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). The neural implementation of this correspondence process is incompletely understood. Current models suggest that the monocular images undergo substantial processing before any binocular comparison is possible, and that the final result of this monocular processing is a simple output (response rate of a single neuron). This processing integrates image information over finite regions of visual space. If binocular comparisons are made after this integration, then only a coarse spatial map of disparities should be visible. We recorded the activity of disparity selective neurons in the visual cortex of awake behaving animals, in response to sinusoidal variations in disparity over space, at different scales (spatial frequencies). The neuronal responses did indeed seem to be limited to a coarse spatial representation of disparity, limited by the size of each neuron's spatial area of integration. This coarse representation in the neuronal signals closely matches psychophysical measures of the ability to detect disparity changes over space. It has been known for may years that the human resolution for such disparity modulations is coarse, but this phenomenon has never been explained. 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 the previously unexplained limitation of human stereo resolution.

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National Eye Institute (NEI)
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U.S. National Eye Institute
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Bredfeldt, C E; Read, J C A; Cumming, B G (2009) A quantitative explanation of responses to disparity-defined edges in macaque V2. J Neurophysiol 101:701-13
Haefner, Ralf M; Cumming, Bruce G (2008) Adaptation to natural binocular disparities in primate V1 explained by a generalized energy model. Neuron 57:147-58
Read, Jenny C A; Cumming, Bruce G (2007) Sensors for impossible stimuli may solve the stereo correspondence problem. Nat Neurosci 10:1322-8
Nienborg, Hendrikje; Cumming, Bruce G (2007) Psychophysically measured task strategy for disparity discrimination is reflected in V2 neurons. Nat Neurosci 10:1608-14
Bredfeldt, Christine E; Cumming, Bruce G (2006) A simple account of cyclopean edge responses in macaque v2. J Neurosci 26:7581-96
Nienborg, Hendrikje; Cumming, Bruce G (2006) Macaque V2 neurons, but not V1 neurons, show choice-related activity. J Neurosci 26:9567-78
Read, Jenny C A; Cumming, Bruce G (2005) Effect of interocular delay on disparity-selective v1 neurons: relationship to stereoacuity and the pulfrich effect. J Neurophysiol 94:1541-53
Read, Jenny C A; Cumming, Bruce G (2005) The stroboscopic Pulfrich effect is not evidence for the joint encoding of motion and depth. J Vis 5:417-34
Read, Jenny (2005) Early computational processing in binocular vision and depth perception. Prog Biophys Mol Biol 87:77-108
Nienborg, Hendrikje; Bridge, Holly; Parker, Andrew J et al. (2004) Receptive field size in V1 neurons limits acuity for perceiving disparity modulation. J Neurosci 24:2065-76

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