AFTERIMAGES AND SUCCESSIVE CONTRAST

Figure 3.8. Afterimages and successive contrast. Stare fixedly at the centre of circle A for at least 20 seconds, then immediately look at the centre of circle B for about ten seconds, noting the changing afterimage. Each sector will display an afterimage the colour of the additive complement of the stimulus colour. Repeat the procedure, this time looking immediately at the centre of circle C. Now the colours of the afterimage in each sector will influence the appearance of the red colour in an example of successive contrast.

The coloured afterimages seen after exposing the eye to coloured light for an interval of time result from changes in the relative adaptation of the three cone types. Adaptation is the main process that permits the eyes to adjust to the dark, and involves changing sensitivity of the receptor cells in response to different levels of illumination. After a period of exposure to coloured light, the cone type or types that are relatively weakly stimulated by that light, due to a paucity of certain wavelengths, become proportionately dark-adapted. When neutral light is restored, a temporary illusion of a light composed of the "missing" wavelengths is seen.

Ewald Hering cited coloured afterimages as evidence for his opponent model of colour vision, and very numerous subsequent authors down to the present have repeated the assertion that afterimages take the colour of Hering's opponent colours. They do not - they in fact take the colour of the additive compliment - that colour of light that mixes with the stimulus to make white light. For example, the afterimage seen after viewing red is cyan, not green. The afterimage takes the colour of the opposite stimulus, not the opposite colour experience.

Coloured afterimages influence the apparent colour of objects viewed subsequently to the stimulus, causing the phenomenon known as successive contrast. Consequently, successive contrast also goes toward the additive complementary. Note in Figure 3.8C that the afterimages of red, yellow and magenta, the three colours that contain red light, dull the appearance of the red, while the afterimage of the other three colours intensifies it.

Successive contrast is the actual explanation of the experience often mislabelled "fatigue" of the eye - for example, the apparent dulling of a high-chroma red surface after being examined fixedly for a few seconds. In looking at a red object the L cones are not being "fatigued" any more than they are in looking at a white object - the effect is caused by the increased sensitivity of the other cone types.

For some effective demonstrations of coloured afterimages see here (link 1,2,3). The demonstration involving flashing colours (link 4) shows that a faint afterimage is noticeable even after a very short interval of viewing a high-chroma surface

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