Wavelength Discrimination in Blindsight

In the circumscribed, long-standing, clinically absolute visual field defects of three patients with vascular lesions that involved the optic radiation and visual cortex, forced-choice discrimination between coloured stimuli was tested. Paired stimuli were matched for luminous efficiency on the basis of previous measurements of increment-threshold spectral sensitivity made in the same patients and at the same retinal positions. To different extents all patients could discriminate between narrowband wavelength stimuli. The results imply that despite the effects of retrograde degeneration on thalamic and retinal colour-processing channels, neurons which process wavelength information are still functional, although the information they transmit is not consciously perceived.