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Being the agent: Memory for action events
Consciousness and Cognition Consciousness and Cognition 12 (2003) 670–683 www.elsevier.com/locate/concog
Being the agent: Memory for action events Elena Daprati,a,b,* Daniele Nico,a,b,c Nicolas Franck,a,d and Angela Sirigua a
d
Institut des Sciences Cognitives, CNRS, 67 Boulevard Pinel, Bron Cedex F-69675, France b Fondazione Santa Lucia, via Ardeatina, 354, Rome I-00179, Italy c Universita La Sapienza, via dei Marsi 78, Rome I-00185, Italy EA 3092 Centre Hospitalier Le Vinatier, 95 Boulevard Pinel, Bron Cedex F-69677, France Received 27 February 2003
Abstract Whoever paid the bill at the restaurant last night, will clearly remember doing it. Independently from the type of action, it is a common experience that being the agent provides a special strength to our memories. Even if it is generally agreed that personal memories (episodic memory) rely on separate neural substrates with respect to general knowledge (semantic memory), little is known on the nature of the link between memory and the sense of agency. In the present paper, we review results from two experiments investigating the effects of agency on both explicit and implicit memory traces. Performance of normal subjects is compared to that of schizophrenic patients in order to explore the role of awareness of action on memory. It is proposed that reliable first-person information is necessary to create a stable and coherent motor memory trace. Ó 2003 Elsevier Inc. All rights reserved.
1. Introduction At the middle of the XVII century, Francßois de La Rochefoucauld smartly described one peculiarity of human memory in a popular maxim ‘‘Why is it that our memory is good enough to retain the least triviality that happens to us, and yet not good enough to recollect how often we have told it to the same person?’’ [Maximes et Reflexions diverses, n. 313]. Today we still lack a comprehensive answer, although experimental psychologists have disclosed many issues concerning early selection processes, encoding strategies and source-memory. The present paper *
Corresponding author. Fax: +39-06-51501482. E-mail address: [email protected] (E. Daprati).
1053-8100/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1053-8100(03)00074-6
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summarises two experimental studies in the attempt to explore the role that awareness may play on memory inconsistencies. Our ability to retain pieces of information can be very peculiar: it is a common experience that we easily forget daily activities that are carried out almost automatically and thus do not benefit from our complete attention or conscious awareness. Even more annoying, our source-memory can be very poor. We are often in doubt when recalling the source of a gossip and may fail to remember the context in which we learnt a relevant piece of information (see Johnson, 1997 for a review). Similarly, we may loose trace of the mechanisms through which an element came to our knowledge: for instance, if we have been engaged in a vivid mental simulation or in the detailed pre-planning of one given act, we might later get confounded in the attempt to decide whether we executed the action or simply imagined to (Anderson, 1984; Ecker & Engelkamp, 1995; Goff & Roediger, 1998). On the contrary, whenever we can relate one piece of information directly to ourselves, we rarely fail to remember it. For example, given a list of common adjectives, we easily remember those that describe our own feelings, but not those requiring a third-person reference or a semantic encoding (‘‘selfreference effect,’’ Rogers, Kuipers, & Kirker, 1977; Symons & Johnson, 1997). Similarly, our memory of past events is influenced by our participation to them—i.e., the level of awareness we have acquired of them. Recollection of how and where we learnt an event, as well as reminiscence of the event itself, seems to be tightly related to our own ability to consciously monitoring it at the time of encoding, thus suggesting a close link between the ‘‘self’’ and our memory traces. A promising approach to test this hypothesis would be to investigate the specific effects of disordered awareness on memory. We will focus here on the motor domain. Several neurological and psychiatric disorders modify relevant aspects of self-consciousness, and affect motor control and awareness of action. One such condition is schizophrenia. It is a well-established notion that patients with schizophrenia experience hallucinations, unwanted intrusions in their thoughts, and delusion of control and of omnipotence, namely false beliefs leading them to either interpret internal phenomena as if they were coming from the external world or to regard external events as their own private experience. The same patient has sometimes been found to develop different symptoms during the course of his illness, the stable feature of the syndrome being a dramatic disruption of the distinction between the self and the external world (Schneider, 1955). If we assume that memory is deeply influenced by awareness, it is legitimate to ask what kind of memory of their own actions do these patients retain. Verbal memory has been widely investigated in schizophrenia, leading to quite inconsistent results: while Bleuler (1911) initially excluded memory defects in the patients, more recent studies reported evidence of severe failures in the memory domain (Aleman, Hijman, de Haan, & Kahn, 1999; Saykin et al., 1991). Such inconsistencies may arise from the fact that schizophrenia seems to affect, quite specifically, the conscious retrieval of encoded stimuli, the patients being often able to produce the relevant information if tested in implicit memory tasks, as is the case for implicit learning or word stemcompletion tasks (Clare, McKenna, Mortimer, & Baddeley, 1993; Danion, Rizzo, & Bruant, 1999; Danion, Meulemans, Kauffmann-Muller, & Vermaat, 2001; Gras-Vincendon et al., 1994; Kazes et al., 1999; Keri et al., 2000). Whether awareness in the encoding phase plays a role in memory deficits in schizophrenia remains an open question, which we will try to address below. One interesting issue that pertains to the memory domain is the capacity to recall the mechanisms through which an element came to our knowledge. This ability likely helps to distinguish those memories that pertain to internally-generated events (i.e., our own thoughts) from memories
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of externally derived actions (i.e., someone elseÕs speech). This peculiar form of source-monitoring process (Johnson, 1997; Johnson, Hashtroudi, & Lindsay, 1993) is of great interest to the domain of schizophrenia, given that ambiguity between internally and externally generated events is thought to play a role in the pathogenesis of typical symptoms, such as hallucinations. It has been proposed (Frith, 1996) that abnormal interactions between speech-generation areas and auditory cortex—namely between pre-frontal cortex and temporal lobes—would induce a faulty monitoring of inner speech production. This disorder would in turn account for a confusion between overt and covert speech, thus leading to the experience of auditory hallucinations. Indeed, in some cases, verbal hallucinations correspond to the content of the patientÕs sub-vocal speech (Bick & Kinsbourne, 1987; Green & Preston, 1981). Similarly, abnormal interactions between the hippocampal formation and dorso-lateral pre-frontal cortex have been recently connected to other manifestations of schizophrenia, particularly to verbal memory impairments (Fletcher, 1998; Heckers et al., 1998; Weiss et al., 2003). In the past few years, a faulty on-line monitoring of selfgenerated actions has been invoked as the core of most productive symptoms of schizophrenia (Frith, 1987, 1992; Frith, Blakemore, & Wolpert, 2000) and weakened awareness has been reported at various levels of control, including that of motor actions. Interestingly, automatic motor control seems to be unimpaired in schizophrenia, as suggested by normal responses to visuomotor adaptation tasks (Fourneret, Franck, Slachevsky, & Jeannerod, 2001; Kopp & Rist, 1994; Pitblado, Shapiro, & Petrides, 1980). On the contrary, conscious action monitoring would be selectively affected. It has been shown (Daprati et al., 1997; Franck et al., 2001) that when direct visual control is prevented and an ambiguous feedback is provided, schizophrenic patients fail to discriminate their own hand movements from similar gestures produced by an actor. Namely, they fail in tasks that likely require an efficient on-line monitoring of motor production as well as the possibility to gain conscious access to it. Here, we suggest that an impoverished or unreliable information on self-generated events may also lead to a defective encoding of the specific context and/or processes through which the information itself had been acquired. A source-memory deficit is thus predictable in schizophrenia, and it is very likely to be closely linked to concomitant hallucinatory behaviour.
