Fig. 1.

Examples of the visual images used in the study.a, One frame from an animation sequence in which the contrast of a sample of 2-D noise is sinusoidally modulated along the radius. The phase of the sinusoid changes smoothly over time to produce expanding or contracting second-order motion. In the experiments, the mean luminance was the same in regions of high and low contrast (luminance distortions may have been introduced by the printing process). b, Similar to a but a case in which the noise is luminance-modulated and the amplitude of the noise remains constant to give first-order motion. c, Ahemifield checkerboard used for retinotopic mapping. The checks reverse polarity at a rate of 8 Hz to give a high-contrast stimulus that is broadband in both spatial and temporal frequency. The flickering hemifield rotates slowly about the central fixationpoint. d, A checkerboardwedge that flickers and rotates in the same way as thehemifield in c does.