Experimental Task . The monkey released a lever when it detected a colour change at fixation. The colour change could occur at any moment in time following fixation onset, starting from 200 ms post-fixation onset to 3 seconds post fixation onset. While the monkey fixated, a stimulus located at 4° of eccentricity was displayed (a physically isoluminant sinusoidal grating; diameter: 1.2°; spatial frequency: 0.4-0.8 cycles/deg; drift velocity: 0.6 deg/s drifting within a circular aperture). Stimulus contrast varied between zero and an 82% contrast value.