NEUROIMAGING STUDIES OF FALSE MEMORY: A SELECTIVE REVIEW

Transcription

1 Psychologia, 2012, 55, NEUROIMAGING STUDIES OF FALSE MEMORY: A SELECTIVE REVIEW Nobuhito ABE Harvard University, USA It is widely recognized that human memory is an imperfect process that sometimes causes various kinds of distortions and illusions. Recently, some light has been shed on the brain mechanisms involved in this false memory phenomenon as a result of research into its neural basis embarked on by cognitive neuroscientists. This article reviews neuroimaging studies that have attempted to distinguish between true and false memory retrieval. It also reviews neuroimaging studies that have measured neural activity during encoding and addresses the question of whether the encodingrelated neural activity predicts subsequent memory distortions. Finally, there is a brief discussion from the cognitive neuroscience perspective about whether the memory distortion reflects deficient cognitive processing or is a by-product of adaptive cognitive processing. Key words: false memory, fmri, PET, recognition, recollection. Memory problems are mainly characterized by the failure to retrieve desired information, but sometimes people have memories of events that did not occur. Studies of this false memory phenomenon have a long history in the field of psychology, and it is now broadly accepted that human memory is prone to various kinds of distortions and illusions (Roediger, 1996; Schacter, 1999; Loftus, 2003). Memory distortions, as well as normal forgetting, provide valuable opportunities for researchers to analyze scientifically memory processing, which would be extremely difficult if memory were perfect. The development of neuroimaging techniques during the past two decades positron emission tomography (PET) and functional magnetic resonance imaging (fmri) has enabled us directly to measure brain activity associated with various cognitive functions. Many researchers in cognitive neuroscience have used functional neuroimaging to delineate the neural correlates of memory distortion and have attempted to distinguish true and false memories. The present paper reviews current progress in the study of the neural basis of false memory. This paper consists of three main sections. The first section discusses attempts to use PET and fmri to distinguish retrieval processes between true and false memories. The second section discusses attempts to measure brain activity during encoding processes and to determine whether encoding-related brain activity predicts subsequent memory distortions. The concluding section of this paper briefly discusses from the cognitive neuroscience perspective whether memory distortion reflects deficient cognitive processing or is a by-product of adaptive cognitive processing. The author was supported by JSPS Postdoctoral Fellowships for Research Abroad. Correspondence concerning this article should be addressed to Nobuhito Abe, Department of Psychology, Harvard University, 33 Kirkland Street, Cambridge, MA 02138, USA ( abe@wjh.harvard.edu). 131

2 132 ABE NEUROIMAGING OF FALSE MEMORY RETRIEVAL-BASED STUDIES Classic models of memory conceptualize memory processes as involving three separate stages: encoding, storage, and retrieval. One of the restrictions in the field of memory research is that whether subjects memories are accurate or not is unknown to the experimenters until they examine the data obtained from the retrieval phase. It is therefore not surprising that much attention has been paid to the retrieval mechanisms of false memory. In fact, many of the functional neuroimaging studies have attempted to distinguish between veridical and illusory memories by measuring brain activity at the retrieval phase. Most neuroimaging experiments to distinguish between true and false memory retrieval have been carried out in the context of what has been termed the sensory reactivation hypothesis. This hypothesis assumes that true memories are accompanied by the retrieval of more sensory/perceptual details than false memories (Schacter & Slotnick, 2004; Schacter, Chamberlain, Gaesser, & Gerlach, in press). The hypothesis originated from behavioral studies showing evidence of the retrieval of more sensory/perceptual details during true rather than false memory retrieval (e.g., Schooler, Gerhard, & Loftus, 1986; Johnson, Foley, Suengas, & Raye, 1988; Mather, Henkel, & Johnson, 1997; Norman & Schacter, 1997; Marche, Brainerd, & Reyna, 2010). The sensory reactivation hypothesis also assumes that reactivation of sensory brain areas occurs during true, but not false, memory retrieval. The origins of this line of thinking are apparent in several neurobiologically based models of memory retrieval, which state that episodic retrieval involves the reinstatement of processes that were active at the time of encoding (Alvarez & Squire, 1994; Rolls, 2000; Shastri, 2002; Norman & O Reilly, 2003). According to such models, recollection of an event occurs when a pattern of cortical activity corresponding to the event is reinstated by activation of a stored representation of that pattern through hippocampal activity. These ideas have been integrated into what has been called the cortical reinstatement hypothesis (Rugg, Johnson, Park, & Uncapher, 2008), and several neuroimaging experiments have provided direct evidence supporting this hypothesis (e.g., Nyberg, Habib, McIntosh, & Tulving, 2000; Persson & Nyberg, 2000; Wheeler, Petersen, & Buckner, 2000; Nyberg et al., 2001; Vaidya, Zhao, Desmond, & Gabrieli, 2002; Johnson & Rugg, 2007; Ueno et al., 2007; Ueno et al., 2009). In the pioneering study of neuroimaging of false memory, Schacter et al. (1996) used PET to clarify the neural correlates of true and false recognition and to test the sensory reactivation hypothesis. False recognition is a process whereby people incorrectly claim that they have recently seen or heard a stimulus they have not encountered (Underwood, 1965). False recognition is not accompanied by a subjective feeling that people are responding untruthfully, and therefore researchers need to be able to detect a difference between true and false recognition that is not apparent to the conscious mind. A necessary condition for conducting such investigations, especially in functional neuroimaging experiments, is to obtain enough trials of false recognition to produce a stable brain activation map. To this end, Schacter et al. (1996) used as experimental stimuli the set of word lists