2. Source-memory in schizophrenia In a recent study, Franck and co-workers (2000) attempted to assess the ability of schizophrenic patients to recover from memory information on whether a given noun had been previously encoded through covert or overt speech production. Participants were 17 hospitalised patients (14 men and 3 women, mean age 29.8 years) who received a clinical diagnosis of schizophrenia according to the DSM-IV. At the time of testing patients were clinically stable. All had a clinical history of auditory hallucinations and eight among them had recent experience of hallucinations at the moment of testing (see Franck et al., 2000 for details). None presented neuropsychological deficits. Seventeen age-matched controls were recruited among hospital staff (15 men and 2 women, mean age 28.4 5.2). All participants were native French speakers. In the experimental session, subjects were presented with a list of sixteen French concrete highfrequency words, i.e., maison (house), crayon (pencil), and table (table), written on separate rectangular cards (see Fig. 1). Cards were shown to the subjects one following the other, in
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random order. Right before uncovering each card, one experimenter asked the subject to read the word either aloud or silently. In both cases, participants were required to memorise the word. Eight words were always read overtly, eight covertly. Then, subjects were engaged in a visuospatial task for approx. 5 min, after which they received a grid containing 24 words, namely the 16 words previously learnt plus 8 new words. For each word, they were required to decide whether they had read it aloud (Overt Reading) or silently (Covert Reading) or whether the word was absent in the previous list (Absent). As can be seen in Fig. 2, patients performed similarly to controls only when recalling those words they had previously read aloud. On the contrary, for covertly read items, they correctly
Fig. 1. Schematic description of stimuli and procedure of the source-memory experiment by Franck et al. (2000). For each trial, instruction on how to read the word was given to the participant before the corresponding card was uncovered.
Fig. 2. Proportion of items correctly recognised as either covertly or overtly read or as new items by schizophrenic patients (black bars) and age-matched controls (white bars). Whiskers represent standard deviations. Significant between-groups comparisons (Mann–Whitney U test) at p < :05 are shown (derived from Franck et al., 2000).
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retrieved a significantly smaller amount of items compared to controls. Moreover, novelty effect was not effective in determining correct recognition of new items. If distribution of errors is analysed (see Fig. 3, large pie charts), the pattern observed in schizophrenic patients (upper panel) differs from that of controls (lower panel): namely, schizophrenics produced significantly more false recognitions, i.e., a new word classified as previously read (see legend to Fig. 3 for details). Furthermore, as shown by the small charts, a ‘‘source advantage’’ is apparent in controls: namely, words that were overtly read by the participant apparently produced a more robust memory trace compared to covertly read items. Indeed, spoken aloud items were significantly less frequently forgotten and in case of error, the subject reported the word as silently produced rather than as a new item. Finally, new words classified among overtly read items were extremely rare. In other words, control subjects appeared to take advantage from information on their overt articulatory behaviour (i.e., their subjective experience of having produced speech) in order to solve the memory task. This was not the case for schizophrenic patients, who as easily forgot spoken items and eventually claimed to have pronounced words they had never read (see legend to Fig. 3 for details). These results are consistent with previous reports of defective source-memory for various events in schizophrenic patients (Bentall, Baker, & Havers, 1991; Harvey, 1985; Keefe, Arnold, Bayen, & Harvey, 1999; Vinogradov et al., 1997). It has been proposed that this impairment may depend on the failure to bind separate aspects of the event—i.e., the content and the source, into an integrated memory representation (Ôrepresentational binding,Õ Danion et al., 1999; Rizzo, Danion, Van der Linden, Grange, & Rohmer, 1996). In the specific case of action events, Franck and coworkers (2000) suggested that source-memory impairment might further arise from a deficit in consciously monitoring speech production, being it inner speech or overt speech. In other words, in the present case, due to defective awareness, the different sources (overt vs. covert speech) may have become undistinguishable. This possibility finds support in the differential role played by overt speech on memory in control subjects compared to schizophrenic patients, as described above. We could speculate that a defective monitoring of overt speech may have resulted in a reduced specificity of the information associated to the corresponding item, thus eliminating differences between the encoded words. Interestingly, in this experiment, within-patients comparisons demonstrated a significantly larger impairment in the group of patients that had a recent experience of hallucinations at the time of testing (see Franck et al., 2000 for details). Besides forgetting a larger amount of words (even words they had spoken aloud), hallucinating subjects made more frequent source-inversion errors. This specificity is in line with the hypothesis that one possible cause of vulnerability to hallucinations might be indeed an incorrect monitoring of speech production (Frith, Friston, & Herold, 1995). The claim we wish to make here is that subjective experience (related, in the previous example, to overt speech-production) may play an important role not only in the on-line monitoring of selfgenerated events, but also in determining the strength of our memory traces. On the one hand, first-person information specific to self-generated events (i.e., motor commands, somato-sensory feed-back) may be used to anticipate the sensory effects of the action. At the same time, this information is supposed to reduce the response (at the neural level) to those components that pertain to self-movement, thus allowing a first discrimination between signals that had been internally generated and information arising from an external source (Blakemore, Wolpert, & Frith, 2002 for a review). Accordingly, this mechanism would enable us to distinguish between our own
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Fig. 3. Distribution of the different error types produced by schizophrenic patients (upper panel) and age-matched controls (lower panel). Main pie charts: percentage of errors in each of the three possible categories. Small pie charts: proportion of inversions (i.e., covertly read items described as overtly read and vice versa), false recognitions (i.e., new items described as overtly or as covertly read), forgetting (i.e., overtly or covertly read items that were defined as absent). Significant between-groups comparisons were observed for inversions and false recognitions (Mann–Whitney U test, p < :02). In the latter case, proportion of new words classified as overtly read was significantly larger in patients compared to controls (Mann–Whitney U test, p < :006). Within the control group, overtly read items were forgotten less often than covertly read ones (Wilcoxon test: T ¼ 3:00, Z ¼ 2:4973, p < :01) and inversions were significantly more frequent in the covert-overt direction than vice versa (Wilcoxon test: T ¼ 16:5, Z ¼ 2:2599, p < :02). None such significant difference was found in patients (more details in Franck et al., 2000, adapted figure).
