The present study was aimed at deciphering whether the delay
in choice reaction time (RT) and the silent period (SP) caused
by transcranial magnetic stimulation (TMS) of the motor cortex
in the ongoing electromyogram are due to the same physiological
mechanism. To this end, the effect of TMS was studied in 6 healthy
volunteers performing a between-hand choice RT task. Specific
predictions were derived from a logic inspired from the
“postponed stages” hypothesis (Pashler & Johnson,
1989). This logic predicts a correlation between SP duration
and RT when the stimulated cortex is involved in the response,
and a stronger correlation when the stimulation is delivered
later during the RT interval. The effect of TMS on RT was twofold:
At early stimulation times, the stimulation shortened the RT
and this effect was independent of the involvement of the
stimulated motor cortex in the subsequent response. At later
stimulation times, TMS had a disruptive effect, provided that
the stimulated cortex was involved in the response. When the
stimulated cortex was involved in the response, there was a
correlation between SP and RT; this correlation was stronger
when the stimulation occurred later. In contrast, there was
no correlation between these two variables when the stimulated
cortex was not involved.