- Email: [email protected]
To imitate or not to imitate? How the brain can do it, that is the question!
Brain and Cognition 53 (2003) 479–482 www.elsevier.com/locate/b&c
To imitate or not to imitate? How the brain can do it, that is the question! Raffaella Ida Rumiatia,* and Harold Bekkeringb a
Programme in Neuroscience, Scuola Internazionale Superiore di Studi Avanzati, Trieste, Italy Nijmegen Institute for Cognition and Information, University of Nijmegen, The Netherlands
b
Accepted 16 July 2003
Abstract In this paper the authors discuss the most prominent views addressing the issue of how we imitate actions. It is argued that the existing theories lay along a continuum, with the direct mapping approach at one end (Butterworth, 1990; Gray, Neisser, Shapiro, & Kouns, 1991) the Active Intermodal Matching approach (Meltzoff & Moore, 1997) in the middle, and the goal-directed theory (Bekkering, Wohlsch€ager, & Gattis, 2000) and the dual route theory (Rumiati & Tessari, 2002) at the opposite end. Interestingly the latter views can account for behaviors that cannot be explained by invoking the direct mapping or the Active Intermodal Matching approach. Ó 2003 Elsevier Inc. All rights reserved.
1. Introduction The study of imitation has a long history and until the last 10 years it was confined within the disciplines of developmental psychology and evolutionary biology, both mostly concerned with who imitates, and what is imitated. But it is now receiving increased attention from neuroscientists. Whether imitation evolved from primates to the human beings or is rather an ability peculiar to our species is still an undecided issue. There is in fact no complete agreement as to whether non-human primates are able to truly imitate actions performed by others. Nagell, Olguin, and Tomasello (1993), for instance, studied the ability to learn how to use a rake in order to retrieve food or toys, respectively, by chimpanzees and 2-yearolds human children. They found that as opposed to children who imitated a model even when the observed behavior had no real advantage (i.e., using the rake position instead of the edge position), chimpanzees focused on the end results of the observed behavior. This type of learning in chimps has been called emulation because it focuses on the changes of state in the envi* Corresponding author. Fax: +39-040-3787249. E-mail address: [email protected] (R.I. Rumiati).
0278-2626/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0278-2626(03)00208-2
ronment produced by others and not on a conspecificÕs behavior or behavioral strategy (Tomasello, 1999). That children can imitate actions that are not useful as long as they resemble the desired result in a global fashion is also proved in a study by Want and Harris (2001) on learning by observation how to perform a tool-use task that consisted of inserting a stick or a marble into a tube in order to retrieve a toy. They found that 3-, but not 2-years-old children, benefited from observing a correct following an incorrect demonstration marked by the demonstrator as accidental with the exclamation ‘‘Oops!.’’ When 3-years-olds are shown only how to succeed but not how to fail, they are likely to reproduce the observed actions without learning the causal relationships involved. However, when they observed the demonstrator making a mistake, they avoided reproducing that mistake themselves suggesting that they can learn from observation. Less concerned with the conditions under which imitation occurs, Byrne and Russon (1998) proposed a theory of imitative behavior in great apes more entailed with the specific mechanisms involved. Based on a particular plant-processing technique observed in populations of mountain gorillas and chimps, they came to argue that two are the processes that can be employed in action imitation: the action-level that allows one to copy
480
R.I. Rumiati, H. Bekkering / Brain and Cognition 53 (2003) 479–482
actions following the surface form, and the program-level thanks to which one can copy the hierarchical organization of a complex action and, ultimately to learn new behaviors. Thus everyday use of imitation seems to be closer to the program-level than to the action-level imitation. However, this claim does not imply that only known actions can be imitated. Indeed there would be no need to imitate an act that we have already acquired. Rather, what needs to be learned is the novel combination of known subunits of the actions. Assuming that imitation is present and that it is likely to be innate in humans and perhaps in non-human primates too, what does it tell us about what is actually involved in successful imitation? To answer this question we have now to turn to neurocognitive research that addresses the issue of how visual information can evoke motor performance. At least three accounts have been offered to explain imitation and they lay on a continuum. On the one end, the direct mapping approach states that the motor system directly is activated by the perception of an action (Butterworth, 1990; Gray et al., 1991). That is, the motor system of the imitator receives direct input from observing the modelÕs movement. In many occasions, advocates of this view have used the finding of the so-called mirror neurons as the neural substrate of direct-mapping. The mirror neuron (MN) system is a brain circuit— including parietal and