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actions and those produced by an external agent (Blakemore et al., 2002; Georgieff & Jeannerod, 1998; Jeannerod, 1999). On the other hand, we suggest that the same specificity may be responsible for the peculiar nature of self-referred memories and thus facilitate recall of the corresponding items. We think that this hypothesis is supported by the above-described research: indeed, patients showing a disruption in self-monitoring benefit only in part from the subjective experience of having pronounced a given word. Although in that condition their recall reaches normal range, quality of the encoded information does not achieve a format sufficient to avoid the large amount of source-inversions and false recognitions that are recorded.
3. Enactment effect and subjective experience At a closer look, data from Franck et al. (2000) might as well be explained by a failure in consciously retrieving from memory a source-information that patients may actually possess. Given that participants were explicitly required to attribute each of the presented items to a given category, one could speculate that voluntary access to the information was prevented, rather than memory of the source itself missing. This conservative interpretation would agree with the reports of severe deficits in accessing stored information in schizophrenic patients (Aleman et al., 1999; Saykin et al., 1991). In a recent study, Daprati and colleagues (submitted manuscript) directly explored this possibility, by analysing the performance of schizophrenic patients in a memory task that did not require an explicit statement of how a given piece of information came to the subjectÕs attention. Experimental psychology provides interesting descriptions of what is called the enactment effect (see Engelkamp, 1998 for a review). This quite common memory effect emerges as a rise in the proportion of correctly remembered items for simple action-phrases (e.g., ‘‘to close the door’’) when encoding is obtained by enactment of the verbal material compared to when the same action-phrases are encoded by an auditory or visual presentation. In other words, subjects retain better semantic information conveyed by a phrase when they are allowed to enact the denoted actions rather than when they simply attend to verbal presentation. One interesting hypothesis suggests that the beneficial effect of enactment might depend on a rich multi-modal encoding linked to the activation and execution of a motor program (Engelkamp & Zimmer, 1994a). This motor specificity has been documented by the reduction or even absence of the effect when the actual motor component is removed, and only imagery of oneself performing the action or observation of the action are allowed (Denis, Engelkamp, & Mohr, 1991; Engelkamp, 1997). Similarly, it has been shown that a selective interference on the recall occurs only when subjects are presented with distracting items whose motor component matches that of the target phrase (Engelkamp & Zimmer, 1994b; Mohr, Engelkamp, & Zimmer, 1989). It has been proposed that the beneficial effect of enactment emerges when ‘‘implicit’’ knowledge is tested, as suggested by the observation that it is not removed by most of the physiological (i.e., ageing, B€ ackman & Nilsson, 1985) and pathological conditions that affect the ability to voluntary retrieve memorised information, such as several forms of amnesia (Karlsson et al., 1989; Mimura et al., 1998). Indeed, in all these cases, although performance may be dramatically impoverished, the enactment effect is still detectable. Accordingly, one could argue that in schizophrenia the existence of a poor conscious retrieval may similarly affect the overall memory span, but should
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nevertheless leave intact the differential effect produced by enactment. This difference would be possibly removed only if one assumes that the poor conscious action monitoring that has been described in schizophrenia (Daprati et al., 1997; Franck et al., 2000, 2001) may affect the construction of a stable memory trace. This latter hypothesis finds support in the results produced in a memory-for-action task by the group of schizophrenic patients described by Daprati et al. (submitted manuscript). The authors administered a list of phrases describing common actions (i.e., ‘‘to wash oneÕs hands’’) to eight patients who received a clinical diagnosis of schizophrenia according to DSM-IV (all men, mean age 38.3 13.4 years, mean educational level 11.3 1.6 years). Patients were clinically stable at the time of testing with prominent negative symptoms (Scale for the Assessment of Negative Symptoms, Andreasen, 1983: mean score 40.4 21.2; Scale for the Assessment of Positive Symptoms, Andreasen, 1984: mean score 26.3 6.5). All received medication and those receiving anti-psychotic drugs reported to interfere with memory functions, were administered the French version of the Grober & BuschkeÕs verbal memory test (Ergis, Van Der Linden, Boller, Degos, & Deweer, 1995) to confirm that they scored within the normal range. None of them showed evidence for a broad cognitive impairment, as suggested by a normal or borderline IQ at either the Wechsler Intelligence Scale or at the Raven Progressive Matrices (PM47). Each participant listened to a manÕs voice reading a list of 30 action-phrases and watched a TVmonitor where the action was pantomimed by a male actor. The task for the patient was to look at the video, carefully listen to the voice, and eventually act out the action according to the colour of a cue that appeared before each target-phrase was presented (see Fig. 4). Fifteen action-phrases were always enacted and 15 were simply observed on the monitor (and listened to). After a 10-min interval, the authors tested the patients recollection ability in a computerised recognition task. Subjects viewed 60 written sentences that corresponded to the previously heard 30 action-phrases
Fig. 4. Schematic description of the procedure followed in the enactment-effect experiment (Daprati et al, submitted manuscript). Each of the 30 action-phrases was preceded by a visual cue (red/green box) indicating whether subjects had to observe or to execute the action. Verbal presentation of the phrase followed the cue by a 800–1000 ms interval and preceded video onset by a 500–800 ms interval.
