, insufficient time or resources may mean that extensive police interviews may not be carried out. Incomplete retrieval of the material known about a witnessed event may, in turn, produce retrieval-induced forgetting for those items of information that had not been the subject of retrieval. Our thinking goes one stage further; that is, the incomplete retrieval of information about an event may also produce exactly those conditions that promote the incorporation of misinformation into memory by honest eyewitnesses. In other words, where misinformation has inadvertently been introduced about an item that has been the subject of retrieval-induced forgetting, only the misinformation may remain available to conscious inspection. In order to explore this possibility, we embarked on a set of studies that examined the relationship between retrieval-induced forgetting and the production of misinformation effects (see Saunders & MacLeod, 2002). Using a variant of the basic retrieval practice paradigm, we set out to investigate whether misinformation effects would be apparent where inconsistent postevent information had been introduced about Rp- items (i.e., items that had been inhibited as a result of selected retrieval practice of other related items). If our reasoning is correct, we could also expect that where misinformation is introduced about non-inhibited items (i.e., Rp+ or Nrp items) significant misinformation effects would fail to emerge. 17 Study 1 Study Phase: Participants were presented with items ostensibly stolen from two houses (i.e., Jones’ and the Smith’s). Information was presented in the form of two narratives about two separate burglaries. Order of presentation of the two narratives was counterbalanced throughout. Information about the burglaries was contained within a booklet that also contained a number of distractor tasks in addition to the retrieval practice questions. Participants were prompted through each stage of the booklet by the experimenter. The first part of each narrative contained scenesetting information about when and where the burglary had occurred. Ten items were described as having been stolen from the Smith’s house and ten items from the Jones’ house. Each item was embedded within a set of sentences describing where the stolen item had originally been in located within the house. (e.g., ‘The television had been in the sitting room, which is at the front of the house. It was sitting in the corner of the room. The remote control for it hadn’t been taken.’) Earlier pilot work had established that the items chosen for each household were considered believable as potential stolen items in a burglary. On completing the first narrative, participants were instructed to read the second narrative. The stolen items were presented in blocked format (i.e., all items about the Jones’ house followed by the Smith’s house or vice versa) and their presentation fully randomized within each block. The information sets for each house were divided into two subgroups (each containing five items) for the purpose of creating a practiced (i.e., Rp+) and an unpracticed (i.e., Rp-) set of items for each theft. Retrieval Practice: Immediately after the study phase, participants were presented with a series of questions about one of the subsets of stolen items from one of the houses, thereby creating Rp+, Rp-, and Nrp items sets. This series of questions comprised three sets of questions about 18 five of the stolen items (i.e., 15 questions in total). In order to maximise the effects of retrieval practice, we adopted the procedure suggested by Landauer and Bjork (1978) whereby, on each successive set of retrieval practice questions, the inherent difficulty of the question sets increased. Question difficulty had been determined in earlier pilot work. Counterbalancing ensured that each item appeared equally often as Rp+, Rp- and Nrp items. Participants were required to write down all the stolen items they could remember from those presented originally. This served as a manipulation check that retrieval-induced forgetting had occurred in all experimental conditions. We also included a control condition in which participants received retrieval practice for the names of capital cities (e.g., ‘The capital city of Cuba is Ha____’ ). In doing so, we ensured that participants in the control were engaged in the same type of task (i.e., retrieval) but that it was ‘non-relevant’ in the sense that the questions concerned material unrelated to the items presented in the study phase. Each of the three retrieval practice phases was followed by a distractor task. On completion of the retrieval practice phase, participants were presented with a piece of misinformation about an Rp+, Rp-, or Nrp item. Participants in the control condition also received a piece of misinformation about one of the thefts, thereby providing a baseline level of misinformation reported in the absence of relevant retrieval practice. Only one misinformation was incorporated into each set of 12 questions so as not to arouse suspicions about the true nature of the study. Critical items that were the subject of misinformation were counterbalanced throughout for each condition. Final Test The final task comprised a series of multiple-choice questions that probed memory for the stolen items. For non-critical items (i.e., where no misinformation had been presented), participants had to choose between