A stimulus artefact undermines the evidence for independent ON and OFF channels in stereopsis

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 the stimuli used in all these experiments contain a subtle artefact. 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. Since our current theories suggest stereopsis is based on a correlation-like computation in primary visual cortex, it is then unsurprising that performance was better for the mixed-polarity stimuli. We conclude that there is currently no evidence supporting separate ON and OFF channels in stereopsis.

Comparisons of Two Variants of the Method of Constant Stimuli for Estimating Difference Thresholds

The two-alternative forced-choice (2AFC) and the reminder tasks are variants of the method of constant stimuli. One or the other task is usually employed for estimating the difference limen (DL) in psychophysical research. Lapid, Ulrich, and Rammsayer (2008) found that the 2AFC task yields larger DLs than the reminder task for duration discrimination judgments. The results of the present paper confirm that this discrepancy also generalizes to discrimination judgments about nontemporal, visual information (Experiment 1: Random dot pattern discrimination; Experiment 2: Line-length discrimination). It is argued that this discrepancy is related to the presentation order of the standard and the comparison stimulus.