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plus an equal amount of new items. For each phrase, patients were asked to decide whether it corresponded to an old or a new item. No information on how subjects experienced the phrase in the first place (i.e., whether the phrase had been enacted or observed) was required. The performance of schizophrenic patients, expressed as the proportion of correctly recognised items and the corresponding response times, was compared to that of eight age-matched controls. Inspection of these results (see Fig. 5) discloses a picture that is suggestive of the hypothesis we proposed at the beginning of this section, namely that poor conscious action monitoring in
Fig. 5. (A) Latencies (RTs) required by age-matched controls (grey bars) and schizophrenic patients (black bars) to give their responses to old and new items. Results for old items are split according to study condition (enacted or observed). Significant between-groups (t test for independent samples) and within-groups comparisons (t test for dependent samples) at p < :05 are shown. (B) Proportion of items correctly identified as either old or new by schizophrenic patients and age-matched controls. More captions as in a (adapted from Daprati et al., submitted manuscript).
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schizophrenia may affect the construction of a stable (motor) memory trace and thus may disrupt the enactment effect. Indeed, in control subjects, the latency required to recognise the enacted items as ‘‘old’’ was significantly shorter compared to that required to recognise the observed ones. On the contrary, patients with schizophrenia seemed not to take advantage of motor encoding requiring a comparable amount of time to recognise as ‘‘old’’ both types of items (a two-way ANOVA, with Group as between factor Type of Encoding as within factor, providing a significant interaction F ð1; 14Þ ¼ 6:288, p < :0251, the two groups differing significantly according to type of encoding, namely for enacted items). Similarly, the proportion of correctly recalled items was significantly larger for the enacted items within the control group, but not among patients (see legend to Fig. 5 for details). Although caution should be used given the small dimension of the analysed sample, these results seem to suggest a reduction of the enactment effect in schizophrenia. Several considerations can be drawn from these results. Compared to previous reports, the small group of patients with schizophrenia described by Daprati et al. (submitted manuscript) is not dramatically impaired in this memory task. Following simple verbal encoding, the amount of items that the patients correctly recognised as old ones did not significantly differ from that of normal controls, suggesting an adequate ability to store information in memory. One could argue that recognition tasks require a less important conscious effort by the patient in order to retrieve memorised information, compared for instance to the previously reported source-discrimination task (Franck et al., 2000). Accordingly, the above-described result could be accounted for by the hypothesis that some implicit knowledge of the presented items actually persists. Indeed, a failure in consciously retrieving from memory a source-information that patients may actually possess has been frequently described in schizophrenia (see for example Clare et al., 1993). In the present case however, no direct request to report how each item had been encoded was made to the patients. The enactment effect is supposed to be effective in the absence of any explicit effort by the subjects to either encode or retrieve, to a differential extent, the corresponding item. In other words, it would arise as a consequence of sort of a procedural learning that implicitly favours the enacted items (Perrig, 1988). Thus, although the sample examined is still small, we think that one appealing explanation for the reduction (or even absence) of the enactment effect observed here may arise from some defective conscious action monitoring in the patients. This, in turn, would have weakened the difference between observed and enacted items. In other words, we can speculate that in the absence of a reliable conscious action monitoring, as has been described in schizophrenia (Daprati et al., 1997; Franck et al., 2000, 2001), the memory traces produced by enacted and observed items do not differ anymore. Consequently, the source effect they normally would induce is affected.
4. Conclusions Psychology—as well as everyday life—suggests that we can be deceived by our memory system, although we usually keep an excellent trace of our own actions. Namely, whenever we personally produce an action (and are conscious of it), our memory trace of the event is strong enough to guarantee an adequate recall. This extreme specificity of enactment is far from being a trivial effect. Indeed, in our continuous interaction with the environment, we experience a huge number
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of both internal and external events, given that we as often perform and observe motor actions. Both events are likely to activate internal action representations and it has been argued that very close neural substrates may even be involved in both processes (Gerardin et al., 2000; Rizzolatti, Fadiga, Gallese, & Fogassi, 1996; Rizzolatti, Fogassi, & Gallese, 2001; Stephan et al., 1995). Nevertheless, overt motor production retains a definite specificity, given that, for instance, observation necessarily lacks those internal signals (such as somatosensory reafferences) that characterise enactment. These same internal signals likely help to define the event as self-generated and, in a sense, they qualify the subject as the agent providing him/her with the subjective experience that he/she is the source of the event (Blakemore et al., 2002; Georgieff & Jeannerod, 1998; Jeannerod, 1999). The particular stability of memories derived from self-generated actions suggests that memory traces maintain such a specificity. Recent neuroimaging studies indicate that similar areas are involved in encoding and retrieval of memory information related to enacted actions. In particular, when subjects recall enacted action-phrases, a significantly increased activity is found in motor areas, suggesting that motor information may have become part of the memory trace (Nilsson et al., 2000; Nyberg et al., 2001). The studies we reviewed here suggest a couple of considerations. First, we believe that one important pre-requisite to the beneficial effect derived from enactment is a reliable first-person experience. In other words, what makes a self-generated event special, even in its memory trace, is the subjectÕs direct involvement. Performance of patients with schizophrenia is suggestive of this assumption: an impoverishment of awareness of action can affect memory. For instance, within the patientsÕ group, latencies required to recognise that a sentence had been previously shown did not differ between enacted and observed items. Namely, the facilitation induced from the previous experience of having overtly performed an action was not evident, as if the private signals generated by motor production were either weaker or missing in the patients, thus reducing differences between self-generated and observed events. This result is consistent with recent reports of the pathogenesis of several schizophrenic symptoms, such as verbal hallucinations (Frith et al., 2000). We further speculate that in the absence of a reliable awareness of actions, memories of internally generated movements would loose their peculiar attributes. Thus, becoming undistinguishable from those of external events, these memories would then be more vulnerable to oblivion. Second, we wish to underlie the observation that the memory facilitation induced by the opportunity to relate an event to oneÕs own private experience likely starts at the level of motor production. In other words, motor-related signals may play a relevant role in characterising memory traces. Thus, rather than being an experimental curiosity, the memory facilitation induced by the enactment effect would more concretely obey to a specific ecological goal. At a basic level, motor-information related to previously performed actions may prevent unwanted repetitions of a given act or, by keeping track of previous movements, may help to improve our motor skills. Moreover, it may as well enable us to tell apart memories of internal events (such as our own speech) from those of external experiences (such as other peopleÕs speeches). In more speculative terms, by strengthening the experience of self-generated actions, motor-related signals may contribute to the peculiar stability of personal memories which, interestingly, can be frequently spared following CNS damage (see Kopelman, 2002 for a review on memory disorders). Indeed, as Jackson pointed out, thinking can be regarded as our most complex motor act (Jackson, 1958). Accordingly, we might as well suppose that the construction of an individual autobiographical experience begins with the extreme specificity of his/her own motor experience.
