Subtractive and divisive adaptation in the human visual system

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Abstract
Sensory systems can adapt to the conditions imposed on them. In the visual system, adapting to a pattern increases the threshold of the ability to see that pattern, and reduces the perceived contrast of the pattern above threshold. Most neurons of the striate cortex reduce their responsiveness after being stimulated for some time by a high-contrast pattern. Such an effect may lie behind these psychophysical adaptation phenomena. These adaptation effects have been reported to be confined to patterns of similar orientation, which is understandable in that the visual neurons that adapt are only excited by a small range of orientations. Neurophysiological evidence suggests that neurons with different orientation preferences have inhibitory interconnections. It is therefore of interest to explore the possible effects of these connections on perception. Here we show that adapting to a horizontal pattern can reduce the perceived contrast of a vertical test pattern more than a horizontal test pattern. These 'cross-orientation' effects are modelled by a division-like process, whereas the more normal 'similar-orientation' effects are modelled by a subtractive process.