Color

Negative Afterimage

Before continuing, try , following the instructions in its caption. The afterimage you saw is largely due to adaptation of cones in the retina and the way the retina’s output influences color-opponent cells in the lateral geniculate nucleus and cortex. In each case, the afterimage is opposite of the adapting color.

Color-Opponent Simulation

simulates the activity of cones and color-opponent cells. After using it to explore how different colors affect the cones and opponent cells, check the box that allows cones to adapt and see how it simulates the negative afterimage.

McCollough Effect

In 1965, Celeste McCollough described a color aftereffect that is still not well understood. Try (it takes 3-5 minutes of adaptation to get a good effect, but you may blink, move your eyes, or rest your eyes during adaptation). After the adaptation period, look at the test patterns. The effect may be stronger if you wait a few minutes. It is subtle but long lasting; come back to it after an hour or two and look at the test patterns again. If you do the adaptation several times, you may even find the aftereffect persisting for several days. This effect is not as easily explained as the usual afterimage; see Humphrey and Goodale (1998) for a review of recent work.

Further Exploration

Use to experiment with the afterimage. This version allows you to set the adaptation period and measure the duration of the afterimage. You can even import your own image for testing. Try some of these activities:

• Plot afterimage duration vs. adaptation time. Try it for different colors. Is the strength of the afterimage the same for all parts of the retina? Try the experiment while fixing your gaze at a corner of the window rather than at the black dot.
• After a period of adaptation, look at a distant white wall or out the window at the sky. Why does the afterimage now appear enormous?
• Adapt to blue, and then look at cyan. What do you expect it to look like? Test your prediction. Try adapting to cyan and then looking at red. Make a prediction and then test it. Are all of these results consistent with the color-opponent explanation? (Check using .)
• Adapt with one eye closed, then test for an aftereffect with the other eye open and the adapted one closed. Is there an afterimage? Would you expect one? Now try it with the McCollough effect (). Does it work any better? What do these two experiments test?
• In , does it matter that the vertical and horizontal stripes are opposing colors (e.g. green and magenta)? Try other combinations (e.g. red and blue).
• Does generalize to other grating sizes? Stimulate with one size and then test with another size; does the aftereffect persist when you change the size? If the effect is general across sizes, can you force it to be size-specific? Stimulate with large stripes in one color set (green vertical and magenta horizontal) until you get a robust afterimage. Next, stimulate with small stripes of the opposite colors (magenta vertical and green horizontal). When you change the size of the test pattern, does it switch tint to fit the color/size combinations you used, or does the tint disappear?

Questions

1. In school, you were taught that red and green are complementary colors, so why does red give a bluish-green (cyan) afterimage instead of a green one? Why does green give a magenta afterimage rather than a red one? Why do blue and yellow work as expected? ( may help you with these questions.)
2. The adaptation image has a hole in it. What color is the hole in the afterimage? What might account for this? (See .)
3. How does adaptation to black cause a “whiter-than-white” afterimage? Would adaptation to white cause a “blacker-than-black” afterimage?
4. Why does the afterimage outlast the adaptation time?
5. Why is the color-opponent theory unable to explain the McCollough effect?

References

• Humphrey GK, Goodale MA (1998). Probing unconscious visual processing with the McCollough effect. Consciousness and Cognition 7:494-519. []
• McCollough C (1965). Color adaptation of edge-detectors in the human visual system. Science 149:1115-1116. []
• McCollough C, Webster MA (2011). McCollough effect. Scholarpedia, 6(2):8175. []