Slide 3

Contrast adaptation in the visual system has traditionally been studied through the presentation of sinewave gratings of varying contrast (but equal average luminance). In this experiment, performed by Blakemore and Campbell (1969), the human subject was asked to adjust a potentiometer that controlled the level of contrast of a visual display. The goal of the subject was to make the display just visible. Then, the subject was adapted for one minute to a high contrast sinewave grating (middle trace in top set). Following this, the subject once again adjusted the test contrast to be just visible. The out put of the potentiometer is shown in the lower right, with down corresponding to an increase in contrast.

This result demonstrates that following adaptation to high contrast, the perceptual ability to see low contrast is markedly reduced and slowly recovers over about 30-40 seconds. This result indicates that the visual system adapts to changes in contrast with a time scale of 10's of seconds.

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Bottom part of the figure is from: Blakemore, C.B., and Campbell, F.W. (1969) On the existence of neurones in the human visual system selectively senstive to the orientation and aize of retinal images. J. Physiol. 203: 237-260.