The role of inhibition in the hierarchical gating of executed and imagined movements.

A theory is presented concerning the neuronal mechanisms which may underlie the organisation of imagined versus executed movements. A review is first presented of previous theoretical and experimental evidence suggesting that the brain can use the same mechanisms for the imagination and the execution of movement. In particular the fact that adaptation of the vestibulo-ocular reflex can be obtained by pure mental effort and not solely by conflicting visual and vestibular cues has been suggestive of the fact that the brain could internally simulate conflicts and use the same adaptive mechanisms used when actual sensory cues were in conflict. The saccadic system is taken as a good model for the study of this question because the mechanisms which underlie saccade generation are now partially understood at different levels from the brain stem to the cortex. The central idea of the theory is based upon the fact that, in parallel with the excitatory mechanisms underlying saccade generation, several inhibitory mechanisms in cascade allow the selective modulation and blockage of saccades. Synaptic inhibition is therefore supposed to play a major role in a hierarchical selective gating of saccade execution not at one but at several levels allowing a variety of different types of "imagined movements' some involving only the higher levels some in which the execution is only blocked at the very immediate premotor level. But in all cases the theory proposes that imagination and execution have many mechanisms in common. PET data showing that indeed the same structures are activated in both types of movements support this idea although the final answer will have to be brought by neuronal data.