premotor cortices and underpinning the ability to observe as well as to produce goal-directed movements—that has been described both in the monkey (Di Pellegrino, Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Rizzolatti et al., 1996) and humans (Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995; Iacoboni et al., 1999). Although, mirror neurons may not be sufficient to support imitative behavior in macaques (Hauser, Chomsky, & Fitch, 2002), in humans it may well be used for imitation. However, this is a simplistic interpretation of what the mirror neurons seem to be coding. In fact, it is reported that in order for them to fire, the animal needs to see a purposeful and object-related action. Surprisingly, the animal does not need to perceive the whole action; as long as it is purposeful, the MN-system also fires if the hand is occluded (Umilt a et al., 2002). Further along the continuum, the Active Intermodal Matching (hereafter AIM) approach put forward by Meltzoff and Moore (1997) states that human infants code human acts within an innate supramodal system that unifies observation and execution of motor acts. In a seminal paper (Meltzoff & Moore, 1977) showed imitation of facial and manual gestures in neonates. This finding changed sharply the view that humans gradually acquire the ability to imitate over the years, in favor of the view that they can indeed imitate from birth. However, at least two sets of behavioral findings from our research on action imitation do not easily fit a
basic version of the direct-mapping approach and the AIM theory. The first one is provided by the work of Bekkering, Wohlschl€ager and colleagues (Bekkering, Wohlschl€ager, & Gattis, 2000; Bekkering & Wohlschl€ager, 2002) which clearly shows that childrenÕs imitation frequently deviates from the model. Children asked to copy movements directed to a right or left ear (the object), using either the ipsilateral or the controlateral hand (the agent), moving parallel to the body or crossing the body midline (the movement paths) or crossing their arms (the salient feature), selected either the object or the agent correctly (i.e., the most important goals), but neglected the movement paths and the salient feature (i.e., less important goals). Bekkering and colleagues proposed a goal-directed theory of imitation to explain this pattern of behavior. Thus children do not merely mimic the actions shown by the demonstrators but first decompose and then reproduce them according to a hierarchy of goals. The reason that children do not simply match an afferent input to an efferent output is that their cognitive resources are limited, allowing them to reproduce only the most important goal at any particular time. Additional evidence for goal-directed imitation derives from a series of experiments in adults (Wohlschl€ager, Gattis, & Bekkering, 2003) and patients with Apraxia (Bekkering, Woschina, Brass, & Jacobs, in press). The second set that does not smoothly fit with the classic direct-mapping approach, is supplied by the research carried out by Rumiati and colleagues. First, Rumiati and Tessari (2002) found that healthy individuals have larger spans for meaningful actions (MF) that for meaningless actions (ML). Second, Tessari and Rumiati (submitted) found that in healthy adult subjects, a number of elements affect action imitation, such as the type of actions (MF or ML), the experimental context (blocked or mixed experimental list), and the relative proportion of the two types of action. In short the authors found that, under time pressure, familiar, meaningful actions are imitated more accurately than novel, meaningless actions when shown in separate blocks, but they are imitated with the same accuracy when they are presented intermingled. This behavioral pattern was obtained whether or not participants were aware of the composition of the experimental list. When the list of stimuli to be imitated contained more ML than MF actions, the same results were obtained as in the mixed condition. When more MF than ML actions were presented intermingled, participants imitated more accurately MF than ML actions. Tessari and Rumiati (submitted) argued that when MF and ML actions are presented intermingled, as well as when there are more ML than MF actions, participants select the most suitable mechanism for reproducing both types of actions, bypassing the long-term, semantic system. In the blocked presentation and in the list contains more MF
R.I. Rumiati, H. Bekkering / Brain and Cognition 53 (2003) 479–482
than ML actions, however, a mechanism that relays on stored representations of the known actions is triggered for reproducing MF actions; in contrast, ML actions can be imitated from the model only. The context (or list) effect may account for the failure to find a dissociation between imitation of MF and ML actions in two large group studies with left-brain damaged patients (De Renzi, Motti, & Nichelli, 1980; Toraldo, Reverberi, & Rumiati, 2001). In both studies, in fact, MF and ML actions were administered intermingled. In contrast, using separate lists of MF and ML actions Goldenberg and Hagmann (1997) described a simple dissociation with MF actions being imitated significantly better than ML actions. According to the dual route theory (Rumiati & Tessari, 2002) this failure to reproduce ML actions could be interpreted as a damage