Stop Signal Task

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).

(a)

(b)

(c)

 

This test can be run on a tablet or on a Java-enabled mobile phone. 

 

PenScreen Home Test Descriptions