with ROB VAN LIER and MARK VERGEER
Gaze steadily at the cross, ALWAYS! Without moving your eyes, note the colors of the squares (red, green, blue, yellow). Every so often, different colors will flash up briefly. These colors are afterimages–not on the screen, but in your head!
The adapting plaid, below, consists of a blue/yellow vertical grating, superimposed on a red/cyan horizontal grating. After adapting to this plaid, vertical black test lines make the afterimage look yellow/blue, while horizontal test lines make the after image look cyan/pink. Thus one and the same adapting pattern gives differently colored afterimages.
Conclusion: the visual system averages after image colors within but not across luminance test contours.
Below, second-order test stripes defined only by motion give the same BY and RC afterimages. These are not first-order test contours defined by luminance, but are second-order contours defined only by motion. But the horizontal bars still look blue/yellow and the vertical bars look red/green.
Above: The “+” test field looks red and green, while the “Tic-Tac-Toe” test field looks blue and yellow. All from one and the same adapting field.