Fig. 1.

Moving stimuli were displayed continuously for 60 s while subjects reported the perceived direction of motion using computer keyboard arrows. Although the dominant direction of motion was always the actual direction, observers also reported perceiving the opposite direction for a considerable fraction of the time. Whether using rotating sunburst patterns under natural, continuous illumination (a) (n = 6), rotating radial sinusoidal patterns (b) (n = 4), horizontally drifting luminance-modulated (first-order motion) (c), or contrast-modulated gratings (second-order motion) (d) on a computer monitor with a 120- or 160-Hz refresh rate (n = 5), the c-WWI effect was always maximal for alternation rates around 10 Hz. This finding points toward a temporally specific mechanism underlying the illusion. The range and mean (± standard deviation) of preferred temporal frequencies over subjects was 10.0–13.3 Hz (mean 11.4 ± 1.6 Hz) for the data in a, 9.2–11.6 Hz (mean 10.8 ± 1.1 Hz) for b, 10.0–14.2 Hz (mean 11.6 ± 2.0 Hz) for c, and 5.8–10.8 Hz (mean 8.0 ± 2.0 Hz) for d.