3 NEUROIMAGING OF FALSE MEMORY 133 Fig. 1. Blood flow increases associated with true versus false recognition. Significant differences were found in the vicinity of the left superior temporal gyrus, temporal plane, and supramarginal gyrus. Adapted from Schacter et al. (1996) with permission from Elsevier. developed by Deese (1959) and Roediger and McDermott (1995). In this Deese- Roediger-McDermott (DRM) paradigm, participants hear a number of lists consisting of semantic associates (e.g., moon, light, shine, bright, hot, gleam, etc). During a subsequent test phase, subjects are presented with previously studied true targets (e.g., hot), nonstudied false targets (e.g., sun) that are semantically related to the studied items (i.e., lures for false recognition), and unrelated new targets (e.g., building). The merits for the use of this paradigm are twofold. The first obvious reason is that this paradigm can produce gist-based false recognition (i.e., false recognition where people fail to recollect specific details of an experience and instead remember general information of the gist of what happened) in response to the false targets with high probability (for a review, see Gallo, 2010). The second reason is that this word-list paradigm, which can be used either in an auditory or a visual modality, is particularly suitable for testing the sensory reactivation hypothesis. By auditorily presenting stimuli at encoding and visually presenting stimuli at retrieval (with scanning), it is possible to determine whether the regions responsible for auditory processing are reactivated during true, but not false, recognition. Schacter et al. (1996) found both commonalities and differences in brain activation between true recognition and false recognition. They found that, compared with a common baseline condition, both true and false recognition were associated with increased blood flow in regions implicated in memory processing such as the anterior prefrontal cortex, dorsolateral prefrontal cortex, medial parietal cortex, and medial temporal lobe. They also found that the left temporoparietal cortex, a region previously implicated in auditory processing, showed greater activation during true than false recognition (Fig. 1). They interpreted this finding in the context of the sensory reactivation hypothesis: because subjects had heard true, but not false, targets during the auditory encoding phase, left

