Monkeys were trained to perform a modified version of a non-match-to-sample visual WM task (Figure 1A). They fixated on a point on the left or right (50% of trials randomly) of a computer screen. An object briefly appeared as a sample in the center of the screen, thus in the right or left visual hemifield, respectively (Figure 1B, left). The sample could be one of two different objects, at one of two different locations slightly above or below the center of the screen. The monkeys were required to remember both the object identity and location over a blank delay and then compare it with a test object. If it did not match the sample in either identity or upper/lower location, they were trained to saccade directly to it. Otherwise, they held fixation until a second, always non-matching test object appeared.
