Figure 1.

Stimulus displays. Participants viewed displays containing a single square-wave grating in the upper left or right visual field. On each trial, the grating was assigned one of nine orientations (0–160° in 20° increments) and one of two colors (yellow or green). In separate scans, participants were instructed to attend either the orientation or luminance of the grating. During attend-luminance scans, participants discriminated the direction (clockwise or anticlockwise) of brief and unpredictable perturbations in stimulus orientation. During attend-luminance scans, participants discriminated the direction (brighter or dimmer) of brief and unpredictable perturbations in stimulus luminance.