Associative sequence learning: the role of experience in the development of imitation and the mirror system

Abstract

A core requirement for imitation is a capacity to solve the correspondence problem; to map observed onto executed actions, even when observation and execution yield sensory inputs in different modalities and coordinate frames. Until recently, it was assumed that the human capacity to solve the correspondence problem is innate. However, it is now becoming apparent that, as predicted by the associative sequence learning model, experience, and especially sensorimotor experience, plays a critical role in the development of imitation. We review evidence from studies of non-human animals, children and adults, focusing on research in cognitive neuroscience that uses training and naturally occurring variations in expertise to examine the role of experience in the formation of the mirror system. The relevance of this research depends on the widely held assumption that the mirror system plays a causal role in generating imitative behaviour. We also report original data supporting this assumption. These data show that theta-burst transcranial magnetic stimulation of the inferior frontal gyrus, a classical mirror system area, disrupts automatic imitation of finger movements. We discuss the implications of the evidence reviewed for the evolution, development and intentional control of imitation.

Figure 1.

An example of two successive trials in the automatic imitation task. A blank screen was followed by a neutral hand stimulus on which the eventual location of the coloured circle was indicated by a white outline of a circle. The coloured circle instructing the response appeared at the same time as the irrelevant finger action stimulus. For participants given orange to index finger and purple to little finger stimulus–response mappings, the first trial is spatially and imitatively compatible, while the second is spatially compatible but imitatively incompatible. Prior to theta-burst stimulation, 144 baseline trials were presented in two blocks (preceded by 24 practice trials). After theta-burst stimulation, 288 trials were presented in four blocks. Each of the four trial types (as listed in table 1) was presented an equal number of times in each block in a pseudo-random order.

Figure 2.

Mean ± s.e.m. of automatic imitation effects (RT in imitatively incompatible − RT in imitatively compatible trials) for spatially compatible (white bars) and spatially incompatible (grey bars) trials in each of the three rTMS conditions.

Figure 3.

Illustration of the delay hypothesis. ‘SC’ and ‘SI’ indicate time of response selection in spatially compatible and spatially incompatible trials, respectively. As time (left to right along the x-axis) passes after the onset of the irrelevant movement stimulus, the automatic imitation effect builds up and then declines. In the baseline condition (solid line), the perceptual-motor translation process is not delayed and therefore the build-up begins immediately; in the rTMS to IFG condition (dashed line), the translation process is delayed and thus the build-up of the automatic imitation effect begins later. SC and SI represent the time points at which responses are selected in spatially compatible and spatially incompatible trials, respectively. In the baseline condition (solid line), response selection at both of these times will result in automatic imitation effects of similar sizes. However, in the delayed IFG condition (dashed line), response selection in spatially compatible trials (early) will result in a smaller automatic imitation effect than response selection in spatially incompatible trials (late).