Neural mechanisms of imitation
Introduction
Over time, the scientific community has markedly changed its view of imitation. Until few decades ago, imitation was definitely not considered to be associated with high forms of intelligence; however, first, the advent of the ‘cognitive revolution’ and, later, the emergence of the ‘embodied cognition’ approach have changed this perception so much that a recent interdisciplinary collection on imitation describes it as, “a rare ability that is fundamentally linked to characteristically human forms of intelligence, in particular to language, culture, and the ability to understand other minds” [1].
With the exception of a very special form of imitation (birdsong learning, the neural correlates of which have been studied extensively over the years [2]), detailed studies of the neural correlates of imitation have emerged only in the past few years. This surge of interest in the neural correlates of imitation has been probably inspired — at least in part — by the discovery of ‘mirror neurons’ in macaques. These neurons are active when the monkey performs a goal-directed action and when the monkey observes somebody else performing the same action [3, 4, 5]. Mirror neurons have been found in the ventral premotor cortex (area F5 [3, 5, 6]) and in the rostral sector of the inferior parietal lobule (area PF [7••]). F5 and PF are anatomically interconnected [8]; in addition, PF connects with the superior temporal sulcus (STS) [9]. In the STS, there are higher-order visual neurons that respond to seeing the actions of others [10]. Thus, in the macaque, there seems to be a circuitry composed of the STS, PF and F5 that codes the actions of others and seems to be able to map these actions onto the motor repertoire of the observer.
These neurophysiological properties seem optimal as precursors of neural mechanisms of imitation in humans. Indeed, functional imaging studies in humans adopting imitative tasks have identified a similar circuitry in the human brain [11, 12, 13, 14]. This circuitry comprises the human STS and the human mirror neuron system — namely, the inferior frontal cortex [11], which seems particularly important for coding the goal of the imitated action [13], and the rostral part of the inferior parietal lobule [15, 16].
Two main questions emerged from the initial work on the neural mechanisms of imitation in humans. First, given the overlap between the areas activated by imitation tasks and classical areas of language (an overlap that supports the hypothesis that mirror neurons might be important neural elements in the evolution of language [17]), is the activation of classical language areas during imitation essential to imitative behavior or is it simply an epiphenomenon due to internal verbalization [18, 19]? Second, given that monkeys have mirror neurons but do not seem to be skilled imitators, what is the role of mirror neurons in monkeys and how do mirror neuron areas in the human brain support imitation? In this review, I will discuss recent studies that have addressed these questions.
Section snippets
Virtual lesion approach to studies of imitation
The advent of functional neuroimaging has made it possible to study in detail the neural correlates of all sorts of human behavior; however, functional neuroimaging cannot provide information on how essential a brain area is to the behavior under investigation. With transcranial magnetic stimulation (TMS), by contrast, it is possible to induce a ‘transient lesion’ in the stimulated area and to measure the behavioral effects of such a lesion [20, 21]. If the area stimulated is essential to the
Mirror neurons and imitation
Mirror neurons fire during both the execution of one's own actions and the observation of the actions of others — properties that seem relevant to imitation. Moreover, functional neuroimaging studies in humans have shown that areas of the human brain that are relevant to imitation seem to be the homolog of the mirror neuron areas in macaques. Taken together, these observations suggest that mirror neurons are important cells in imitation. However, the degree of imitative ability in monkeys is
Neural correlates of human imitative learning
Recent attempts have been made to clarify the role of the mirror neuron system in imitative learning. These attempts can be categorized into two domains: theoretical and empirical. Initial imaging work suggested that there is a flow of information processing across the three principal cortical areas for human imitation — namely, the human STS and the two frontoparietal mirror neuron areas. First, the STS provides a higher-order visual description of the observed action. Second, this description
Imitation as a form of social interaction
Imitation also seems to be a pervasive form of social communication and identification [40]. People tend to imitate each other during social interactions [41]. The more that people tend to imitate others, the more that they tend to be empathic and concerned with the emotional states of others [41]. This suggests that the core circuitry for imitation might interact with the limbic system (the neural system concerned with emotions) during social mirroring [42]. A recent fMRI study of the
Conclusion
Recent advances in our understanding of the neural mechanisms of imitation suggest that imitative learning and social mirroring are both associated with activity in a core ‘imitation’ circuit comprising the frontoparietal mirror neuron system and the STS. However, during imitative learning the core circuitry for imitation interacts with dorsolateral prefrontal cortex (BA46) and motor preparation areas, whereas during social mirroring it interacts with the insula and the limbic system. Future
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This work was supported, in part, by the Brain Mapping Medical Research Organization, Brain Mapping Support Foundation, Pierson-Lovelace Foundation, Ahmanson Foundation, Tamkin Foundation, Jennifer Jones-Simon Foundation, Capital Group Companies Charitable Foundation, Robson Family and Northstar Fund, and grants from the National Center for Research Resources (RR12169 and RR08655) and the National Institute of Mental Health (MH63680).
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