New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
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- Visual cortex
Figure 32. The relationship of the eye to the brain.
Local adaptation - afterimages
Observation of intense coloured areas for any length of time affects subsequent vision on the
corresponding areas of the retina. If we stare fixedly at a yellow object and shift our gaze after
twenty seconds to a neutral background, we shall see the complementary colour blue. This afterimage
is due to local adaptation. In the area of the retina where the yellow field was first imaged,
the sensitivities of the green and red receptor systems were reduced by prolonged exposure to a
mixture of red and green light. Thus, when the yellow field was replaced by a sheet of white
paper, red and green were subtracted from neutral white, leaving a blue image. As the receptor
systems recover their sensitivities, the afterimage fades.
Lateral adaptation
A turquoise object having a green background appears bluer than when placed against a blue
background, which gives it a green tinge. Further, it appears lighter against a black background
than a white one.
General colour adaptation
In viewing a given scene, the visual system adapts its colour sensitivity in such a way that the
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