Abstract

My thesis aims at demonstrating the existence of two parallel networks that are involved in stimulus-dependent and rule-dependent visuomotor associations. In the first experiment we demonstrated a biphasic time-course of motor cortical excitability during a counter-imitative task: a stimulus-dependent automatic simulation at 150 ms from the stimulus onset and a rule-dependent voluntary motor preparation at 300 ms. Moreover we identified two regions involved in this biphasic pattern by means of the repetitive transcranial magnetic stimulation (rTMS). The double-dissociation between the effects of offline rTMS to the lateral prefrontal and parietal cortices, on the early and late components, allowed us to hypothesize the presence of two different anatomo-functional pathways: a parieto-(premotor) network mediating early stimulus-dependent responses and a (temporo)-prefrontal network producing late rule-dependent responses. Thus, the first experiment adds important information on the mechanisms by which the brain is both tuned to produce imitative responses in a fast automatic way but is also capable of overriding them by means of a parallel, more flexible, visuomotor coupling that follows arbitrary visuomotor associations. The fast imitative tendencies seemingly persist also during the online performance of a counter-imitative task. The two processes access the motor output by two partially independent neural substrates. We took a step forward in the second experiment where we investigated, by means of the combined rTMS and fMRI techniques, the functional connectivity in the rule-dependent executive pathway. We used the condition-and-map approach to highlight the brain regions where this pathway passes through to give a counter-imitative response. We hypothesised that this region could be the premotor cortex, and probably the dorsal part of it. Our data showed that exactly in the dorsal premotor cortex there is an interaction between the TMS stimulations and the tasks. Taken together these results allowed us to add a new piece in the puzzle of the rule-dependent visuomotor pathway. The further step will be to use the same paradigm to test the stimulus-dependent automatic pathway and finally to test where these two systems converge either in the premotor cortex or in the motor cortex.