Fig. 1.

Sequence of events during the discrimination tasks. (A) CD task; the fixation target (FT) and the 2 circles appeared simultaneously in the center and at both sides of the monitor screen, respectively. The monkey initiated the trial by fixating the FT. Fixation had to be maintained during the trial otherwise it was aborted. When the FT disappeared and after a variable prestimulus delay (PSD) (100–300 ms) 2 stimuli (oriented lines S1, S2) each of 500 ms duration, appeared in sequence separated by a delay of 1 s. Once S2 had disappeared, the monkey made an eye movement to one of the two circles to indicate whether the orientation of S2 was to the left or to the right of S1. Correct discriminations were rewarded. Masking white noise was present during the trial. S1 and S2 changed randomly from trial to trial. (B) FDIR task ; same temporal sequence and stimuli set as in the CD task, except that S1 was not shown (i.e., it was implicit) and had to be recovered from LTM by trial and error. The implicit stimulus changed from block to block of about 90 trials each and only S2 changed trial by trial. The interval between trials changed randomly in the 2 tasks between 1.5–3.5s. (C) Distribution of the orientation of the stimuli used in the CD and FDIR tasks. We used 3 S1 and 8 S2 for each S1. (D) Sketch of the brain showing the localization of the recording area.