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Seiji Tanabe, Ichiro Fujita; Reduced binocular disparity selectivity of V4 neurons to anti-correlated random-dot stereograms. Journal of Vision 2003;3(9):62. doi: 10.1167/3.9.62.
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A plane lying in depth is vividly perceived when a random-dot stereogram (RDS) is fused with slight binocular disparity. By reversing the contrast of dots seen by one of the eyes to generate an anti-correlated RDS, the perception of depth is lost or diminished, and the stimulus is perceived as merely rivalrous. This perceptual phenomenon became a useful tool for testing whether neural responses in the striate cortex (Cumming & Parker 1997) and extrastriate cortical areas MT and MST (Krug et al. 2000; Takemura et al. 2001) correlate with stereoscopic depth perception. When anti-correlated RDSs are used as stimuli, only a small number of neurons in these areas lose their disparity selectivity while the majority still modulate their responses dependent on binocular disparities. Neuronal activities in these areas thus do not correlate with depth perception. We examined neural responses to anti-correlated RDSs in area V4, where disparity selectivity of neurons to bar stimuli has recently been demonstrated (Hinkle & Connor 2001; Watanabe et al. 2002). We recorded extracellular action potentials from 52 and 27 V4 neurons in two awake, fixating monkeys (Macaca fuscata). Twenty-four cells (12 each from the 2 monkeys) significantly modulated their responses according to disparities in correlated RDSs covering their receptive fields (ANOVA p<0.05). The majority of these cells (N=18/24) lost their selectivity for disparity when the RDS was anti-correlated (ANOVA p>0.05). For a subset of the cells that were also sensitive to disparities in anti-correlated RDS (3 out of the 6 cells), the disparity tuning curves followed a Gabor function. The tuning curves were shifted by π between correlated and anti-correlated RDSs, which is reminiscent of the characteristic of striate neurons. Our results suggest that responses to false-matches between contrast-reversed dots in the left and right eye images elicited in the striate cortex are substantially reduced in or before V4.
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