Figure 1.

Visual stimuli and the behavioral task. A, Direction range stimuli (left and middle dot fields): stimuli consisted of random dots displaced in directions chosen from a predetermined distribution. The width of the distribution of directions that determined the range of directions within which, individual dots moved was varied between 0° (all dots moving in the same direction) and 360° (dots moving in all directions). Motion signal thresholds (left and right dot fields): stimuli consisted of a proportion of dots moving in the same direction in a field of dots moving in random directions (signal dots, filled circles; noise dots, open circles). B, Behavioral task: the animals maintained fixation on a target at the center of the display and reported whether the two stimuli, S1 and S2, separated by a delay moved in the same or in different directions by pressing one of the two response buttons (M123 and M601) or making a saccade to one of the two targets (M908, M202) which appeared after the offset of the fixation target. S1 and S2 were 4°–5° diameter and moved at 10°–15°/s. In separate blocks of trials, stimuli were presented in the upper visual field on either side of the vertical meridian, in the ipsilesional and contralesional quadrants. On each trial, the direction of S1 was chosen at random from a set of eight directions; S2 moved either in the same or opposite direction as S1.