Leon Festinger (Festinger, Allyn and White) did research on Benham's top. The article (I can't find the exact reference) was in Vision Research and it was, I think, in 1971. Essentially, they imagined that the visual system was coding the transitions from black to white as the disk rotated. Half the disk is all black and half is white but with sectors composed of black arcs. They thought the visual system treated these arcs as gray. So the sector that looked red be off (black) then on at half maximum brightness for 1/8 of a cycle (an arc), then fully on (the white). Then they coded white lights (with square wave onsets and offsets) to go on and off in the appropriate codes. This set-up generated the appropriate, though weak, color sensations. Sorry I can't find the reference. Paul Schulman
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