Fig. 1.

Stimuli and task. Sequence of events in a trial: trials began with a central white fixation point (0.3 × 0.3° visual angle) presented simultaneously with a surrounding red circle (radius, 1° visual angle; duration, 800/850 or 1,200/1,250 ms) against a black background. Two superimposed circular, high-contrast red dot patterns then appeared, one rotating clockwise and the other counterclockwise (radius, 3.5° visual angle; rotation 1, 500 ms). Sixty percent of the dots in 1 of the dot fields then translated in 1 of 4 possible directions, selected at random (translation 1, 100 ms) while the remaining dots in this dot field, and dots in the other dot field, continued rotating in their original directions. This first translation served to exogenously direct attention to the translating dot field. The dot pattern undergoing this first translation is therefore referred to as the “cued surface.” After this first translation, both surfaces rotated as before (rotation 2, 50 ms), and then a second brief translation occurred; 60% of dots in 1 of the dot fields translated in 1 of 4 possible directions, selected at random (translation 2, 100 or 50 ms). After this second translation, both dot fields again rotated as before (rotation 3, 250 ms). Subjects could respond during translation 2, up to 500 ms after the onset of rotation 3. They reported the direction of the second translation by pressing 1 of 4 keys.