Reafferent copies of imitated actions in the right superior temporal cortex

Abstract

Imitation is a complex phenomenon, the neural mechanisms of which are still largely unknown. When individuals imitate an action that already is present in their motor repertoire, a mechanism matching the observed action onto an internal motor representation of that action should suffice for the purpose. When one has to copy a new action, however, or to adjust an action present in one's motor repertoire to a different observed action, an additional mechanism is needed that allows the observer to compare the action made by another individual with the sensory consequences of the same action made by himself. Previous experiments have shown that a mechanism that directly matches observed actions on their motor counterparts exists in the premotor cortex of monkeys and humans. Here we report the results of functional magnetic resonance experiments, suggesting that in the superior temporal sulcus, a higher order visual region, there is a sector that becomes active both during hand action observation and during imitation even in the absence of direct vision of the imitator's hand. The motor-related activity is greater during imitation than during control motor tasks. This newly identified region has all the requisites for being the region at which the observed actions, and the reafferent motor-related copies of actions made by the imitator, interact.

Figure 1

Coronal, transverse, and sagittal views of the STS area (in red, 45 voxels) in the right hemisphere in which signal intensity is reliably bigger during imitation compared with motor control tasks and during action observation compared with visual control tasks.

Figure 2

Time series of the active STS area in the first (Top) and second (Bottom) experiments. These graphs represent the average time series of all runs in all subjects participating in the two different experiments. Thus, each data point in the top graph is the average of 48 data points, and each data point in the bottom graph is the average of 20 data points. The order of tasks was counterbalanced across subjects in the real experiments but is obviously displayed as a fixed order in this figure. The first graph is composed of seven rest periods alternated with six tasks periods. The first three task periods correspond to the observation/execution tasks, and the last three task periods correspond to the observation tasks. The small pictures correspond to the type of stimulus presented and are used here for display purposes only. The hand with the lifted finger corresponds to the animated hand, the geometric figure corresponds to the geometric figure condition, and the hand with the small black cross on the finger corresponds to the static hand condition. The second graph is composed of nine rest periods alternated with eight task periods. The first four task periods correspond to the observation/execution tasks, and the last four task periods correspond to the observation tasks. The hand with the lifted finger corresponds to the animated hand, and the hand with the small black cross on the finger corresponds to the static hand condition. There are two left hand stimuli (animated and static) and two right hand stimuli (animated and static). In both graphs, the signal is reliably higher for imitation tasks than relative motor control tasks and for action observation tasks relative to visual control tasks (see Results).