the originally presented items and two new erroneous items. For the critical item, however, participants were required to choose between the original, the 19 misinformation and a new item. On completion, participants were debriefed, thanked for their participation, and dismissed. (See Saunders & MacLeod, 2002 for further details of the materials and procedures employed). Results Retrieval practice rates were 90.6%, 88.8% and 87.7% for the MisRp+ (i.e., where misinformation had been introduced about a practiced item), MisRp- (i.e., where misinformation had been introduced about a non-practiced item from the practiced set), and MisNrp conditions (i.e., where misinformation had been introduced about a non-practiced item from the unpracticed set), respectively. In order to establish an adequate test of our hypothesis that retrieval-induced forgetting facilitates the production of misinformation effects, we first confirmed that retrievalinduced forgetting had occurred in all the experimental conditions (see Figure 2). Indeed, across all treatment conditions, mean recall performance for unpracticed items from the unpracticed set was .43 whereas for unpracticed items from the practiced set mean recall performance was only .26. The magnitude of the difference between Rp- and Nrp items (i.e., the retrieval-induced forgetting effect) was also found to be highly comparable to other studies of retrieval-induced forgetting (Anderson et al., 1994; Anderson & Spellman, 1995; MacLeod, 2002; MacLeod & Macrae, 2001; Macrae & MacLeod, 1999). __________________________________________ Figure 2 about here __________________________________________ Additionally, we confirmed that Nrp recall performance in each of the treatment conditions (overall M = 0.43) was significantly lower than that achieved in the control condition 20 (M = 0.56). Thus, we can be confident that the observed differences between Rp- and Nrp items in the treatment conditions was not due to an inflation of the Nrp baseline performance but rather to a real drop in Rp- recall performance. Having demonstrated the presence of retrieval-induced forgetting in all three treatment conditions, we then turned to consider the critical issue of whether the misinformation effect is dependent upon retrieval-induced forgetting. The principal comparison of interest here is between the level of misinformation reported in the MisRp- condition where misinformation was introduced about an item that was subject to retrieval-induced forgetting versus the levels of misinformation reported in the MisRp+ and MisNrp conditions where misinformation was introduced about items that had not been subject to retrieval-induced forgetting. We found that in the MisRp- condition, 60% of participants chose the misinformation in comparison to 16% in the MisRp+ and 20% in the MisNrp conditions (see Figure 3). Chi-square analysis confirmed that there was no significant difference in the proportion of participants reporting misinformation in either the MisRp+ or MisNrp conditions compared with controls where no relevant retrieval practice had taken place. _________________________________________ Figure 3 about here _________________________________________ We also noted that there was no significant difference in the final test performance across conditions for non-critical items (85%, 89%, 86% and 91% correct for the MisRp+, MisRp-, MisNrp, and control conditions, respectively). Thus, it would seem unlikely that the misinformation effect produced in the MisRp- conditions was due to poorer overall recall performance at time of test. 21 In summary, this study provides us with a clear illustration that the retrieval practice of a subset of items can create conditions ideal for the promotion of misinformation effects. It would appear that when misinformation is introduced about items that have been subject to retrieval-induced forgetting, participants are significantly more likely to choose the misleading information during final test. In contrast, where misinformation is introduced about an item that has not been subject to retrieval-induced forgetting, the level of misinformation reported is no different from controls where no relevant retrieval practice had occurred. Study 2 In order to test our hypothesis further, we posed an even more stringent test. As we and others have argued that retrieval-induced forgetting is the result of an active inhibitory mechanism (Anderson & Spellman 1995; Anderson & Green, 2001; Saunders & MacLeod, in press), it could be expected that misinformation effects would emerge only where that inhibition remains active. MacLeod and Macrae (2001) had previously demonstrated that, at least for certain kinds of information, the effects of retrieval-induced forgetting are transient. Specifically, they showed that where a 24-hour delay had been inserted between retrieval practice and final recall, the typical pattern of retrieval-induced forgetting disappeared. In contrast, the insertion of a 24-hour delay between original presentation and retrieval practice produced the typical retrieval-induced forgetting effect. MacLeod and Macrae argued that the transient nature of retrieval-induced forgetting reflects the need for flexibility in the selection of the material retrieved from memory in order to meet current or anticipated information processing goals. From one day to the next, we have little way of knowing what tasks we will