Figure 1.

Psychophysical procedure. (A) Half cycle of sinusoidal stimulus modulation (1/32 Hz presented at 88 frames/sec depicted for this figure at 1.45 seconds and 0.09 π radian intervals). (B) Approximate percepts corresponding to the stage of stimulus modulation immediately above. The two halves reach perceptual identity before they become physically identical. The afterimage lasts for a significant period of time after the cessation of the stimulus modulation. (C) Segment of trial around the point of identity expanded to show how the clock face is used to make the measurement.