The go/nogo reaction time task has been frequently used to assess volitiona
l inhibition. Psychophysiological studies of the correlates of the go/nogo
decision have almost exclusively been concerned with N2 and P3 potentials o
f the event-related potentials (ERPs). However, in studies where the EMG on
set latency was available, it was obvious that this latency was shorter tha
n or at least equal to the latencies of the studied cerebral potentials. In
this study, by concurrent recording of the EEG and EMG activity we aimed t
o better define the temporal relationship between cortical activity and mot
or response. Particularly, we wanted to identify the early (i.e. pre EMG-on
set) electrophysiological correlates of the nogo decision. We used a modifi
ed S1-S2 paradigm that involved a two-stage go/nogo decision. In this task
both S1 and S2 were informative and required the subject to make a decision
, but the nature of the decision differed. The decision at SI involved whet
her to prepare a movement, whereas the decision at S2 involved whether to i
nitiate or inhibit an already prepared response. To better visualise the no
go decision related components of the ERPs, the go ERPs were subtracted fro
m the corresponding nogo ERPs and difference ERPs were formed. Before EMG o
nset, a small negative component common to both go/nogo difference traces a
nd corresponding roughly with the N1 wave was detected. It is suggested tha
t this early negativity may be a more specific electrophysiological reflect
ion of the nogo decision proper.