Independent neural mechanisms for bright and dark information in binocular stereopsis.

Early visual processing is organized into a number of independent channels. In the retina, increments and decrements of brightness are processed independently by different groups of neurons. For psychophysical measurements of human vision, independence can be tested statistically. Using this criterion in a depth judgement task, we show here that, for binocular stereo vision, increments and decrements are treated independently, at least as far as the level at which information from the left and right eyes is first combined. At later stages of stereo processing, the information from the two channels is no longer independent. Because the signals for stereo vision are first combined at the visual cortex, these results suggest that the neural 'on' and 'off' channels remain independent right up to early cortical stages. Theoretical studies of stereo vision have proposed that visual features in the views of the two eyes are matched on the basis of 'similarity'. Our results show that stereo matching treats features as statistically independent (and therefore dissimilar) if they appear perceptually bright and dark relative to the background. If features differ perceptually but only in the degree of brightness or darkness, human stereo vision treats them as similar.