be required to perform and what information from memory will ultimately prove critical for the completion of these tasks. Thus, MacLeod and Macrae have argued that it would not be particularly adaptive to 22 render some forms of information permanently inaccessible given that this information may be just what is required in order to complete a future task. We used the transient nature of retrieval-induced forgetting to test our hypothesis further that misinformation effects are dependent upon retrieval-induced forgetting. Specifically, we predicted that where misinformation was introduced about a critical item that was still subject to retrieval-induced forgetting, the typical misinformation effect would emerge. In contrast, the introduction of misinformation about an item that had been subject to retrieval-induced forgetting but which was no longer inhibited, should fail to produce a misinformation effect. In order to test our hypothesis, we used the same materials as those employed in our first study. In this particular study, however, misinformation was introduced about Rp- items only (i.e., those items that were subject to retrieval-induced forgetting). Misinformation was introduced either following a free recall procedure in which participants experienced a 24-hour delay between study and retrieval practice or following a 24-hour delay between retrieval practice and free recall. Manipulation checks were consistent with MacLeod and Macrae’s original findings. Importantly, we found that misinformation effects emerged only where retrieval-induced forgetting remained active; that is, where no delay had been inserted between any stages of the retrieval practice paradigm, or where a 24-hr delay was inserted between study and retrieval practice. No significant misinformation effects emerged where a 24-hour delay had been inserted between retrieval practice and recall (see Figure 4). ______________________________________________ Figure 4 about here ______________________________________________ 23 Discussion Our programme of research was designed to explore the extent to which retrievalinduced forgetting could promote the production of misinformation effects. In the present article we have detailed two of our initial studies in this field (Saunders & MacLeod, 2002). Both of these studies provide strong inference that retrieval-induced forgetting is a potent mechanism in facilitating misinformation effects. It is unlikely that the misinformation effects observed in our study can be attributed to demand characteristics (cf. McCloskey & Zaragoza, 1985) as it could have been expected that misinformation effects would have emerged across all conditions given that any demand characteristics would have been equivalent. Instead, we found in Study 1 that misinformation effects emerged only where misinformation had been introduced about items that had been subject to retrieval-induced forgetting. Similarly, it would be difficult to argue that the misinformation effect observed in the MisRp- condition (Study 1) was due to a failure to encode the critical item given the high level of recall performance for non-critical items across all conditions, or that some form of spontaneous forgetting might have occurred given that new erroneous information tended not to be chosen at test. Our interpretation that misinformation effects are facilitated by retrieval-induced forgetting is further supported by our second study in which we demonstrated that misinformation effects emerged only where retrieval-induced forgetting remained active. Also, the observed pattern of retrieval-induced forgetting observed in this study supports MacLeod and Macrae’s (2001) earlier findings that the effects of retrieval-induced forgetting are, at least under certain conditions, transient. One interpretation for this transient aspect of inhibitory processing is that memory needs to be flexible in order to meet the demands of a complex and constantly changing social world. For many kinds of information, it would make little sense to inhibit information on a more permanent basis as unknown future processing goals may require the processing of such information. This feature of inhibitory processing, however, also permitted us 24 to explore further the role of retrieval-induced forgetting in the production of misinformation effects. From our studies it is clear that misinformation effects were produced only where there was evidence that retrieval-induced forgetting remained active. Where it had dissipated, in contrast, misinformation effects failed to emerge. Inevitably, however, our findings raise further questions and issues of theoretical and applied interest. The most obvious of these concerns the nature of the mechanism underlying retrieval-induced forgetting. As we intimated earlier, retrieval-induced forgetting can be caused by both non-inhibitory and inhibitory means. The present studies illustrate a strong relationship between retrieval-induced forgetting and the production of misinformation effects but do not provide conclusive evidence that the underlying mechanism is inhibitory. While there is good reason to think that the kind of retrieval-induced forgetting found using the retrieval practice paradigm is inhibitory and that output interference and source confusion were not significant contributors to the observed effects (see Saunders & MacLeod, 2002), there remains a need to provide more