of the visuo-motor conversion route. Based on the goal-directed theory, the effect of type of stimuli on imitation can be regarded in terms of subjects finding easier to imitate MF actions over ML actions because they are purposeful, e.g., they contain a goal that is the use of an object. The opposite pattern, i.e., spared imitation of ML actions but impaired reproduction of MF actions, has been reported once by Bartolo, Cubelli, Della Sala, Drei, and Marchetti (2001) cannot easily be explained by either of the theories. In fact, the dual route theory predicts that the visuo-motor conversion mechanism can be used to imitate both ML and MF actions. Thus if the indirect semantic route is damaged, then MF actions could still be reproduced based on the direct route, unless one posits that once the damaged semantic route is activated by the seen action, the system cannot switch to the unimpaired direct route. Likewise, it is not clear how the goal-directed theory may account for a better imitation performance with ML actions. In conclusion, an extended theory of imitation should specify not only a sensory-motor level and an intentional level of processing in imitation, but it should also take into account other factors influencing imitation performance. One such effect is that of enhanced imitation of MF versus ML actions (Rumiati & Tessari, 2002; Tessari & Rumiati, submitted). Using Bekkering et al.Õs (2000) terminology, the advantage of MF over ML actions may relate to the fact that MF actions have a concrete action goal likely to be represented in the observerÕs brain. Reproducing the already acquired actions does not involve visuo-motor mapping but activation of movements associated with how to accomplish the same action goal. But for ML actions we cannot rely on these stored associations and therefore the imitator needs to actively apply visuo-motor mapping (see also Gleissner, Meltzoff, & Bekkering, 2000). Another issue concerns the limitation of cognitive resources. For instance, Rumiati and Tessari (2002) found shorter action spans for ML than for MF actions in healthy adults. Under time pressure, and thus with
481
reduced cognitive resources, normal subjects showed to avoid switching costs in a task in which they were requested to imitate mixed MF and ML actions (Tessari & Rumiati, submitted). Likewise Wohlschl€ager et al. (2003) observed that healthy adults committed errors in imitation when the number of goals challenged their cognitive resources. Together these findings stress the importance of working memory and long-term memory involvement in imitation. However, as compared with the ontogenetic and the phylogenetic approaches, there are relatively fewer studies exploring the functional architecture of how imitation is accomplished and the underlying brain structures and physiological mechanisms. The aim of the present issue on Perception and Imitation of Actions is to contribute to fulfill this gap.
2. Overview of the issue Four are the contributions that the reader will find in this issue. The first paper is by Bartolo, Cubelli, Della Sala, and Drei (this issue) who reported the case of a brain damaged patient, VL, showing an impairment in pantomiming the use of objects and in imitating meaningless gestures, as well as a reduced working memory. In contrast VLÕs ability to produce meaningful actions was preserved. Bartolo et al. (this issue) argued that pantomimes are to some extent novel gestures and that, in order to produce them, VL would need to integrate in a workspace the functional knowledge about objects (i.e., action semantics) with stored procedural programs (i.e., action output lexicon). This is the reason why a faulty working memory may selectively affect pantomime performance. Edwards, Humphreys, and Castiello (this issue) tested the idea that observation and execution of actions share a common neural system by studying the effect of action observation on subsequent execution in normal subjects. Besides the reliable priming effect of prior action observation on the kinematics of reaching and grasping a target object, the authors also found a priming effect even when observers saw just the object. Not only do these results speak in favor of a common coding for seen and performed actions, but they also give credit to the argument that seen objects afford actions (see also Tessari & Rumiati, 2002). The study by Flach Knoblich and Prinz (this issue) investigated whether perceiving our own actions differs from perceiving actions of others. They found that synchronizing with oneÕs own action effects is different from synchronizing with somebody elseÕs action effects. More precisely, Flach et al. (this issue) observed that subjects responded earlier to self-generated than to etero-generated action effects, providing that the motion stimuli were rather unpredictable.
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R.I. Rumiati, H. Bekkering / Brain and Cognition 53 (2003) 479–482
The last paper is by Fadiga and Craighero (this issue) who summarized the neurophysiological work done in the last two decades to elucidate the integrative role of the monkey ventro-rostral premotor cortex. In addition the authors present some data indicating that both in humans and in monkeys, action-related sensorimotor transformations are not limited to visual information but concern also acoustic information.
