In each trial, a fixation point is first presented
for 500 ms, followed by a letter stimulus which is either an O or an
X, together with two on-screen buttons. The task is to tap the left
button for an X, the right button for an O, as quickly as possible
(Fig a,b). In one trial in four, a stop-signal is presented. The
stop-signal consists if two horizontal red lines superimposed on the
letter display (Fig c), and appears after the onset of the letter.
When a stop-signal appears, the user should not respond to the
letter stimulus. The time between the appearance of the letter and
the stop-signal is referred to as the stop-signal delay (SSD) and
varies in increments of 50 ms. It is initially set to 250 ms, and
increases by 50 ms each time the user responds to a letter with a
stop signal (making the task easier on the next stop-signal trial)
and decreases by 50 ms if the subject successfully withholds the
response (making the task harder).