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Antony B. Morland, Michael Hoffmann; Retinotopic organisation of the visual cortex in human albinism. Journal of Vision 2002;2(10):46. doi: 10.1167/2.10.46.
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The albino visual cortex receives input from the ipsilateral visual field. To investigate how the visual cortex of humans albinos organizes this abnormal input we applied retinotopic mapping fMRI procedures. Two subjects with albinism and only small nystagmus and two control subjects underwent T2* MRI scanning of the occipital lobe during visual stimulation. In separate experiments we stimulated monocularly the nasal and temporal retina with phase encoded visual stimuli (Engel et al. 1997). BOLD responses were projected to the flattened representation of T1 weighted images, fourier analysed and correlated with the stimulus fundamental frequency. Retinotopic mapping yielded phase maps that allowed the identification of V1 and dorsal and ventral representations of V2 and V3 in both controls and albino subjects. In the controls V1 comprised a representation of the contralateral visual field, while it comprised a representation of both the contralateral and the ipsilateral visual field in the albino subjects. The normal contralateral and the abnormal ipsilateral representations are, at the resolution of fMRI, arranged as an overlay. We obtained evidence for a similar arrangement in other early visual areas. Our results indicate that, in the albinos tested, there has been no reordering of the geniculostriate projection in human as has been reported in other species. Furthermore, there appears to be an absence of the complete suppression of the abnormal input to the cortex that has also been documented in cat and ferret. In the human albino, we conclude that representations of mirror symmetric positions in the visual field occupy neighbouring regions of the cortex. This feature may have behavioural significance for tasks performed in regions of the visual field where fibres project aberrantly.
Engel, S.A., Glover, G.H. & Wandell, B.A. (1997) Retinotopic organization in human visual cortex and the spatial precision of functional MRI. Cereb. Cortex, 7, 181–92.
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