4 134 ABE temporoparietal activation for true recognition might be a sensory signature that reflects memory traces for auditory aspects of previously studied words. Although their follow-up study using fmri (Schacter, Buckner, Koutstaal, Dale, & Rosen, 1997) failed to replicate the greater activation in the left temporoparietal area during true than false recognition (for discussion, see Johnson et al., 1997), a subsequent study by Cabeza, Rao, Wagner, Mayer, and Schacter (2001) again supported the sensory reactivation hypothesis. Cabeza et al. conducted an fmri study using the DRM paradigm with study conditions that promoted the encoding of sensory information. First, subjects viewed videotapes in which a male voice spoke half the words and a female voice spoke the other half. Second, subjects were instructed to remember not only the presented words but also which speaker presented them. The logic of this paradigm is that these study conditions would encourage the encoding of sensory information such as speakers voices and faces and therefore lead to increased differences in brain activations between true and false recognition. Cabeza et al. (2001) found that the left parietal cortex was activated during true recognition compared with false recognition. They also found that the parahippocampal gyrus, a region within the medial temporal lobe, showed increased brain activity during true recognition compared with false recognition. They concluded that these activations might reflect the recovery of sensory information during the encoding of word lists that were accompanied by rich contextual, sensory information. A subsequent study carried out by Slotnick and Schacter (2004), who used abstract shapes as stimuli in an old-new recognition memory task, also provided evidence supporting the sensory reactivation hypothesis. During fmri scanning, subjects studied exemplar shapes and later made recognition memory decisions in response to studied shapes (shapes identical to those presented at encoding), nonstudied lures (shapes similar to but not identical to those presented at encoding), and new shapes (shapes not presented at encoding). Slotnick and Schacter (2004) found that true recognition of previously studied shapes, compared with false recognition of nonstudied lures, was accompanied by increased activation of visual processing regions which would reflect a sensory signature of true memory. A study by Kensinger and Schacter (2006), who used a reality monitoring paradigm, provided further evidence for the sensory reactivation hypothesis (for a related study, see Kensinger & Schacter, 2005b). Before scanning, subjects were presented with a series of printed names of objects and formed mental images of the named objects. Half of the names were followed by photographs of the objects. During the scan, participants saw the object names and indicated whether the corresponding photograph had been studied. Kensinger and Schacter (2006) found that activity in the visual processing regions such as the precuneus corresponded with the attribution of an item to a pictorial presentation. Based on previous evidence, they suggested that the precuneus activity might reflect retrieval of perceptual details about the perceived picture. Another strand of evidence comes from the study carried out by Okado and Stark (2003). In their study, subjects were presented with verbal labels of common objects followed either by a picture of the object or instructions to imagine the object. They were then given a lie test in which they were requested to indicate whether they had seen an

5 NEUROIMAGING OF FALSE MEMORY 135 actual picture of the object during the preceding study phase, and were strongly encouraged to lie about having seen a picture even when they had not. In a later memory test with scanning, subjects indicated whether they had actually seen a picture of an object during the study phase. Okado and Stark (2003) found that the occipital cortex and the posterior parahippocampal gyrus showed greater activation for the correct attribution of pictures than for the incorrect attribution of mentally imagined items, suggesting the recovery of sensory information for veridical versus illusory memories. Recently, Stark, Okado, and Loftus (2010) reported an fmri study using a post-event misinformation paradigm. In this paradigm, subjects are presented with erroneous information following the initial encoding of an event, which increases later endorsement of that misinformation on a memory test of the original event. In the study by Stark et al. (2010), subjects were presented with a series of photographs and later listened to an auditory narrative describing them, which included misleading information. They were then given a test of their memory for the photographs during scanning. Neuroimaging data revealed that the pattern of brain activity associated with true memory was similar to that associated with false memory, but the true memories formed by visual information showed greater activation in the visual cortex, while the false memories derived from the auditory narrative showed greater activation in the auditory cortex. These results are again in line with the sensory reactivation hypothesis. My colleagues and I also conducted an fmri study with the DRM paradigm to shed light on the neural correlates of false memory (Abe et al., 2008). The main purpose of our study was to clarify the differences in neural correlates between deception and false memory, but we also compared brain activity during true recognition with that during false recognition. Activity difference between true recognition and false recognition was found in the left temporoparietal regions probably engaged in the encoding of auditorily presented words (Fig. 2). The results were thus highly consistent with those of the aforementioned previous studies and with the sensory reactivation hypothesis. However, Kahn, Davachi, and Wagner (2004) reported findings that do not support the sensory reactivation hypothesis. In their study, subjects were visually presented with a series of adjectives (e.g., dirty, happy) before scanning. Half of the stimuli were encoded via an orienting task requiring mental imagery ( Image task) and the remaining half via an orienting task requiring orthographic-to-phonological transformation ( Read task). That is, during the Image task, subjects formed a mental image of a spatial scene described by the adjective (e.g., for dirty, the subject might imagine a garbage dump). During the Read task, subjects silently pronounced the word backwards (e.g., happy might be pronounced ip-pæ ). In a later memory test with scanning, subjects indicated whether they recognized the word as having been studied and which encoding task was performed with the item when studied (i.e., Old-Image, Old-Read, or New). The results revealed that the bilateral parahippocampal cortex, the region probably engaged in the formation of visual imagery at encoding, was activated during accurate recollection of having engaged in the Image task, whereas the left premotor posterior ventrolateral prefrontal cortex, the region probably engaged in silent reading at encoding, was activated during recollection of having performed the Read task. Importantly, the reactivation effects observed in the

