Figure 1. Schematic drawing of the sequence of events in the different trial types presented to subjects.

Target trial: The trial started with the appearance of a fixation cross for a randomized duration followed by the appearance of two empty ‘boxes’, one at the center of the screen and at a 9-deg eccentric position. After the appearance of the two boxes, a target was flashed in the central one for 50 milliseconds. Extinction of the central target marked the beginning of the delay period that could last either 400, 900, 1400 or 1900 ms. At the end of the delay period, a target appeared for 50 ms in the eccentric box and the subject had to make a saccade to the eccentric box within a 400 ms grace period. Cue&target trial: The trial started with a fixation cross (same duration as target condition) that was followed by the cue period when a red disk was presented on the screen for one of the four durations tested randomly. A short fixation period followed disk appearance and the two empty boxes appeared on the screen. The end of the trial was similar as in a target trial but delay duration was always equal to cue duration. Probe trial: same as sequence as cue&target trial but the eccentric target appeared with a fixed probability at the end of the delay period.