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Being the agent: Memory for action events Elena Daprati,a,b,* Daniele Nico,a,b,c Nicolas Franck,a,d and Angela Sirigua a
d
Institut des Sciences Cognitives, CNRS, 67 Boulevard Pinel, Bron Cedex F-69675, France b Fondazione Santa Lucia, via Ardeatina, 354, Rome I-00179, Italy c Universita La Sapienza, via dei Marsi 78, Rome I-00185, Italy EA 3092 Centre Hospitalier Le Vinatier, 95 Boulevard Pinel, Bron Cedex F-69677, France Received 27 February 2003
Abstract Whoever paid the bill at the restaurant last night, will clearly remember doing it. Independently from the type of action, it is a common experience that being the agent provides a special strength to our memories. Even if it is generally agreed that personal memories (episodic memory) rely on separate neural substrates with respect to general knowledge (semantic memory), little is known on the nature of the link between memory and the sense of agency. In the present paper, we review results from two experiments investigating the effects of agency on both explicit and implicit memory traces. Performance of normal subjects is compared to that of schizophrenic patients in order to explore the role of awareness of action on memory. It is proposed that reliable first-person information is necessary to create a stable and coherent motor memory trace. Ó 2003 Elsevier Inc. All rights reserved.
1. Introduction At the middle of the XVII century, Francßois de La Rochefoucauld smartly described one peculiarity of human memory in a popular maxim ‘‘Why is it that our memory is good enough to retain the least triviality that happens to us, and yet not good enough to recollect how often we have told it to the same person?’’ [Maximes et Reflexions diverses, n. 313]. Today we still lack a comprehensive answer, although experimental psychologists have disclosed many issues concerning early selection processes, encoding strategies and source-memory. The present paper *
Corresponding author. Fax: +39-06-51501482. E-mail address: [email protected] (E. Daprati).
1053-8100/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1053-8100(03)00074-6
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summarises two experimental studies in the attempt to explore the role that awareness may play on memory inconsistencies. Our ability to retain pieces of information can be very peculiar: it is a common experience that we easily forget daily activities that are carried out almost automatically and thus do not benefit from our complete attention or conscious awareness. Even more annoying, our source-memory can be very poor. We are often in doubt when recalling the source of a gossip and may fail to remember the context in which we learnt a relevant piece of information (see Johnson, 1997 for a review). Similarly, we may loose trace of the mechanisms through which an element came to our knowledge: for instance, if we have been engaged in a vivid mental simulation or in the detailed pre-planning of one given act, we might later get confounded in the attempt to decide whether we executed the action or simply imagined to (Anderson, 1984; Ecker & Engelkamp, 1995; Goff & Roediger, 1998). On the contrary, whenever we can relate one piece of information directly to ourselves, we rarely fail to remember it. For example, given a list of common adjectives, we easily remember those that describe our own feelings, but not those requiring a third-person reference or a semantic encoding (‘‘selfreference effect,’’ Rogers, Kuipers, & Kirker, 1977; Symons & Johnson, 1997). Similarly, our memory of past events is influenced by our participation to them—i.e., the level of awareness we have acquired of them. Recollection of how and where we learnt an event, as well as reminiscence of the event itself, seems to be tightly related to our own ability to consciously monitoring it at the time of encoding, thus suggesting a close link between the ‘‘self’’ and our memory traces. A promising approach to test this hypothesis would be to investigate the specific effects of disordered awareness on memory. We will focus here on the motor domain. Several neurological and psychiatric disorders modify relevant aspects of self-consciousness, and affect motor control and awareness of action. One such condition is schizophrenia. It is a well-established notion that patients with schizophrenia experience hallucinations, unwanted intrusions in their thoughts, and delusion of control and of omnipotence, namely false beliefs leading them to either interpret internal phenomena as if they were coming from the external world or to regard external events as their own private experience. The same patient has sometimes been found to develop different symptoms during the course of his illness, the stable feature of the syndrome being a dramatic disruption of the distinction between the self and the external world (Schneider, 1955). If we assume that memory is deeply influenced by awareness, it is legitimate to ask what kind of memory of their own actions do these patients retain. Verbal memory has been widely investigated in schizophrenia, leading to quite inconsistent results: while Bleuler (1911) initially excluded memory defects in the patients, more recent studies reported evidence of severe failures in the memory domain (Aleman, Hijman, de Haan, & Kahn, 1999; Saykin et al., 1991). Such inconsistencies may arise from the fact that schizophrenia seems to affect, quite specifically, the conscious retrieval of encoded stimuli, the patients being often able to produce the relevant information if tested in implicit memory tasks, as is the case for implicit learning or word stemcompletion tasks (Clare, McKenna, Mortimer, & Baddeley, 1993; Danion, Rizzo, & Bruant, 1999; Danion, Meulemans, Kauffmann-Muller, & Vermaat, 2001; Gras-Vincendon et al., 1994; Kazes et al., 1999; Keri et al., 2000). Whether awareness in the encoding phase plays a role in memory deficits in schizophrenia remains an open question, which we will try to address below. One interesting issue that pertains to the memory domain is the capacity to recall the mechanisms through which an element came to our knowledge. This ability likely helps to distinguish those memories that pertain to internally-generated events (i.e., our own thoughts) from memories
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of externally derived actions (i.e., someone elseÕs speech). This peculiar form of source-monitoring process (Johnson, 1997; Johnson, Hashtroudi, & Lindsay, 1993) is of great interest to the domain of schizophrenia, given that ambiguity between internally and externally generated events is thought to play a role in the pathogenesis of typical symptoms, such as hallucinations. It has been proposed (Frith, 1996) that abnormal interactions between speech-generation areas and auditory cortex—namely between pre-frontal cortex and temporal lobes—would induce a faulty monitoring of inner speech production. This disorder would in turn account for a confusion between overt and covert speech, thus leading to the experience of auditory hallucinations. Indeed, in some cases, verbal hallucinations correspond to the content of the patientÕs sub-vocal speech (Bick & Kinsbourne, 1987; Green & Preston, 1981). Similarly, abnormal interactions between the hippocampal formation and dorso-lateral pre-frontal cortex have been recently connected to other manifestations of schizophrenia, particularly to verbal memory impairments (Fletcher, 1998; Heckers et al., 1998; Weiss et al., 2003). In the past few years, a faulty on-line monitoring of selfgenerated actions has been invoked as the core of most productive symptoms of schizophrenia (Frith, 1987, 1992; Frith, Blakemore, & Wolpert, 2000) and weakened awareness has been reported at various levels of control, including that of motor actions. Interestingly, automatic motor control seems to be unimpaired in schizophrenia, as suggested by normal responses to visuomotor adaptation tasks (Fourneret, Franck, Slachevsky, & Jeannerod, 2001; Kopp & Rist, 1994; Pitblado, Shapiro, & Petrides, 1980). On the contrary, conscious action monitoring would be selectively affected. It has been shown (Daprati et al., 1997; Franck et al., 2001) that when direct visual control is prevented and an ambiguous feedback is provided, schizophrenic patients fail to discriminate their own hand movements from similar gestures produced by an actor. Namely, they fail in tasks that likely require an efficient on-line monitoring of motor production as well as the possibility to gain conscious access to it. Here, we suggest that an impoverished or unreliable information on self-generated events may also lead to a defective encoding of the specific context and/or processes through which the information itself had been acquired. A source-memory deficit is thus predictable in schizophrenia, and it is very likely to be closely linked to concomitant hallucinatory behaviour.
