The psychophysics of stereopsis can be explained without invoking independent ON and OFF channels.

Jenny C A Read,Bruce G Cumming

doi:10.1167/19.6.7

Jenny C A Read, Bruce G Cumming

Open Access

https://doi.org/10.1167/19.6.7

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Abstract

Early vision proceeds through distinct ON and OFF channels, which encode luminance increments and decrements respectively. It has been argued that these channels also contribute separately to stereoscopic vision. This is based on the fact that observers perform better on a noisy disparity discrimination task when the stimulus is a random-dot pattern consisting of equal numbers of black and white dots (a “mixed-polarity stimulus,” argued to activate both ON and OFF stereo channels), than when it consists of all-white or all-black dots (“same-polarity,” argued to activate only one). However, it is not clear how this theory can be reconciled with our current understanding of disparity encoding. Recently, a binocular convolutional neural network was able to replicate the mixed-polarity advantage shown by human observers, even though it was based on linear filters and contained no mechanisms which would respond separately to black or white dots. Here, we show that a subtle feature of the way the stimuli were constructed in all these experiments can explain the results. The interocular correlation between left and right images is actually lower for the same-polarity stimuli than for mixed-polarity stimuli with the same amount of disparity noise applied to the dots. Because our current theories suggest stereopsis is based on a correlation-like computation in primary visual cortex, this postulate can explain why performance was better for the mixed-polarity stimuli. We conclude that there is currently no evidence supporting separate ON and OFF channels in stereopsis.

Highlights

Harris and Parker (1995) made a striking claim about stereoscopic vision
We have long been puzzled by the evidence apparently supporting independent neural mechanisms for bright and dark information—ON and OFF channels—in human stereopsis
This evidence seemed to show that disparity was easier to extract in mixedpolarity random-dot stereograms—made up of black and white dots on a gray background—than in samepolarity stereograms, where all dots are black or all white

Summary

Introduction

Harris and Parker (1995) made a striking claim about stereoscopic vision. They argued for the existence of neural mechanisms for bright and dark information that make independent contributions to stereopsis. Their evidence came from the performance of their observers on a depth discrimination task made challenging by the addition of disparity noise. The stimulus was a random-dot stereogram with a vertical step edge. The mean disparity was opposite on either side of the edge, but each dot was given a noise disparity drawn from a Gaussian distribution with a given standard deviation. Observers performed better when the stimulus was made up of equal numbers of black and white dots on a gray background (Figure 1A) than when all dots were black (Figure 1B) or white (Figure 1C)

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