Abstract
Background: Absolute and relative disparity cues are crucial for coarse and fine depth encoding, respectively. In human and non-human primates (NHPs), relative disparity cues are preferentially encoded within thick-type cortical columns, distributed within extrastriate visual cortex. In NHPs, electrophysiological evidence for absolute disparity encoding is limited to areas V1 and MT. However, neural mechanisms underlying absolute disparity encoding in human extrastriate cortex are largely unknown. This information is crucial in terms of defining the cortical sites affected by amblyopia, a developmental disorder caused by disruption of symmetric binocular visual input early in life, with significant impact on stereopsis. Methods: We used high resolution fMRI (7T) to test the response to absolute vs. relative disparity in seven individuals with normal vision, plus five amblyopic (3 strabismic and 2 anisometropic) individuals with impaired stereopsis (stereoacuity >250 arc sec; randot test). Stimuli were generated using random dot stereograms. In each individual, motion- and color-selective clusters were localized within areas V2, V3 and V3A based on independent scans (see also Kennedy et al. abstract). Results: In addition to area MT, absolute disparity evoked a significant response within V3 and V3A (but not V2) motion-selective clusters. This activity was significantly stronger than the response to relative disparity. Outside motion-selective clusters, those clusters that showed a significant response to relative disparity showed a weaker response to absolute disparity. Color-selective clusters did not show any significant response to either relative or absolute disparity. Motion-selective clusters were detected across V3 and V3A in amblyopic (as in non-amblyopic) individuals (see also Kennedy et al.). However, in amblyopic individuals, we did not find any significant response to absolute/relative disparity within/outside motion-selective clusters. Conclusion: Absolute and relative disparity are encoded within different neuronal clusters across areas V3 and V3A. Development of absolute and relative disparity encoding mechanisms are impaired by amblyopia.