2. Source-memory in schizophrenia In a recent study, Franck and co-workers (2000) attempted to assess the ability of schizophrenic patients to recover from memory information on whether a given noun had been previously encoded through covert or overt speech production. Participants were 17 hospitalised patients (14 men and 3 women, mean age 29.8 years) who received a clinical diagnosis of schizophrenia according to the DSM-IV. At the time of testing patients were clinically stable. All had a clinical history of auditory hallucinations and eight among them had recent experience of hallucinations at the moment of testing (see Franck et al., 2000 for details). None presented neuropsychological deficits. Seventeen age-matched controls were recruited among hospital staff (15 men and 2 women, mean age 28.4 5.2). All participants were native French speakers. In the experimental session, subjects were presented with a list of sixteen French concrete highfrequency words, i.e., maison (house), crayon (pencil), and table (table), written on separate rectangular cards (see Fig. 1). Cards were shown to the subjects one following the other, in
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random order. Right before uncovering each card, one experimenter asked the subject to read the word either aloud or silently. In both cases, participants were required to memorise the word. Eight words were always read overtly, eight covertly. Then, subjects were engaged in a visuospatial task for approx. 5 min, after which they received a grid containing 24 words, namely the 16 words previously learnt plus 8 new words. For each word, they were required to decide whether they had read it aloud (Overt Reading) or silently (Covert Reading) or whether the word was absent in the previous list (Absent). As can be seen in Fig. 2, patients performed similarly to controls only when recalling those words they had previously read aloud. On the contrary, for covertly read items, they correctly
Fig. 1. Schematic description of stimuli and procedure of the source-memory experiment by Franck et al. (2000). For each trial, instruction on how to read the word was given to the participant before the corresponding card was uncovered.
Fig. 2. Proportion of items correctly recognised as either covertly or overtly read or as new items by schizophrenic patients (black bars) and age-matched controls (white bars). Whiskers represent standard deviations. Significant between-groups comparisons (Mann–Whitney U test) at p < :05 are shown (derived from Franck et al., 2000).
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retrieved a significantly smaller amount of items compared to controls. Moreover, novelty effect was not effective in determining correct recognition of new items. If distribution of errors is analysed (see Fig. 3, large pie charts), the pattern observed in schizophrenic patients (upper panel) differs from that of controls (lower panel): namely, schizophrenics produced significantly more false recognitions, i.e., a new word classified as previously read (see legend to Fig. 3 for details). Furthermore, as shown by the small charts, a ‘‘source advantage’’ is apparent in controls: namely, words that were overtly read by the participant apparently produced a more robust memory trace compared to covertly read items. Indeed, spoken aloud items were significantly less frequently forgotten and in case of error, the subject reported the word as silently produced rather than as a new item. Finally, new words classified among overtly read items were extremely rare. In other words, control subjects appeared to take advantage from information on their overt articulatory behaviour (i.e., their subjective experience of having produced speech) in order to solve the memory task. This was not the case for schizophrenic patients, who as easily forgot spoken items and eventually claimed to have pronounced words they had never read (see legend to Fig. 3 for details). These results are consistent with previous reports of defective source-memory for various events in schizophrenic patients (Bentall, Baker, & Havers, 1991; Harvey, 1985; Keefe, Arnold, Bayen, & Harvey, 1999; Vinogradov et al., 1997). It has been proposed that this impairment may depend on the failure to bind separate aspects of the event—i.e., the content and the source, into an integrated memory representation (Ôrepresentational binding,Õ Danion et al., 1999; Rizzo, Danion, Van der Linden, Grange, & Rohmer, 1996). In the specific case of action events, Franck and coworkers (2000) suggested that source-memory impairment might further arise from a deficit in consciously monitoring speech production, being it inner speech or overt speech. In other words, in the present case, due to defective awareness, the different sources (overt vs. covert speech) may have become undistinguishable. This possibility finds support in the differential role played by overt speech on memory in control subjects compared to schizophrenic patients, as described above. We could speculate that a defective monitoring of overt speech may have resulted in a reduced specificity of the information associated to the corresponding item, thus eliminating differences between the encoded words. Interestingly, in this experiment, within-patients comparisons demonstrated a significantly larger impairment in the group of patients that had a recent experience of hallucinations at the time of testing (see Franck et al., 2000 for details). Besides forgetting a larger amount of words (even words they had spoken aloud), hallucinating subjects made more frequent source-inversion errors. This specificity is in line with the hypothesis that one possible cause of vulnerability to hallucinations might be indeed an incorrect monitoring of speech production (Frith, Friston, & Herold, 1995). The claim we wish to make here is that subjective experience (related, in the previous example, to overt speech-production) may play an important role not only in the on-line monitoring of selfgenerated events, but also in determining the strength of our memory traces. On the one hand, first-person information specific to self-generated events (i.e., motor commands, somato-sensory feed-back) may be used to anticipate the sensory effects of the action. At the same time, this information is supposed to reduce the response (at the neural level) to those components that pertain to self-movement, thus allowing a first discrimination between signals that had been internally generated and information arising from an external source (Blakemore, Wolpert, & Frith, 2002 for a review). Accordingly, this mechanism would enable us to distinguish between our own
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Fig. 3. Distribution of the different error types produced by schizophrenic patients (upper panel) and age-matched controls (lower panel). Main pie charts: percentage of errors in each of the three possible categories. Small pie charts: proportion of inversions (i.e., covertly read items described as overtly read and vice versa), false recognitions (i.e., new items described as overtly or as covertly read), forgetting (i.e., overtly or covertly read items that were defined as absent). Significant between-groups comparisons were observed for inversions and false recognitions (Mann–Whitney U test, p < :02). In the latter case, proportion of new words classified as overtly read was significantly larger in patients compared to controls (Mann–Whitney U test, p < :006). Within the control group, overtly read items were forgotten less often than covertly read ones (Wilcoxon test: T ¼ 3:00, Z ¼ 2:4973, p < :01) and inversions were significantly more frequent in the covert-overt direction than vice versa (Wilcoxon test: T ¼ 16:5, Z ¼ 2:2599, p < :02). None such significant difference was found in patients (more details in Franck et al., 2000, adapted figure).