References Bartolo, A., Cubelli, R., Della Sala, S., Drei, S., & Marchetti, C. (2001). Double dissociation between meaningful and meaningless gesture reproduction in apraxia. Cortex, 37(5), 696–699. Bekkering, H., Wohlsch€ager, A., & Gattis, M. (2000). Imitation of gestures in children is goal-directed. The Quarterly Journal of Experimental Psychology, 53A, 153–164. Bekkering, H., & Wohlschl€ager, A. (2002). Action perception and imitation: a tutorial. In W. Prinz & B. Hommel (Eds.), Attention and Performance XIX. Common mechanisms in Perception and Action (pp. 294–333). Oxford: Oxford University Press. Bekkering, H., Woschina, S., Brass, M., & Jacobs, A. (in press). Goaldirected imitation in patients with Ideomotor Apraxia, Cognitive Neuropsychology. Butterworth, G. (1990). On reconceptualizing sensori-motor coordination in dynamic system terms. In H. Bloch & B. I. Bertenthal (Eds.), Sensory motor organizations and development in infancy and early childhood (pp. 57–73). The Netherlands: Kluwer Academic Press. Byrne, R. W., & Russon, A. E. (1998). Learning by imitation: A hierarchical approach. Behavioral and Brain Sciences, 21, 667–684. De Renzi, E., Motti, F., & Nichelli, P. (1980). Imitating gestures: A quantitative approach to ideomotor apraxia. Archives of Neurology, 37, 6–10. Di Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V., & Rizzolatti, G. (1992). Understanding motor events: a neurophysiological study. Experimental Brain Research, 91, 176–180. Fadiga, L., Fogassi, L., Pavesi, G., & Rizzolatti, G. (1995). Motor facilitation during action observation: A magnetic stimulation study. Journal of Neurophysiology, 73, 2608–2611. Gallese, V., Fadiga, L., Fogassi, L., & Rizzolatti, G. (1996). Action recognition in the premotor cortex. Brain, 119, 593–609.
Gleissner, B., Meltzoff, A. N., & Bekkering, H. (2000). ChildrenÕs coding of human action: Cognitive factors influencing imitation in 3-year-olds. Developmental Science, 3, 405–414. Goldenberg, G., & Hagmann, S. (1997). The meaning of meaningless gestures: A study of visuo-imitative apraxia. Neuropsychologia, 35, 333–341. Gray, J. T., Neisser, U., Shapiro, B. A, & Kouns, S. (1991). Observational learning of ballet sequences: The role of kinematic information. Ecological Psychology, 3, 121–134. Hauser, M. D., Chomsky, N., & Fitch, W. T. (2002). The faculty of language: What is it, and how did it evolve? Science, 298, 1569– 1579. Meltzoff, A. N., & Moore, M. K. (1977). Imitation of facial and manual gestures by human neonates. Science, 198, 75–78. Meltzoff, A. N., & Moore, M. K. (1997). Explaining facial imitation: A theoretical model. Early Development Parenting, 6, 179–192. Nagell, K., Olguin, R. S., & Tomasello, M. (1993). Processes of social learning in the tool use of chimpanzees (pan troglodytes) and human children (homo sapiens). Journal of Comparative Psychology, 107, 174–186. Rumiati, R. I., & Tessari, A. (2002). Imitation of novel and wellknown actions: The role of short-term memory. Experimental Brain Research, 142, 425–433. Tessari, A., & Rumiati, R. I. (2002). Motor distal component and pragmatic representation of objects. Cognitive Brain Research, 14(2), 218–227. Tessari, A., & Rumiati, R. I. How endogenous and exogenous factors play tricks on imitation. Journal of Experimental Psychology: Human Perception and Performance, submitted. Tomasello, M. (1999). The cultural origins of human cognition. Cambridge: Harvard University Press. Toraldo, A., Reverberi, C., & Rumiati, R. I. (2001). Critical dimensions affecting imitation performance of patients with ideomotor apraxia. Cortex, 37, 737–740. Umilta, M. A., Kohler, E., Gallese, V., Fogassi, L., Fadiga, L., Keysers, C., & Rizzolatti, G. (2002). I know what you are doing. A neurophysiological study. Neuron, 31, 155–165. Want, S. C., & Harris, P. L. (2001). Learning from other peopleÕs mistakes: Causal understanding in learning to use a tool. Child Development, 72, 431–443. Wohlschl€ager, A., Gattis, M., & Bekkering, H. (2003). Action generation and action perception in imitation: An instance of the ideomotor principle. Philosophical Transitions of the Royal Society of London B: Biological Science, 358, 501–515.