6 136 ABE Fig. 2. Brain regions showing greater activity during true recognition than during false recognition. (a) Left supramarginal gyrus, (b) left superior temporal sulcus/middle temporal gyrus, and (c) left middle temporal gyrus. Note that these regions were also more activated during true recognition than during correct rejection (i.e., new response to unrelated new targets), indicating stronger evidence in support of the sensory reactivation hypothesis. TR, true recognition; CR, correct rejection; FR, false recognition. Adapted from Abe et al. (2008) with permission from Oxford University Press. parahippocampal cortex and premotor ventrolateral prefrontal cortex were also observed during false recognition accompanied by an erroneous Image or Read judgment, respectively. These results indicate that, at least in this experimental paradigm, the reactivation of sensory brain areas does not necessarily distinguish true and false memory retrieval. It should be noted that not all neuroimaging studies of false memory have focused on the sensory reactivation hypothesis. For example, some researchers have focused on the different types of false recognition. In their study, Garoff-Eaton, Slotnick, and Schacter (2006) examined the neural correlates of related false recognition false alarms to items that are similar but not identical to previously seen items and unrelated false recognition false alarms to novel items. True recognition and related false recognition were associated with similar patterns of neural activity, but there was no overlap between true recognition and unrelated false recognition. In another study, Garoff-Eaton, Kensinger, and Schacter (2007) examined the differences in neural correlates between conceptual false recognition false alarms that result from semantic or associative similarities between studied and tested items and perceptual false recognition false alarms that result from physical similarities between studied and tested items. Garoff-Eaton et al.

7 NEUROIMAGING OF FALSE MEMORY 137 (2007) found that several regions in the frontal lobe showed significant activation during conceptual false recognition compared with true recognition, but not during perceptual false recognition compared with true recognition. Taken together, both of the studies by Garoff-Eaton et al. (2006; 2007) indicate that different types of false recognition can rely on dissociable neural substrates. Recent neuroimaging studies of false memory have utilized confidence rating (high or low) or Remember/Know procedure (recollection or familiarity) to delineate further the neural basis of false memory retrieval. For example, in the study by Kim and Cabeza (2007b), subjects read short lists of categorized words, and neural activity was measured while they performed a recognition test with confidence rating. Kim and Cabeza (2007b) found that high-confidence true recognition was associated with increased medial temporal lobe activity, whereas high-confidence false recognition was associated with increased frontoparietal activity. These results indicate that when one focuses exclusively on high-confidence responses, the neural correlates of true and false memory are different. In a more recent study by Dennis, Bowman, and Vandekar (2012), subjects were presented with a series of pictures at encoding and were later asked to make a recognition memory judgment in response to target pictures (pictures identical to those seen at encoding), related lures (novel pictures similar to but not identical to those seen at encoding), and unrelated lures (novel pictures not similar to those seen at encoding). Subjects were told to respond Remember if they could recollect specific details about the picture or their thoughts/feelings during its initial presentation, Know if the picture looked familiar but they could not recollect any specific details of its prior presentation, or New if they believed the picture was not presented during the encoding session. Direct comparisons between true and false recollection (i.e., Remember response to target picture and related lure, respectively) showed greater activity in right hippocampus and early visual cortex for true recollection, whereas no region exhibited greater activity for false recollection. These results indicate that there are distinctions in the neural networks contributing to the two recollection processes. In summary, the evidence reviewed in this section suggests that there is a difference in the neural correlates of true and false memory retrieval (see also Heun et al., 2000; von Zerssen, Mecklinger, Opitz, & von Cramon, 2001; Heun et al., 2004; Umeda et al., 2005; Marchewka et al., 2008). Many neuroimaging studies have reported findings that support the sensory reactivation hypothesis, indicating that the reactivation of sensory brain regions might be regarded as a neural signature that distinguishes true from false memory retrieval. However, some of the previous studies have failed to detect the neural signature of true memory. Further studies are required to answer the question of what kinds of factors result in those inconsistencies. NEUROIMAGING OF FALSE MEMORY ENCODING-BASED STUDIES Brain mechanisms underlying false memory have been examined by measuring brain activity not only at retrieval phase, but also at encoding phase. The purpose of encoding-

