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Pinglei Bao, Chris Purington, Bosco S. Tjan; Overlap but not interact: fine grain organization of neural populations in the visual cortex of achiasmia revealed with long-term fMRI adaption. Journal of Vision 2012;12(9):790. doi: https://doi.org/10.1167/12.9.790.
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In achiasmia (lack of optic chiasm), each brain hemisphere receives retinal inputs of the full visual field from the ipsilateral eye. Testing a 23-year-old male with achiasmia (confirmed with MRI) revealed that the retinotopic representation of the ipsilateral visual field in V1-V3 is a mirror image of the contralateral field representation. The two are superimposed such that two symmetrical points in the visual field across the vertical meridian are represented at the same cortical location. Nevertheless, psychophysical testing showed no detectable interaction between the left and right visual field, suggesting that even though the neurons with receptive fields in different hemifields are closely packed in V1-V3, they do not interact. We tested this hypothesis with a long-term fMRI adaptation paradigm (Fang et al., 2005, 2007). The subject was tested monocularly while performing a demanding fixation task. The adaptor and test stimuli were counter-flickering Gabors presented in the lower field on either side of the fixation. As expected, we observed in V1-V3 a release from adaptation when the test was at the same location as the adaptor but with orthogonal orientation. Critically, we also observed a release from adaptation when the test was at the location symmetric to the adaptor across the vertical meridian, irrespective of orientation. For V2 and V3, the amplitude of release for a symmetric-location test was the same as for the same-location test with orthogonal orientation. For V1, however, the release for the symmetric-location test was significantly higher than the same-location orthogonal test. These results are consistent with the hypothesis that inputs from the different hemifields might be organized in columns akin to the ocular dominance columns (ODC) in V1, which are coarser in size than the orientation columns. Such "hemifield dominance columns" disappears in V2 just as ODC is absent from V2 in normally sighted individuals.
Meeting abstract presented at VSS 2012
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