To imitate or not to imitate? How the brain can do it, that is the question! Raffaella Ida Rumiatia,* and Harold Bekkeringb a
Programme in Neuroscience, Scuola Internazionale Superiore di Studi Avanzati, Trieste, Italy Nijmegen Institute for Cognition and Information, University of Nijmegen, The Netherlands
b
Accepted 16 July 2003
Abstract In this paper the authors discuss the most prominent views addressing the issue of how we imitate actions. It is argued that the existing theories lay along a continuum, with the direct mapping approach at one end (Butterworth, 1990; Gray, Neisser, Shapiro, & Kouns, 1991) the Active Intermodal Matching approach (Meltzoff & Moore, 1997) in the middle, and the goal-directed theory (Bekkering, Wohlsch€ager, & Gattis, 2000) and the dual route theory (Rumiati & Tessari, 2002) at the opposite end. Interestingly the latter views can account for behaviors that cannot be explained by invoking the direct mapping or the Active Intermodal Matching approach. Ó 2003 Elsevier Inc. All rights reserved.
1. Introduction The study of imitation has a long history and until the last 10 years it was confined within the disciplines of developmental psychology and evolutionary biology, both mostly concerned with who imitates, and what is imitated. But it is now receiving increased attention from neuroscientists. Whether imitation evolved from primates to the human beings or is rather an ability peculiar to our species is still an undecided issue. There is in fact no complete agreement as to whether non-human primates are able to truly imitate actions performed by others. Nagell, Olguin, and Tomasello (1993), for instance, studied the ability to learn how to use a rake in order to retrieve food or toys, respectively, by chimpanzees and 2-yearolds human children. They found that as opposed to children who imitated a model even when the observed behavior had no real advantage (i.e., using the rake position instead of the edge position), chimpanzees focused on the end results of the observed behavior. This type of learning in chimps has been called emulation because it focuses on the changes of state in the envi* Corresponding author. Fax: +39-040-3787249. E-mail address: [email protected] (R.I. Rumiati).
0278-2626/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0278-2626(03)00208-2
ronment produced by others and not on a conspecificÕs behavior or behavioral strategy (Tomasello, 1999). That children can imitate actions that are not useful as long as they resemble the desired result in a global fashion is also proved in a study by Want and Harris (2001) on learning by observation how to perform a tool-use task that consisted of inserting a stick or a marble into a tube in order to retrieve a toy. They found that 3-, but not 2-years-old children, benefited from observing a correct following an incorrect demonstration marked by the demonstrator as accidental with the exclamation ‘‘Oops!.’’ When 3-years-olds are shown only how to succeed but not how to fail, they are likely to reproduce the observed actions without learning the causal relationships involved. However, when they observed the demonstrator making a mistake, they avoided reproducing that mistake themselves suggesting that they can learn from observation. Less concerned with the conditions under which imitation occurs, Byrne and Russon (1998) proposed a theory of imitative behavior in great apes more entailed with the specific mechanisms involved. Based on a particular plant-processing technique observed in populations of mountain gorillas and chimps, they came to argue that two are the processes that can be employed in action imitation: the action-level that allows one to copy
480
R.I. Rumiati, H. Bekkering / Brain and Cognition 53 (2003) 479–482
actions following the surface form, and the program-level thanks to which one can copy the hierarchical organization of a complex action and, ultimately to learn new behaviors. Thus everyday use of imitation seems to be closer to the program-level than to the action-level imitation. However, this claim does not imply that only known actions can be imitated. Indeed there would be no need to imitate an act that we have already acquired. Rather, what needs to be learned is the novel combination of known subunits of the actions. Assuming that imitation is present and that it is likely to be innate in humans and perhaps in non-human primates too, what does it tell us about what is actually involved in successful imitation? To answer this question we have now to turn to neurocognitive research that addresses the issue of how visual information can evoke motor performance. At least three accounts have been offered to explain imitation and they lay on a continuum. On the one end, the direct mapping approach states that the motor system directly is activated by the perception of an action (Butterworth, 1990; Gray et al., 1991). That is, the motor system of the imitator receives direct input from observing the modelÕs movement. In many occasions, advocates of this view have used the finding of the so-called mirror neurons as the neural substrate of direct-mapping. The mirror neuron (MN) system is a brain circuit— including parietal and premotor cortices and underpinning the ability to observe as well as to produce goal-directed movements—that