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F.G. Rauscher, G. Plant, J.L. Barbur; Patterns of Colour Vision Loss That Result From Damage to Pre–Striate and Extra Striate Visual Pathways . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2669.
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Neurones that exhibit red–green (r–g) or yellow–blue (y–b) colour opponent signals can be found both in the retina and in the lateral geniculate nucleus. The generation of r–g and y–b, polarity sensitive signals in pre–striate pathways forms the basis for r–g and y–b chromatic channels. The aim of this study was to investigate the extent to which the properties associated with r–g and y–b chromatic channels change at higher levels by examining differences in colour vision loss caused by selective damage to striate and extra striate areas of the visual cortex.
Thresholds for detection of moving, colour–defined stimuli at the fovea and in each of the four quadrants of the visual field (∼ 6o in the periphery) were measured in 24 subjects with damage to the visual pathway using the CAD (Colour Assessment & Diagnosis) test. We measured colour detection thresholds along directions in colour space that isolate the use of the r–g and y–b chromatic channels. MRI scans were used to quantify the extent and the location of the lesions.
Eleven of the 24 subjects had pre–geniculate and optic radiation damage. Nine of the eleven subjects showed symmetric, single channel loss of chromatic sensitivity. In eight of these subjects the loss of chromatic sensitivity was only in the r–g channel and in one subject loss was detected only along the y–b channel. Both the r–g and the y–b channels were affected severely in the remaining two subjects. Thirteen subjects had striate and extra–striate damage. Seven of these showed significant asymmetric loss in one of the two opponent channels. One subject exhibited no colour loss and two subjects showed symmetric r–g colour loss. The remaining three subjects had no colour vision in their affected quadrant. All the results showed that the losses were location specific (i.e. the loss was restricted to either the foveal region and/ or to one or more of the four quadrants).
The results suggest that the processing of r–g and y–b chromatic signals in the cortex involves distinct neural substrates that map different regions of the visual field. The processing of different colour categories (for example red, green, yellow or blue colours) must therefore be carried out in distinct neural substrates. Selective damage to striate and / or extra striate areas of the brain can, in some cases, result in selective loss of vision for single colour categories.
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