compelling data that the misinformation effects produced using this modified misinformation paradigm are a function of inhibitory control. In order to establish if this is the case, we must first determine the nature of the processes underlying the retrieval-induced forgetting effect found with our materials. To date, the best means of establishing whether retrieval-induced forgetting is caused by inhibition or not is via the independent cue technique (cf. Anderson and Spellman, 1995). This procedure involves the use of novel retrieval cues at final test rather than employing those retrieval cues used during initial study and retrieval practice. Anderson and Spellman’s rationale is that if the drop in recall performance for Rp- items is a consequence of non-inhibitory mechanisms (e.g., blocking) then it could be expected that the use of novel cues at final test would permit the Rp- items to be accessed (i.e., they should become retrievable). In other words, the typical retrieval-induced forgetting should fail to emerge if non-inhibitory mechanisms are responsible for its production. 25 Anderson and Spellman and others (Anderson & Green, 2001; Saunders & MacLeod, in press), have demonstrated that Rp- items fail to emerge at final test despite the use of such novel retrieval cues, thereby providing strong inference that the Rp- items themselves had been inhibited. See Figure 5 for an example of independent cueing. _______________________________________________ Figure 5 about here _______________________________________________ Of course, if the active inhibition of the memorial trace is a necessary prerequisite for the production of misinformation effects, then the Rp- item is not the only item that may potentially be vulnerable to misleading post-event information. Anderson and Spellman (1995) previously demonstrated that items from an unrelated and unpracticed category can be inhibited due to their relationship with items from the practiced category. Specifically, items from the unpracticed category that share a retrieval cue with Rp- items (i.e., Nrp-similar items) are themselves vulnerable to inhibition. While these Nrp-similar items may not directly compete with Rp+ items for retrieval they are, nevertheless, related to items that do compete (i.e., Rpitems) and, thus, Nrp-similar items may suffer the same fate as that of Rp- items. Conversely, items from the unpracticed category that do not share a retrieval cue with any of the exemplars from the practiced category (i.e., Nrp-dissimilar items) are not subject to inhibitory control, presumably because such items neither compete for retrieval with Rp+ items, nor are they related to items inhibited due to retrieval competition (i.e., Rp- items). Extending this line of research, Saunders and MacLeod (in press) have recently determined (using the independent cue technique, cf. Anderson & Spellman, 1995) that items 26 from the unpracticed category that share retrieval cues with items from the practiced category are susceptible to inhibitory effects. Consistent with Anderson and Spellman, we found that Nrp items that share retrieval cues with Rp- items were vulnerable to inhibitory effects but, additionally, we found that Nrp items that share retrieval cues with Rp+ items were also inhibited. Recall performance for items from the unpracticed category that were semantically dissimilar to items from the practiced category were not subject to inhibitory control and were recalled at a level similar to that observed for the between-subjects baseline control. Having determined which items are most likely to be inhibited, our next step is to evaluate the inhibitory account of misinformation effects by adapting our misinformation paradigm for use with the independent cue method. To address this goal, we need to add the necessary misinformation and recognition phases to the retrieval practice paradigm that we employed in Saunders and MacLeod (in press). If we are correct in our assertion that the inhibition of the original item facilitates the introduction of post-event misleading information, then all the items we determine to be inhibited should also be susceptible to misleading postevent information. Specifically, misinformation effects should occur in those conditions where misleading information is introduced about Rp- items, and also Nrp items that are semantically related to either Rp+ or Rp- items. Conversely, misinformation effects should not arise in conditions where misleading information is introduced about non-inhibited items (i.e., Nrpdissimilar items). While our research provides a novel theoretical account of how misinformation effects are produced it also has the potential to inform us about possible solutions to practical memory problems. Specifically, experimental research that examines the influence of inhibitory retrieval processes, and their role in misinformation effects, may help to explain some of the memory distortions that typify honest eyewitness errors memory and even false memory syndrome, as well as providing insight into the design of more effective interview techniques. As 27 argued elsewhere (e.g., MacLeod, 2002; Saunders & MacLeod, 2002), any interview technique that requires the selective retrieval of information may prove vulnerable to inhibitory control, and the production of misinformation effects. A practical concern, therefore, is that the method employed to solicit information