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actions and those produced by an external agent (Blakemore et al., 2002; Georgieff & Jeannerod, 1998; Jeannerod, 1999). On the other hand, we suggest that the same specificity may be responsible for the peculiar nature of self-referred memories and thus facilitate recall of the corresponding items. We think that this hypothesis is supported by the above-described research: indeed, patients showing a disruption in self-monitoring benefit only in part from the subjective experience of having pronounced a given word. Although in that condition their recall reaches normal range, quality of the encoded information does not achieve a format sufficient to avoid the large amount of source-inversions and false recognitions that are recorded.
3. Enactment effect and subjective experience At a closer look, data from Franck et al. (2000) might as well be explained by a failure in consciously retrieving from memory a source-information that patients may actually possess. Given that participants were explicitly required to attribute each of the presented items to a given category, one could speculate that voluntary access to the information was prevented, rather than memory of the source itself missing. This conservative interpretation would agree with the reports of severe deficits in accessing stored information in schizophrenic patients (Aleman et al., 1999; Saykin et al., 1991). In a recent study, Daprati and colleagues (submitted manuscript) directly explored this possibility, by analysing the performance of schizophrenic patients in a memory task that did not require an explicit statement of how a given piece of information came to the subjectÕs attention. Experimental psychology provides interesting descriptions of what is called the enactment effect (see Engelkamp, 1998 for a review). This quite common memory effect emerges as a rise in the proportion of correctly remembered items for simple action-phrases (e.g., ‘‘to close the door’’) when encoding is obtained by enactment of the verbal material compared to when the same action-phrases are encoded by an auditory or visual presentation. In other words, subjects retain better semantic information conveyed by a phrase when they are allowed to enact the denoted actions rather than when they simply attend to verbal presentation. One interesting hypothesis suggests that the beneficial effect of enactment might depend on a rich multi-modal encoding linked to the activation and execution of a motor program (Engelkamp & Zimmer, 1994a). This motor specificity has been documented by the reduction or even absence of the effect when the actual motor component is removed, and only imagery of oneself performing the action or observation of the action are allowed (Denis, Engelkamp, & Mohr, 1991; Engelkamp, 1997). Similarly, it has been shown that a selective interference on the recall occurs only when subjects are presented with distracting items whose motor component matches that of the target phrase (Engelkamp & Zimmer, 1994b; Mohr, Engelkamp, & Zimmer, 1989). It has been proposed that the beneficial effect of enactment emerges when ‘‘implicit’’ knowledge is tested, as suggested by the observation that it is not removed by most of the physiological (i.e., ageing, B€ ackman & Nilsson, 1985) and pathological conditions that affect the ability to voluntary retrieve memorised information, such as several forms of amnesia (Karlsson et al., 1989; Mimura et al., 1998). Indeed, in all these cases, although performance may be dramatically impoverished, the enactment effect is still detectable. Accordingly, one could argue that in schizophrenia the existence of a poor conscious retrieval may similarly affect the overall memory span, but should
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nevertheless leave intact the differential effect produced by enactment. This difference would be possibly removed only if one assumes that the poor conscious action monitoring that has been described in schizophrenia (Daprati et al., 1997; Franck et al., 2000, 2001) may affect the construction of a stable memory trace. This latter hypothesis finds support in the results produced in a memory-for-action task by the group of schizophrenic patients described by Daprati et al. (submitted manuscript). The authors administered a list of phrases describing common actions (i.e., ‘‘to wash oneÕs hands’’) to eight patients who received a clinical diagnosis of schizophrenia according to DSM-IV (all men, mean age 38.3 13.4 years, mean educational level 11.3 1.6 years). Patients were clinically stable at the time of testing with prominent negative symptoms (Scale for the Assessment of Negative Symptoms, Andreasen, 1983: mean score 40.4 21.2; Scale for the Assessment of Positive Symptoms, Andreasen, 1984: mean score 26.3 6.5). All received medication and those receiving anti-psychotic drugs reported to interfere with memory functions, were administered the French version of the Grober & BuschkeÕs verbal memory test (Ergis, Van Der Linden, Boller, Degos, & Deweer, 1995) to confirm that they scored within the normal range. None of them showed evidence for a broad cognitive impairment, as suggested by a normal or borderline IQ at either the Wechsler Intelligence Scale or at the Raven Progressive Matrices (PM47). Each participant listened to a manÕs voice reading a list of 30 action-phrases and watched a TVmonitor where the action was pantomimed by a male actor. The task for the patient was to look at the video, carefully listen to the voice, and eventually act out the action according to the colour of a cue that appeared before each target-phrase was presented (see Fig. 4). Fifteen action-phrases were always enacted and 15 were simply observed on the monitor (and listened to). After a 10-min interval, the authors tested the patients recollection ability in a computerised recognition task. Subjects viewed 60 written sentences that corresponded to the previously heard 30 action-phrases
Fig. 4. Schematic description of the procedure followed in the enactment-effect experiment (Daprati et al, submitted manuscript). Each of the 30 action-phrases was preceded by a visual cue (red/green box) indicating whether subjects had to observe or to execute the action. Verbal presentation of the phrase followed the cue by a 800–1000 ms interval and preceded video onset by a 500–800 ms interval.
