Binocular stereopsis is the ability to use differences between the images presented to the two eyes (binocular disparities) to perceive the three dimensional structure of the outside world. In order to detect that an object has a binocular disparity, it is first necessary to correctly match up the images of that object in the two eyes (the stereo correspondence problem). Humans are able to do this very robustly, even when the two eyes are shown random patterns generated by computers (random dot stereograms). However, there are certain striking limitations on the human ability to solve this correspondence problem. As investigated last year, the ability to detect spatial variation in disparity has poor acuity. We have now extended the same principles in an attempt to understand why humans are poor at detecting temporal variation in disparity: if two disparities are shown alternating over time, even at moderate frequencies (8Hz or so), human observers are unable to detect the alternation. We explored the ability of cortical neurons to detect this sort of disparity modulation, and found that single neurons (recorded from the primary visual cortex of awake behaving monkeys) are also poor at representing such signals: Neurons that can modulate their responses rapidly to changing luminance are unable to modulate their responses so rapidly to changing disparities. Thus, neurons at the earliest stage of binocular processing seem to be responsible for the limit in temporal acuity for disparity modulation. We developed a model that accounts quantitatively for this phenomenon, and showed that this sluggish temporal reponse is in fact an inevitable result of the mechanism by which neurons generate reponses that are selective for different disparities. Thus, it appears that the mechanism by which disparity signals are generated early in cortical processing, as part of solving the stereo correspondence problem, accounts for previously unexplained temporal limitations of human stereo resolution.
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