has been described both in the monkey (Di Pellegrino, Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Rizzolatti et al., 1996) and humans (Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995; Iacoboni et al., 1999). Although, mirror neurons may not be sufficient to support imitative behavior in macaques (Hauser, Chomsky, & Fitch, 2002), in humans it may well be used for imitation. However, this is a simplistic interpretation of what the mirror neurons seem to be coding. In fact, it is reported that in order for them to fire, the animal needs to see a purposeful and object-related action. Surprisingly, the animal does not need to perceive the whole action; as long as it is purposeful, the MN-system also fires if the hand is occluded (Umilt a et al., 2002). Further along the continuum, the Active Intermodal Matching (hereafter AIM) approach put forward by Meltzoff and Moore (1997) states that human infants code human acts within an innate supramodal system that unifies observation and execution of motor acts. In a seminal paper (Meltzoff & Moore, 1977) showed imitation of facial and manual gestures in neonates. This finding changed sharply the view that humans gradually acquire the ability to imitate over the years, in favor of the view that they can indeed imitate from birth. However, at least two sets of behavioral findings from our research on action imitation do not easily fit a
basic version of the direct-mapping approach and the AIM theory. The first one is provided by the work of Bekkering, Wohlschl€ager and colleagues (Bekkering, Wohlschl€ager, & Gattis, 2000; Bekkering & Wohlschl€ager, 2002) which clearly shows that childrenÕs imitation frequently deviates from the model. Children asked to copy movements directed to a right or left ear (the object), using either the ipsilateral or the controlateral hand (the agent), moving parallel to the body or crossing the body midline (the movement paths) or crossing their arms (the salient feature), selected either the object or the agent correctly (i.e., the most important goals), but neglected the movement paths and the salient feature (i.e., less important goals). Bekkering and colleagues proposed a goal-directed theory of imitation to explain this pattern of behavior. Thus children do not merely mimic the actions shown by the demonstrators but first decompose and then reproduce them according to a hierarchy of goals. The reason that children do not simply match an afferent input to an efferent output is that their cognitive resources are limited, allowing them to reproduce only the most important goal at any particular time. Additional evidence for goal-directed imitation derives from a series of experiments in adults (Wohlschl€ager, Gattis, & Bekkering, 2003) and patients with Apraxia (Bekkering, Woschina, Brass, & Jacobs, in press). The second set that does not smoothly fit with the classic direct-mapping approach, is supplied by the research carried out by Rumiati and colleagues. First, Rumiati and Tessari (2002) found that healthy individuals have larger spans for meaningful actions (MF) that for meaningless actions (ML). Second, Tessari and Rumiati (submitted) found that in healthy adult subjects, a number of elements affect action imitation, such as the type of actions (MF or ML), the experimental context (blocked or mixed experimental list), and the relative proportion of the two types of action. In short the authors found that, under time pressure, familiar, meaningful actions are imitated more accurately than novel, meaningless actions when shown in separate blocks, but they are imitated with the same accuracy when they are presented intermingled. This behavioral pattern was obtained whether or not participants were aware of the composition of the experimental list. When the list of stimuli to be imitated contained more ML than MF actions, the same results were obtained as in the mixed condition. When more MF than ML actions were presented intermingled, participants imitated more accurately MF than ML actions. Tessari and Rumiati (submitted) argued that when MF and ML actions are presented intermingled, as well as when there are more ML than MF actions, participants select the most suitable mechanism for reproducing both types of actions, bypassing the long-term, semantic system. In the blocked presentation and in the list contains more MF
R.I. Rumiati, H. Bekkering / Brain and Cognition 53 (2003) 479–482
than ML actions, however, a mechanism that relays on stored representations of the known actions is triggered for reproducing MF actions; in contrast, ML actions can be imitated from the model only. The context (or list) effect may account for the failure to find a dissociation between imitation of MF and ML actions in two large group studies with left-brain damaged patients (De Renzi, Motti, & Nichelli, 1980; Toraldo, Reverberi, & Rumiati, 2001). In both studies, in fact, MF and ML actions were administered intermingled. In contrast, using separate lists of MF and ML actions Goldenberg and Hagmann (1997) described a simple dissociation with MF actions being imitated significantly better than ML actions. According to the dual route theory (Rumiati & Tessari, 2002) this failure to reproduce ML actions could be interpreted as a damage of the visuo-motor conversion route. Based on