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plus an equal amount of new items. For each phrase, patients were asked to decide whether it corresponded to an old or a new item. No information on how subjects experienced the phrase in the first place (i.e., whether the phrase had been enacted or observed) was required. The performance of schizophrenic patients, expressed as the proportion of correctly recognised items and the corresponding response times, was compared to that of eight age-matched controls. Inspection of these results (see Fig. 5) discloses a picture that is suggestive of the hypothesis we proposed at the beginning of this section, namely that poor conscious action monitoring in
Fig. 5. (A) Latencies (RTs) required by age-matched controls (grey bars) and schizophrenic patients (black bars) to give their responses to old and new items. Results for old items are split according to study condition (enacted or observed). Significant between-groups (t test for independent samples) and within-groups comparisons (t test for dependent samples) at p < :05 are shown. (B) Proportion of items correctly identified as either old or new by schizophrenic patients and age-matched controls. More captions as in a (adapted from Daprati et al., submitted manuscript).
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schizophrenia may affect the construction of a stable (motor) memory trace and thus may disrupt the enactment effect. Indeed, in control subjects, the latency required to recognise the enacted items as ‘‘old’’ was significantly shorter compared to that required to recognise the observed ones. On the contrary, patients with schizophrenia seemed not to take advantage of motor encoding requiring a comparable amount of time to recognise as ‘‘old’’ both types of items (a two-way ANOVA, with Group as between factor Type of Encoding as within factor, providing a significant interaction F ð1; 14Þ ¼ 6:288, p < :0251, the two groups differing significantly according to type of encoding, namely for enacted items). Similarly, the proportion of correctly recalled items was significantly larger for the enacted items within the control group, but not among patients (see legend to Fig. 5 for details). Although caution should be used given the small dimension of the analysed sample, these results seem to suggest a reduction of the enactment effect in schizophrenia. Several considerations can be drawn from these results. Compared to previous reports, the small group of patients with schizophrenia described by Daprati et al. (submitted manuscript) is not dramatically impaired in this memory task. Following simple verbal encoding, the amount of items that the patients correctly recognised as old ones did not significantly differ from that of normal controls, suggesting an adequate ability to store information in memory. One could argue that recognition tasks require a less important conscious effort by the patient in order to retrieve memorised information, compared for instance to the previously reported source-discrimination task (Franck et al., 2000). Accordingly, the above-described result could be accounted for by the hypothesis that some implicit knowledge of the presented items actually persists. Indeed, a failure in consciously retrieving from memory a source-information that patients may actually possess has been frequently described in schizophrenia (see for example Clare et al., 1993). In the present case however, no direct request to report how each item had been encoded was made to the patients. The enactment effect is supposed to be effective in the absence of any explicit effort by the subjects to either encode or retrieve, to a differential extent, the corresponding item. In other words, it would arise as a consequence of sort of a procedural learning that implicitly favours the enacted items (Perrig, 1988). Thus, although the sample examined is still small, we think that one appealing explanation for the reduction (or even absence) of the enactment effect observed here may arise from some defective conscious action monitoring in the patients. This, in turn, would have weakened the difference between observed and enacted items. In other words, we can speculate that in the absence of a reliable conscious action monitoring, as has been described in schizophrenia (Daprati et al., 1997; Franck et al., 2000, 2001), the memory traces produced by enacted and observed items do not differ anymore. Consequently, the source effect they normally would induce is affected.
4. Conclusions Psychology—as well as everyday life—suggests that we can be deceived by our memory system, although we usually keep an excellent trace of our own actions. Namely, whenever we personally produce an action (and are conscious of it), our memory trace of the event is strong enough to guarantee an adequate recall. This extreme specificity of enactment is far from being a trivial effect. Indeed, in our continuous interaction with the environment, we experience a huge number
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of both internal and external events, given that we as often perform and observe motor actions. Both events are likely to activate internal action representations and it has been argued that very close neural substrates may even be involved in both processes (Gerardin et al., 2000; Rizzolatti, Fadiga, Gallese, & Fogassi, 1996; Rizzolatti, Fogassi, & Gallese, 2001; Stephan et al., 1995). Nevertheless, overt motor production retains a definite specificity, given that, for instance, observation necessarily lacks those internal signals (such as somatosensory reafferences) that characterise enactment. These same internal signals likely help to define the event as self-generated and, in a sense, they qualify the subject as the agent providing him/her with the subjective experience that he/she is the source of the event (Blakemore et al., 2002; Georgieff & Jeannerod, 1998; Jeannerod, 1999). The particular stability of memories derived from self-generated actions suggests that memory traces maintain such a specificity. Recent neuroimaging studies indicate that similar areas are involved in encoding and retrieval of memory information related to enacted actions. In particular, when subjects recall enacted action-phrases, a significantly increased activity is found in motor areas, suggesting that motor information may have become part of the memory trace (Nilsson et al., 2000; Nyberg et al., 2001). The studies we reviewed here suggest a couple of considerations. First, we believe that one important pre-requisite to the beneficial effect derived from enactment is a reliable first-person experience. In other words, what makes a self-generated event special, even in its memory trace, is the subjectÕs direct involvement. Performance of patients with schizophrenia is suggestive of this assumption: an impoverishment of awareness of action can affect memory. For instance, within the patientsÕ group, latencies required to recognise that a sentence had been previously shown did not differ between enacted and observed items. Namely, the facilitation induced from the previous experience of having overtly performed an action was not evident, as if the private signals generated by motor production were either weaker or missing in the patients, thus reducing differences between self-generated and observed events. This result is consistent with recent reports of the pathogenesis of several schizophrenic symptoms, such as verbal hallucinations (Frith et al., 2000). We further speculate that in the absence of a reliable awareness of actions, memories of internally generated movements would loose their peculiar attributes. Thus, becoming undistinguishable from those of external events, these memories would then be more vulnerable to oblivion. Second, we wish to underlie the observation that the memory facilitation induced by the opportunity to relate an event to oneÕs own private experience likely starts at the level of motor production. In other words, motor-related signals may play a relevant role in characterising memory traces. Thus, rather than being an experimental curiosity, the memory facilitation induced by the enactment effect would more concretely obey to a specific ecological goal. At a basic level, motor-information related to previously performed actions may prevent unwanted repetitions of a given act or, by keeping track of previous movements, may help to improve our motor skills. Moreover, it may as well enable us to tell apart memories of internal events (such as our own speech) from those of external experiences (such as other peopleÕs speeches). In more speculative terms, by strengthening the experience of self-generated actions, motor-related signals may contribute to the peculiar stability of personal memories which, interestingly, can be frequently spared following CNS damage (see Kopelman, 2002 for a review on memory disorders). Indeed, as Jackson pointed out, thinking can be regarded as our most complex motor act (Jackson, 1958). Accordingly, we might as well suppose that the construction of an individual autobiographical experience begins with the extreme specificity of his/her own motor experience.
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