8 138 ABE based studies of false memory is to identify the patterns of brain activity that predict later memory distortion. The functional neuroimaging technique, especially event-related fmri, is a powerful tool for addressing this issue, because it allows for the use of the subsequent memory paradigm (for a review, see Kim, 2011). In this paradigm, subjects are first presented with a series of encoding stimuli such as words and pictures during scanning. The encoding stimuli are then sorted into those that are successfully remembered and those that are forgotten, based on participants memory performances on a later memory test. The brain activity that is greater for the stimuli later remembered than for those later forgotten indicates the presence of neural mechanisms supporting successful encoding (e.g., Brewer, Zhao, Desmond, Glover, & Gabrieli, 1998; Wagner et al., 1998). Similarly, by analyzing the neural responses for the stimuli that were later successfully remembered and for those later falsely remembered, researchers can clarify how specific brain areas are involved in the formation of true and false memories. Using the subsequent memory paradigm, Gonsalves et al. (2004) examined the encoding-related brain activity associated with false memory (for a related study, see Kensinger & Schacter, 2005a). During scanning, subjects were presented with a series of printed names of objects and were asked to generate a mental image corresponding to the object. For half of the trials, a photograph of the object was presented 2 seconds later, and for the other half, a blank rectangle was presented 2 seconds later. Gonsalves et al. (2004) examined this encoding-related activity as a function of whether the subjects subsequently showed accurate memory or reality-monitoring errors (i.e., they claimed to have seen an object that they only imagined). The results revealed that the regions responsible for visual imagery, including the precuneus and inferior parietal cortex, were significantly activated when subjects subsequently and wrongly claimed to have seen the corresponding object compared to when a false memory for that object was not subsequently produced. Gonsalves et al. (2004) argued that brain activity associated with visual imagery can lead to falsely remembering something that was only imagined. Aminoff, Schacter, and Bar (2008) also identified the brain regions that cause memory distortion. During scanning, subjects studied a series of object pairs: either two contextually related objects with the same context (e.g., a bulldozer and a construction cone) or two objects that are not associated with a specific context or contextually related to each other (e.g., a camera and a pair of scissors). Subjects were asked to try to link the objects mentally in a context. They were then given a recognition memory test that included previously studied objects, unrelated new objects, and new objects that were contextually related to one of the previously studied context pairs (e.g., a construction helmet). Aminoff et al. (2008) found that encoding-related neural activity in several brain regions linked with contextual processing, including the retrosplenial cortex, medial prefrontal cortex, and lateral parietal cortex, was associated with subsequent false recognition of new items that were contextually related to items presented at encoding. They proposed that this type of false recognition is a result of the coactivation of contextually associated information at the time of encoding. Another line of encoding-based studies has shed light on the neural origins of gistbased memory distortions. For example, Garoff, Slotnick, & Schacter (2005) examined

9 NEUROIMAGING OF FALSE MEMORY 139 the neural basis of memory for specific details of a prior encounter with the object and more general memory for the type of object previously encountered. During event-related fmri, participants were presented with a series of common objects and made size judgments about them. In a subsequent recognition memory test, they viewed the same objects (objects identical to those seen at encoding), similar objects (novel objects similar to but not identical to those seen at encoding), and new objects (novel pictures not similar to those seen at encoding), labeling each one as same, similar, or new. Specific recognition was indicated by a same response to a same object, whereas general and non-specific recognition was indicated by either a same response to a similar object (false memory) or a similar response to a same object (partial memory). Garoff et al. (2005) reported that decreased activity in the right fusiform gyrus predicts general and non-specific recognition memory, indicating the involvement of this region in gist-based false recognition. Using the modified version of the DRM paradigm, Kim and Cabeza (2007a) also examined the encoding-related activity of gist-based memory distortion. During scanning, subjects read short lists of categorized words. In a later retrieval phase, they made an old/new recognition memory judgment with confidence rating (for a retrievalbased study, see Kim & Cabeza, 2007b). Encoding-related brain activity was analyzed as a function of whether it predicted subsequent true recognition of target words or subsequent false recognition of critical lures. Kim and Cabeza (2007a) reported that increased activity in the medial temporal lobe and early visual cortex is associated with true, but not false, memory formation, suggesting that false memory is due, at least in part, to processes that take place at the time of encoding. Functional neuroimaging studies using the post-misinformation paradigm have also shown differential activity between true and false memory formation. For example, in the study by Okado and Stark (2005), subjects were presented with photographs depicting an original event. They were then presented with photographs of what was ostensibly the same event but in fact included some modified photographs in a misinformation phase. Okado and Stark (2005) reported that during the original event phase, activity in the left hippocampus tail and perirhinal cortex was greater for true than for false memories, whereas during the misinformation phase, activity in the left hippocampus tail was greater for false than for true memories. They also showed that activity in the right hippocampus head and body, left hippocampus body, and left parahippocampal cortex was greater for true than for false memories during the misinformation phase. These results indicate that how the encoding processes occur in the medial temporal lobe memory system is an important determinant for true and false memory outcome in the misinformation paradigm. In another study, Baym and Gonsalves (2010) used a modified version of the misinformation paradigm of Okado and Stark (2005) to examine how the presence of verbal misinformation and visual imagery may affect the formation of false memories. Subjects first viewed photographs depicting an original event. They were then presented with written descriptions of the original photographs, some of which contained misinformation. Baym and Gonsalves (2010) found that, in the regions responsible for

