Parieto-occipital cortex and planning of reaching movements: a transcranial magnetic stimulation study.

A large amount of evidence supports a role for the parietal and frontal cortex in the planning of reaching movements. Nevertheless, neither the timing of involvement of these areas nor if and how their activity can be influenced by external stimuli has been clarified. The parieto-occipital cortex has been investigated by applying transcranial magnetic stimulation (TMS) at 25% (Time 1), 50% (Time 2) and 75% (Time 3) of the reaction time from a go signal to hand movement. No local effect was found with Time 1, since pulses were administered before subjects opened their eyes. Reduction of reaction time was observed at Time 2 when stimuli were applied over the anterior occipital lobe, parieto-occipital cortex and posterior parietal cortex. The effect on the posterior parietal cortex reverted when Time 3 was used. The present data confirm the existence, in humans, of a dorso-medial set of areas involved in on-line planning of reaching movements. Moreover, they provide novel evidence on the time course of this involvement. Finally, present data show that it is possible to interact with the flow of activity along this stream by appropriately delivering TMS pulses.