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Abstract:
Partial loss of the primary visual cortex (V1) and/or its inputs leads to a scotoma of the contralateral visual hemifield, the extent of which corresponds retinotopically to the region affected. However, some patients have been found to retain a small amount of residual visual sensitivity within the blind field, a phenomenon termed blindsight, suggesting the existence of alternate pathways that transmit information from the retina to cortex effectively bypassing V1. Blindsight has been associated with activity observed in the middle temporal area complex (V5/MT+) following V1 lesions. An important issue is how the properties of area hV5/MT+, like retinotopic organization and sensitivity to motion, change following V1 lesions. We measured responses in human area V5/MT+ in 5 patients with homonymous visual field defects as a result of area V1 or optic radiation lesions using functional magnetic resonance imaging (fMRI). First, we investigated whether the organization of area hV5/MT+ changes following V1 damage. To do so, we used a recent method that estimates population receptive field (pRF) topography in the visual cortex (Lee et al., A new method for estimating population receptive field topography in visual cortex, NeuroImage, 2013). FMRI measurements were obtained during the presentation of a moving bar stimulus while the subjects were fixating. The pRF topography of area hV5/MT+ was compared with that of control subjects stimulated with matching “artificial scotomas”. In addition, we measured the sensitivity of area hV5/MT+ to coherent motion using random dot kinematograms (RDK) for both patients and controls. RDK patches were presented either inside the visual field scotoma or in the contralateral healthy part of the visual field. Subjects were instructed to report the direction of motion of the presented RDK while fixating. In both cases we found responses in hV5/MT+ arising inside the scotoma, independent of area V1 input, suggesting the existence of a functional alternate pathway bypassing area V1. The retinotopic organization of hV5/MT+ differed between patients and controls under the artificial scotoma condition, suggesting a degree of reorganization. The blood oxygen level-dependent (BOLD) response of area hV5/MT+ to RDK coherent motion stimuli also differed between patients and controls, and was dependent on which side was attended. Studying how the properties of visual areas change after injury may allow us to design better rehabilitative strategies in the future.
