SUBTRACTIVE AND DIVISIVE ADAPTATION IN THE HUMAN VISUAL-SYSTEM

SENSORY systems can adapt to the conditions imposed on them 1. 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 2-4. Most neurons of the striate cortex reduce their responsiveness after being stimulated for some time by a high-contrast pattern 5-7. Such an effect may lie behind these psychophysical adaptation phenomena 2-4. 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 8. Neurophysiological evidence suggests that neurons with different orientation preferences have inhibitory interconnections 9-13. 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.