10 140 ABE visual processing, greater activity predicted subsequent true memories as opposed to subsequent false memories, indicating that differences in encoding may contribute to later susceptibility to misinformation. However, in the right frontal cortex and bilateral medial temporal lobe, activity was greater for both true and false memories than for forgetting. Based on these findings, Baym and Gonsalves (2010) argued that the formation of false memories requires that at least some information about the original event be encoded in order for misinformation to affect later memory. In a recent variation of the misinformation paradigm, Edelson, Sharot, Dolan, & Dudai (2011) attempted to clarify the effects of social conformity on the formation of transient and persistent memory distortion. Subjects initially viewed a movie with several other observers and completed a memory test (Test 1) individually three days later. During fmri scanning, which occurred four days after the initial memory test, subjects again answered questions about the movie (Test 2), either paired with no answers or paired with answers that they were led to believe had been given by their co-observers. Critically, these answers included fabrications. The results revealed more errors on questions paired with fabricated answers, suggesting that social conformity influenced Fig. 3. (a) Effects of social conformity manipulation on memory errors. Subjects showed few errors on the initial memory test (Test 1). During scanning (Test 2), subjects showed more errors on trials paired with fabricated answers by co-observers (manipulation) compared to on trials paired with noanswers (no-manipulation). The conformity effects persisted on the final memory test (Test 3), despite warnings that the answers provided on test 2 were randomly generated and not to be trusted. (b) Medial temporal lobe activations during manipulation that predicts long-term socially induced memory errors. Persistent memory errors were associated with activations in the bilateral amygdala, hippocampus, and parahippocampal gyrus compared with no-answer trials or transient error trials in which participants corrected their answers on the final memory test. L, left; R, right; amyg, amygdala; ant, anterior; hipp, hippocampus; PHG, parahippocampal gyrus; Δβ, delta beta. Adapted from Edelson et al. (2011) with permission from The American Association for the Advancement of Science.

