Stimuli and procedure.

The face was presented on one of the four quadrants in one eye and the masks were presented for 100 ms per frame in the other eye. The contrast of faces increased gradually from 0% to 100% within the first second and stayed constant until the end of that trial. After 1 second from the onset of the trial, the contrast of masks was ramped down from 100% to 0% for 5 seconds. The trial stopped when participants pressed a key to indicate detection of the face, or after 6 seconds if no response was made. After detection, participants answered which side (left or right) they perceived the face.