Menz, Freeman (2003)
Stereoscopic Depth Processing in the Visual Cortex: a Coarse-to-Fine Mechanism.
Nature Neuroscience.
6: 59-65
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For animals with frontally positioned eyes, left and right views of a scene in depth are slightly different. This binocular disparity is interpreted by the brain to provide stereoscopic depth perception. To do this, the visual cortex must combine the correct features between the two eyes and avoid incorrect or false matches. This choice has to take into account the level of detail in the visual field since cortical neurons are selective for spatial frequency. The spatial scale analysis includes both coarse and fine processes. In principal, a coarse scale can unambiguously be used for a rough estimate of depth, but a fine scale is required for adequate resolution. A fundamental question concerning the neural processing of depth is how information is combined across spatial scale. An important theoretical conjecture is that coarse processing should precede and constrain that of fine detail . We present here three types of single cell neurophysiological data that are consistent with a temporal coarse-to-fine tuning of disparity information. First, the disparity tuning of cortical cells generally sharpens during the time course of response. Second, cells responsive to large and small spatial scale have relatively short and long temporal latencies, respectively. Third, analysis of cross-correlation data between simultaneously recorded pairs of cortical cells shows that neural connections between disparity-tuned neurons are generally stronger for coarse-to-fine processing. Cross-correlation data also suggest a role for fine-to-coarse connections. These results are consistent with theoretical and behavioral studies that suggest a special role for coarse-to-fine processing. Our findings are compatible with a scheme of visual processing in which a rapid coarse percept is refined over time.