from witness memory may also prove to be the very procedure that leaves witnesses vulnerable to forgetting previously unsolicited details and the subsequent report of misleading post-event information. Our concern with the combined effects of inhibition and misleading post-event information seems particularly timely given the proliferation of alternative investigative methods. Over the past few decades, interview methods such as the cognitive interview, guided memory interview and structured interview, have been championed by basic and applied researchers alike for their ability to enhance retrieval performance in comparison to the standard police interview. While many studies suggest that the specific techniques employed in such interviews can significantly increase the recall of details about a target event (e.g., Fisher, Geiselman & Raymond, 1987; Koehnken, Thurer & Zorberbier, 1994), and the recognition accuracy of suspects (e.g., Malpass & Devine, 1981), all these techniques tend to result in only a sub-set of details about the witnessed event being solicited. Additionally, if questioning results in the unintentional inhibition of memory traces that have not been subject to retrieval practice, then such interview methods may be particularly ill-equipped for dealing with kind of problem outlined in this article. For example, the retrieval methods employed by the cognitive interview assume that the inability of a witness to remember certain details is due largely to cue-dependent forgetting. Specifically, it is assumed that the failure to retrieve specific information is due to the use of inappropriate retrieval cues or interference having occurred along the retrieval route between the cue and the memory trace. Such cue-dependent forgetting can be overcome simply through employing more appropriate retrieval cues that utilise retrieval routes free from such interference. Unfortunately, if details of an event have been forgotten due to inhibition of the 28 memory trace then such retrieval methods will not prove effective. Thus, irrespective of the quality or the quantity of the retrieval cues and retrieval routes provided, inhibited memories may remain stubbornly unavailable to retrieval. In sum, alternative questioning methods to the standard police interview may be just as susceptible to inhibitory control and, in turn, the unwitting introduction of misinformation. 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Misleading postevent information and recall of the original event: Further evidence against the memory impairment hypothesis. Journal of Experimental Psychology: Learning, Memory & Cognition, 13, 36-44. 35 0.8 0.7 0.6 0.5 Proportion 0.4 recall 0.3 0.2 0.1 0 Rp+ Rp- Nrp Item type Figure 1. Guided retrieval practice and retrieval-induced forgetting. The presence of retrieval-induced forgetting is measured by comparing recall performance for Rp- items with Nrp items (i.e., (Rp-) – Nrp). Rp+ = practiced items from practiced category. Rp- = unpracticed items from practiced category. Nrp = unpracticed items from unpracticed category. From Macrae and MacLeod (1999, Study 1). 36 0.9 0.8 0.7 0.6 Proportion 0.5 recall 0.4 Rp+ 0.3 Rp- 0.2 Nrp 0.1 0 MisRp+ MisRp- MisNrp MisControl Condition Figure 2. Proportion recall of each item type across misinformation conditions. Significant retrieval-induced forgetting effects occurred in each condition where participants engaged in relevant retrieval practice. Rp+ = practiced items from practiced category. Rp- = unpracticed items from practiced category. Nrp = unpracticed items from unpracticed category. From Saunders and MacLeod (2002, Study 1). 37 0.7 0.6 0.5 Proportion of 0.4 misinformation effects 0.3 0.2 0.1 0 MisRp+ MisRp- MisNrp MisControl Condition Figure 3. Proportion of misinformation effects reported. Significantly more misinformation effects were found when misleading post-event information was introduced on an item that had been subject to retrieval-induced forgetting (i.e., MisRp- condition) than where it was presented about items that had not been subject to retrieval-induced forgetting (i.e., MisRp+, MisNrp, and MisControl conditions). MisRp+ = misinformation presented about Rp+ item. MisRp- = misinformation presented about Rp- item. MisNrp = misinformation presented about Nrp item. MisControl = misinformation presented about Control item. From Saunders and MacLeod (2002, Study 1). 38 0.9 0.8 0.7 0.6 Rp+ 0.5 Rp- Proportion recall 0.4 Nrp 0.3 Misinfo Effect 0.2 0.1 0 Delayed Test Delayed Practice No Delay Condition Figure 4. Proportion of participants reporting each item type on the free recall and forced-choice recognition test. When free recall was delayed by 24 hours retrievalinduced forgetting and misinformation effects were absent. When retrieval practice was delayed by 24 hours, or no delay occurred, both retrieval-induced forgetting and misinformation effects were detected. From Saunders and MacLeod (2002, Study 2). 39 Retrieval Practice Cue Independent Cue FRUIT RED fire truck strawberry Rp+ Rp- Figure 5. Cue-independent forgetting. Inhibitory theories of retrieval-induced forgetting assume that the memory trace for the Rp- item (e.g., RED- strawberry) is actively inhibited due to its prior competition with the Rp+ items for retrieval. If the memory trace has been inhibited, the employment of independent retrieval cues (e.g., FRUIT) will be unsuccessful in overcoming inhibition. 40