Figure 4.

Stimuli for Experiment 4: Two luminance sine waves with spatial and temporal frequencies (f_x, f_t) and (2f_x, 2f_t) are added or subtracted to produce peaks-add or valleys-add waveforms that are tested for motion-direction perception. (a, b) Two consecutive frames of spatial sine waves of frequencies 2f_x and f_x. (b) The sine wave f_x is shifted 90° rightward in Frame 2 as indicated by the arrow. (a) The 2f_x sine wave is shifted the same physical amount as f_x but this is now 180°, so the direction of shift is ambiguous, as indicated by the two arrows. (c) Two successive frames of f_x + 2f_x (as above) in peaks-add configuration (left), (d) valleys-add configuration. The arrow (t) indicates the direction of time, the horizontal arrows indicate the direction of motion. (e) Five frames of a motion stimulus illustrating left-to-right movement (from top to bottom) of peaks-add (left) and (f) valleys-add (right) configurations.