the goal-directed theory, the effect of type of stimuli on imitation can be regarded in terms of subjects finding easier to imitate MF actions over ML actions because they are purposeful, e.g., they contain a goal that is the use of an object. The opposite pattern, i.e., spared imitation of ML actions but impaired reproduction of MF actions, has been reported once by Bartolo, Cubelli, Della Sala, Drei, and Marchetti (2001) cannot easily be explained by either of the theories. In fact, the dual route theory predicts that the visuo-motor conversion mechanism can be used to imitate both ML and MF actions. Thus if the indirect semantic route is damaged, then MF actions could still be reproduced based on the direct route, unless one posits that once the damaged semantic route is activated by the seen action, the system cannot switch to the unimpaired direct route. Likewise, it is not clear how the goal-directed theory may account for a better imitation performance with ML actions. In conclusion, an extended theory of imitation should specify not only a sensory-motor level and an intentional level of processing in imitation, but it should also take into account other factors influencing imitation performance. One such effect is that of enhanced imitation of MF versus ML actions (Rumiati & Tessari, 2002; Tessari & Rumiati, submitted). Using Bekkering et al.Õs (2000) terminology, the advantage of MF over ML actions may relate to the fact that MF actions have a concrete action goal likely to be represented in the observerÕs brain. Reproducing the already acquired actions does not involve visuo-motor mapping but activation of movements associated with how to accomplish the same action goal. But for ML actions we cannot rely on these stored associations and therefore the imitator needs to actively apply visuo-motor mapping (see also Gleissner, Meltzoff, & Bekkering, 2000). Another issue concerns the limitation of cognitive resources. For instance, Rumiati and Tessari (2002) found shorter action spans for ML than for MF actions in healthy adults. Under time pressure, and thus with
481
reduced cognitive resources, normal subjects showed to avoid switching costs in a task in which they were requested to imitate mixed MF and ML actions (Tessari & Rumiati, submitted). Likewise Wohlschl€ager et al. (2003) observed that healthy adults committed errors in imitation when the number of goals challenged their cognitive resources. Together these findings stress the importance of working memory and long-term memory involvement in imitation. However, as compared with the ontogenetic and the phylogenetic approaches, there are relatively fewer studies exploring the functional architecture of how imitation is accomplished and the underlying brain structures and physiological mechanisms. The aim of the present issue on Perception and Imitation of Actions is to contribute to fulfill this gap.
2. Overview of the issue Four are the contributions that the reader will find in this issue. The first paper is by Bartolo, Cubelli, Della Sala, and Drei (this issue) who reported the case of a brain damaged patient, VL, showing an impairment in pantomiming the use of objects and in imitating meaningless gestures, as well as a reduced working memory. In contrast VLÕs ability to produce meaningful actions was preserved. Bartolo et al. (this issue) argued that pantomimes are to some extent novel gestures and that, in order to produce them, VL would need to integrate in a workspace the functional knowledge about objects (i.e., action semantics) with stored procedural programs (i.e., action output lexicon). This is the reason why a faulty working memory may selectively affect pantomime performance. Edwards, Humphreys, and Castiello (this issue) tested the idea that observation and execution of actions share a common neural system by studying the effect of action observation on subsequent execution in normal subjects. Besides the reliable priming effect of prior action observation on the kinematics of reaching and grasping a target object, the authors also found a priming effect even when observers saw just the object. Not only do these results speak in favor of a common coding for seen and performed actions, but they also give credit to the argument that seen objects afford actions (see also Tessari & Rumiati, 2002). The study by Flach Knoblich and Prinz (this issue) investigated whether perceiving our own actions differs from perceiving actions of others. They found that synchronizing with oneÕs own action effects is different from synchronizing with somebody elseÕs action effects. More precisely, Flach et al. (this issue) observed that subjects responded earlier to self-generated than to etero-generated action effects, providing that the motion stimuli were rather unpredictable.
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The last paper is by Fadiga and Craighero (this issue) who summarized the neurophysiological work done in the last two decades to elucidate the integrative role of the monkey ventro-rostral premotor cortex. In addition the authors present some data indicating that both in humans and in monkeys, action-related sensorimotor transformations are not limited to visual information but concern also acoustic information.
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