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 froma go signal to handmovement.No local effectwas found with Time 1, since pulseswere
administered before subjects opened their eyes. Reduction of reaction timewas observed at Time 2 when
stimuliwere 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.
