Residual processing of chromatic signals in the absence of a geniculostriate projection.

We have investigated the residual processing of chromatic signals in a subject with unilateral damage to the primary visual cortex using psychophysical, pupillometric and functional magnetic resonance imaging (fMRI) methods. Of particular interest was to establish the correlation between the subject's ability to make use of chromatic signals in the blind hemifield to discriminate between different coloured targets, the corresponding residual pupil colour responses and the level and location of cortical activation generated by the same stimuli as revealed by fMRI. The results obtained using the three different experimental approaches are consistent and suggest that retrograde degeneration of thalamic and retinal chromatic processing mechanisms caused by damage to the primary visual cortex in man does not abolish completely the ability to process chromatic signals particularly when large, long-wavelength stimuli are employed.