Subjects performed a delayed nonmatch-to-sample working memory (WM) task (Figure 1A). They began task trials by holding gaze for 700 ms on a fixation point randomly displayed at 4.5° left or right of the center of a computer screen. A sample object was then shown for 700 ms in the center of the screen, thus in the right or left visual hemifield, respectively. Two sample objects were used each session, chosen from a commercial photo library (Hemera Photo-Objects). Sample objects were displayed in one of two positions, 3.4° above or below the screen center. After a 1.6 s delay, a test object was displayed for 400 ms. The monkeys were required to saccade to it if it did not match the remembered sample in either object identity or upper/lower location. If the test was identical to the sample, they withheld response. Then, after a 100 ms blank period, a nonmatching test object was always shown, which required a saccade. Response to the non-match was rewarded with juice, followed by a 3.2 s inter-trial interval.
