Abstract
Stereo “3D” depth perception requires the visual system to extract binocular disparities between the two eyes' images. Several current models of this process, based on the known physiology of primary visual cortex (V1), do this by computing a piecewise-frontoparallel local cross-correlation between the left and right eye's images. The size of the “window” within which detectors examine the local cross-correlation corresponds to the receptive field size of V1 neurons. This basic model has successfully captured many aspects of human depth perception. In particular, it accounts for the low human stereoresolution for sinusoidal depth corrugations, suggesting that the limit on stereoresolution may be set in primary visual cortex. An important feature of the model, reflecting a key property of V1 neurons, is that the initial disparity encoding is performed by detectors tuned to locally uniform patches of disparity. Such detectors respond better to square-wave depth corrugations, since these are locally flat, than to sinusoidal corrugations which are slanted almost everywhere. Consequently, for any given window size, current models predict better performance for square-wave disparity corrugations than for sine-wave corrugations at high amplitudes. We have recently shown that this prediction is not borne out: humans perform no better with square-wave than with sine-wave corrugations, even at high amplitudes. The failure of this prediction raised the question of whether stereoresolution may actually be set at later stages of cortical processing, perhaps involving neurons tuned to disparity slant or curvature. Here we extend the local cross-correlation model to include existing physiological and psychophysical evidence indicating that larger disparities are detected by neurons with larger receptive fields (a size/disparity correlation). We show that this simple modification succeeds in reconciling the model with human results, confirming that stereoresolution for disparity gratings may indeed be limited by the size of receptive fields in primary visual cortex.
Highlights
Human 3D depth perception is highly precise, with the ability to detect disparities between the two retinal images of less than the width of one photoreceptor [1]
Given that we are claiming our results show that disparity resolution is limited by activity in primary visual cortex, it is important to be clear how the idealized cross-correlation computed in our model relates to more realistic models of individual neurons
We have found that incorporating a size/disparity correlation into a correlation-based model makes it perform consistently for random-dot patterns depicting both square-wave and sine-wave disparity gratings
Summary
Human 3D depth perception is highly precise, with the ability to detect disparities between the two retinal images of less than the width of one photoreceptor [1]. It has very poor spatial resolution [2,3,4]. This can be demonstrated, for example, by using random-dot patterns to depict a corrugation in depth. The disparities between dots visible to the left eye (red) and right eye (green) vary sinusoidally as a function of vertical position in the image. We shall refer to this kind of stimulus, pioneered by Tyler [3], as a sinusoidal disparity grating, by analogy with the luminance gratings pioneered by Schade [5]
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