11 NEUROIMAGING OF FALSE MEMORY 141 memory accuracy, because subjects had answered the questions correctly on the initial test. Furthermore, the conformity effects persisted on a later memory test (Test 3), despite warnings in debriefing that the answers provided in the scanning session were randomly generated and untrustworthy. The neuroimaging results revealed that persistent memory errors were associated with increased activity in the bilateral medial temporal lobe compared with no-answer trials or trials in which participants corrected their answers in the final memory test. These results suggest that the misinformation, which caused persistent memory distortion, resulted in memory updating with additional encoding processes supported by the medial temporal lobe (Fig. 3). In summary, the evidence reviewed in this section suggests that, although some of the studies have reported overlapping activity associated with true and false memory formation (e.g., Baym and Gonsalves, 2010; see also Kim & Cabeza, 2007a), most studies have consistently shown differences in the neural correlates of true and false memory formation. However, the regions showing significant differences in encoding-related activity between true and false memory are diverse across studies, possibly because these studies vary from one another in their experimental paradigms. Such diversity does not allow us to make firm conclusions about the encoding mechanisms underlying memory distortion. Further studies are needed to determine whether some or all of them can be replicated. CONCLUDING REMARKS This review indicates that the neural correlates of true and false memories are distinguishable. Specifically, many of the retrieval-based studies have provided evidence to support the sensory reactivation hypothesis: the reactivation of sensory brain areas that were engaged in memory formation occurred during true, but not false, memory retrieval. The encoding-based studies have also demonstrated that several brain regions were differentially activated during true and false memory formation, although the precise locations of activated regions vary across the studies. To address this issue, more evidence from hypothesis-driven studies with a unified theoretical framework is required. One of the important debates in this field is about whether the false memory phenomenon reflects deficient cognitive processing or is a by-product of adaptive cognitive processing. In particular, adaptive accounts of false memory have recently been advanced from both cognitive and neuroscience studies (Schacter & Addis, 2007; Newman, 2009; Hardt, Einarsson, & Nader, 2010; Howe & Derbish, 2010). In a recent review by Schacter, Guerin, & St Jacques (2011), it was proposed that several types of memory distortion do reflect adaptive cognitive processes that contribute to the efficient functioning of memory. Schacter et al. argued that the flexible and constructive nature of memory processing underlies a broad range of adaptive processing, such as simulation of future events, semantic and contextual encoding, creativity, and memory updating, but that it also causes miscombining of elements of memory and susceptibility to external information.

12 142 ABE Although it is tempting to rely on the adaptive account, the evidence from neuropsychological studies still supports the idea of deficient cognitive processing. For example, it has been shown that patients with Alzheimer s disease, compared with healthy controls, show difficulty using recollection (i.e., the mental reinstatement of experienced events during which unique details of memory are recalled) to counteract false recognition (e.g., Budson, Daffner, Desikan, & Schacter, 2000; Gallo, Sullivan, Daffner, Schacter, & Budson, 2004; Abe et al., 2011; Hanaki et al., 2011). These findings lead to the conclusion that the memory distortions observed in the patients are caused by cognitive deficits due to damage to specific brain areas. This in turn leads to the question of whether false recognition observed in healthy individuals is associated with inefficient activation of specific brain regions, but current evidence is limited. More information could be obtained by using a complementary approach involving both neuroimaging and neuropsychology to address this issue. Future neuroimaging studies of false memory also need to explore new avenues of investigation. The sensory reactivation hypothesis is not necessarily useful in this respect, because the absence of reactivation during false recognition does not imply either deficient cognitive processing or adaptive cognitive processing. The reactivation of sensory brain areas during true versus false recognition only implies the neural signature of true memory. Therefore it would be more useful to look at a neural signature of false memory to provide insights into how and why the memory errors occur, although activity in a variety of brain regions has been associated with false memory retrieval in individual studies (Schacter et al., in press). In addition to the improvement of experimental paradigms, more sophisticated analyses, such as multi-voxel pattern analysis (Norman, Polyn, Detre, & Haxby, 2006) might be more informative for future research. Another interesting avenue for future research is to examine the effects of development (e.g., Paz- Alonso, Ghetti, Donohue, Goodman, & Bunge, 2008) or aging (e.g., Dennis, Kim, & Cabeza, 2007, 2008; Duarte, Graham, & Henson, 2010; Giovanello, Kensinger, Wong, & Schacter, 2010) on the neural correlates of memory distortion. Data on maturation or decline in cognitive and neural processing would provide unique insights into how these processes contribute to the positive or negative aspects of the false memory phenomenon. I am hopeful that the use of multiple approaches will enable the functional neuroimaging of false memory to progress further during the coming years. REFERENCES Abe, N., Fujii, T., Nishio, Y., Iizuka, O., Kanno, S., Kikuchi, H., et al False item recognition in patients with Alzheimer s disease. Neuropsychologia, 49, Abe, N., Okuda, J., Suzuki, M., Sasaki, H., Matsuda, T., Mori, E., et al Neural correlates of true memory, false memory, and deception. Cerebral Cortex, 18, Alvarez, P., & Squire, L. R Memory consolidation and the medial temporal lobe: a simple network model. Proceedings of the National Academy of Sciences of the United States of America, 91, Aminoff, E., Schacter, D. L., & Bar, M The cortical underpinnings of context-based memory distortion. Journal of Cognitive Neuroscience, 20,

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