There has been an explosion of research in the development, evolution, and brain basis of imitation. Human beings are highly imitative. Recent discoveries reveal that newborn infants have an innate ability to imitate facial expressions. This has important implications for theories of FOLK PSYCHOLOGY, MEMORY, CULTURE, and LANGUAGE.
Classical theories of COGNITIVE DEVELOPMENT postulated that newborns did not understand the similarity between themselves and others. Newborns were said to be "solipsistic," experiencing their own internal sensations and seeing the movements of others without linking the two (Piaget 1962). According to Jean PIAGET, the imitation of facial gestures was first possible at one year of age, a landmark development that was a prerequisite for representational abilities. In sharp contrast, modern empirical work has shown that infants as young as forty-two minutes old successfully imitate adult facial gestures (Meltzoff and Moore 1983). Imitation is innate in humans (figure 1).
Figure 1 Photographs of two- to three-week-old infants imitating facial acts demonstrated by an adult. From A. N. Meltzoff and M. K. Moore (1977). Science 198 : 75-78.
Facial imitation presents a puzzle. Infants can see an adult's face but cannot see their own faces. They can feel their own faces move but have no access to the feelings of movement in another person. How is facial imitation possible? One candidate is "active intermodal mapping." The crux of the view is that an infant represents the adult facial expression and actively tries to make his or her own face match that target. Of course, infants do not see their own facial movements, but they can use proprioception to monitor their own unseen actions and to correct their behavior. According to this view, the perception and production of human acts are represented in a common "supramodal" framework and can be directly compared to one another. Meltzoff and Moore (1997) provide a detailed account of the metric used for establishing cross-modal equivalence of human acts and its possible brain basis.
The findings on imitation suggest a common coding for perception and production. Work with adults analyzing brain sites and cognitive mechanisms involved in the imitation, perception, and imagination of human acts suggests they tap common processes (Jeannerod and Decety 1995; Prinz 1990). Neurophysiological studies show that in some cases the same neurons become activated when monkeys perform an action as when they observe a similar action made by another (Rizzolatti et al. 1996). Such "mirror neurons" may provide a neurophysiological substrate for imitation.
Early imitation has implications for the philosophical problem of other minds. Imitation shows that young infants are sensitive to the movements of themselves and other people and can map self-other isomorphisms at the level of actions (see INTERSUBJECTIVITY). Through experience they may learn that when they act in particular ways, they themselves have certain concomitant internal states (proprioception, emotions, intentions, etc.). Having detected this regularity, infants have grounds for making the inference that when they see another person act in the same way that they do, the person has internal states similar to their own. Thus, one need not accept Fodor's (1987) thesis that the adult THEORY OF MIND must be innate in humans (because it could not be learned via classical reinforcement procedures. Imitation of body movements, vocalizations, and other goal-directed behavior provides a vehicle for infants discovering that other people are "like me," with internal states just like the SELF. Infant imitation may be a developmental precursor to developing a theory of mind (Meltzoff and Moore 1995, 1997; Gopnik and Meltzoff 1997; see also SIMULATION VS. THEORY-THEORY).
What motivates infants to imitate others? Imitation serves many cognitive and social functions, but one possibility is that very young infants use behavioral imitation to sort out the identity of people. Young infants are concerned with determining the identity of objects as they move in space, disappear, and reappear (see INFANT COGNITION). Research shows that young infants use imitation of a person's acts to help them distinguish one individual from another and reidentify people on subsequent encounters (Meltzoff and Moore 1998). Infants use the distinctive actions of people as if they were functional properties that can be elicited through interaction. Thus, infants identify a person not only by featural characteristics (lips, eyes, hair), but by how that individual acts and reacts.
As adults, we ascribe mental states to others. One technique for investigating the origins of theory of mind capitalizes on the proclivity for imitation (Meltzoff 1995a). Using this technique, the adult tries but fails to perform certain target acts. The results show that eighteen-month-olds imitate what the adult "is trying to do," not what that adult literally does do. This establishes that young children are not strict behaviorists, attuned only to the surface behavior of people. By eighteen months of age children have already adopted a fundamental aspect of a folk psychology -- actions of persons are understood within a framework involving goals and intentions.
Imitation illuminates the nature of preverbal memory (Meltzoff 1995b). In these tests infants are shown a series of acts on novel objects but are not allowed to touch the objects. A delay of hours or weeks is then imposed. Infants from six to fifteen months of age have been shown to perform deferred imitation after the delay, which establishes preverbal recall memory, not simply recognition of the objects. The findings suggest that infants operate with what cognitive neuroscientists call declarative memory as opposed to procedural or habit memory (Sherry and Schacter 1987; Squire, Knowlton, and Musen 1993), inasmuch as learning and recall of novel material occurs after one brief observation with no motor training. Research is being directed to determining the brain structures that mediate deferred imitation. Amnesic adults (see MEMORY, HUMAN NEUROPSYCHOLOGY) are incapable of the same deferred imitation tasks accomplished by infants, suggesting that it is mediated by the HIPPOCAMPUS and related anatomical structures (McDonough et al. 1995). Compatible evidence comes from a study showing that children with AUTISM have a deficit in imitation, particularly deferred imitation, compared to mental-age matched controls (Dawson et al. 1998).
Comparative psychologists hotly debate the nature and scope of imitation in nonhuman animals (Heyes and Galef 1996). Imitation among nonhuman animals is more limited than human imitation (Tomasello, Kruger, and Ratner 1993; Tomasello and Call 1997). Animals modify their behavior when observing others, but even higher primates are most often limited to duplicating the goal rather than the detailed means used to achieve it. Moreover, social learning in animals is typically motivated by obtaining food and other extrinsic rewards, whereas imitation is its own reward for the human young. In humans, aspects of language development depend on imitation. Vocal imitation is a principal vehicle for infants' learning of the phonetic inventory and prosodic structure of their native language (Kuhl and Meltzoff 1996; see PHONOLOGY, ACQUISITION OF and SPEECH PERCEPTION).
Human beings are the most imitative species on earth. Imitation plays a crucial role in the development of culture and the distinctively human ability to pass on learned abilities from one generation to another. A current challenge in artificial intelligence is to create a robot that can learn through imitation (Demiris and Hayes 1996). Creating more "humanlike" devices may hinge on embodying one of the cornerstones of the human mind, the ability to imitate.
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