Imitation: is cognitive neuroscience solving the correspondence problem?

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Imitation poses a unique problem: how does the imitator know what pattern of motor activation will make their action look like that of the model? Specialist theories suggest that this correspondence problem has a unique solution; there are functional and neurological mechanisms dedicated to controlling imitation. Generalist theories propose that the problem is solved by general mechanisms of associative learning and action control. Recent research in cognitive neuroscience, stimulated by the discovery of mirror neurons, supports generalist solutions. Imitation is based on the automatic activation of motor representations by movement observation. These externally triggered motor representations are then used to reproduce the observed behaviour. This imitative capacity depends on learned perceptual-motor links. Finally, mechanisms distinguishing self from other are implicated in the inhibition of imitative behaviour.

Introduction

Could you learn to tango by telephone? Maybe, but it would be much easier to learn by watching the steps than by listening to instructions. When movements are visible, we can copy them; we can just do what we see. Imitation – copying body movement – appears to be simple. However, the ease with which humans imitate raises a question, sometimes known as the correspondence problem [1], that is proving difficult to answer: When we observe another person moving we do not see the muscle activation underlying their movement but rather the external consequences of that activation. So how does the observer's motor system ‘know’ which muscle activations will lead to the observed movement?

It is important to solve the correspondence problem because imitation provides a foundation for language acquisition, skill learning, socialisation, and enculturation. A review of current progress on this problem is timely because the discovery of mirror neurons has stimulated considerable interest in action observation and imitation among cognitive neuroscientists, and the function of these intriguing cells is itself a focus of controversy (Box 1). In this review, we outline recent accounts of the correspondence problem, considering whether imitation can be understood within a general learning and motor control framework (generalist theories) or whether it depends on a special purpose mechanism (specialist theories). We then review research on the role of learning in imitation and observation of biological motion. Finally, we discuss a problem for generalist theories: If imitation depends on shared representations of perception and action, how can we distinguish between internally generated and externally triggered motor representations.

Section snippets

Generalist and specialist theories of imitation

Current theories of imitation offer specialist or generalist solutions to the correspondence problem (see Figure 1). Specialist theories suggest that imitation is mediated by a special purpose mechanism. The most prominent specialist theory is the active intermodal matching (AIM) model 2, 3 (Figure 1a). AIM proposes that, when a body movement is observed with the intention to imitate, the initial, visual representation of the movement is converted into a ‘supramodal’ representation which

Motor activation by movement observation

As outlined above, generalist theories assume that imitation is based on general purpose learning and motor control mechanisms. They also assume that imitation is achieved by activation of motor representations through observation of action. One would not expect the operation of such a mechanism to be restricted to situations where imitation is intended. By contrast, one would expect an efficient specialist imitation mechanism to be ‘switched on’ only when needed.

Behavioural evidence of motor

Are there special imitation mechanisms?

The previous section summarized evidence that movement observation leads to specific activation of motor-related neuronal and functional representations. Activation of this kind might provide a sufficient basis for imitation or, as the specialist view would suggest, there might be neural circuits specific to imitation.

Many neuroimaging studies have now investigated the brain regions involved in imitation. They have identified a limited number of areas – including the inferior frontal gyrus

The role of learning in imitation

Generalist theories, particularly the ASL model, predict that whether and how well a person imitates will depend on their past experience. I will be able to imitate an observed action, A, only if I have had the opportunity to form a link between visual and motoric representations of A. Specialist theories do not necessarily deny the importance of learning, but the most prominent contemporary example, the AIM model, suggests that the capacity to imitate is innate.

Relatively few behavioural

Why don't we imitate all the time?

We have argued that there is strong evidence for the assumption that the observation of an action leads to activation of an internal motor representation. However, if, as the ideomotor theory suggests, the action representations that prompt imitation are of the same kind, and have the same origins, as those that guide internally-generated action, why don't we confuse observed actions with our own intentions and copy every movement that we see 7, 40?

Research involving neurological patients

Conclusion

More than a century of research on imitation has left us with a crucial functional problem: how are we able to transform a visual representation of an action into motor output. In the present review we have considered whether recent work in cognitive neuroscience has helped us to solve this correspondence problem. Even though research in cognitive neuroscience has rarely been designed to address the problem, it seems to be accumulating evidence that the key to solution is automatic activation

Acknowledgements

This work has received research funding from the European Community's Sixth Framework Programme under contract number: NEST 012929.

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