Figure 1

Computerised delayed matching to sample task. (a) In the DMS used for testing monkeys, each trial commenced by the appearance of a start-cue (an image), and subjects pressed and held a switch. Pressing the switch changed the start-cue to a fixation point, which was replaced by a sample (coloured shape) if the switch was still held. The sample was shown for 1000 ms before disappearing. Following a delay, the four test items appeared. The delay period randomly varied between 1500 ms, 3000 ms, and 4500 ms. If the correct matching item (target) was selected, feedback was provided (the target item flashed off (for 200 ms) and on (for 200 ms). The selection of the wrong target, early switch release, or failure to respond within the time window (4000 ms) were erroneous responses and led to the disappearance of all items, and an error signal was displayed for 500 ms (pink annulus). In shape-DMS trials, the test items had the same colour as the sample but differed in shape, and therefore the participants were required to match by shape. However, in colour-DMS trials, the test items had the same shape as the sample but differed in colour, and therefore the participants had to match by colour. Shape- and colour-DMS trials were intermingled. In the DMS used in monkeys, three test items were presented on the left, right and bottom of the screen during the matching period. In Experiment 1, the delay was fixed at 1500 ms, while in Experiment 2, the delay was randomly varied (1500 ms, 3000 ms, or 4500 ms). Upon selection of the correct target, the monkeys received a liquid reward (water). (b) In the DMS used in humans, the trial structure was similar, however, four test items were presented on the left, right, top and bottom of the screen during the matching period (900 ms).