HUMAN COLOR DISCRIMINATION BASED ON A NON-PARVOCELLULAR PATHWAY

Background: Traditionally, colour information is assumed to be carried by neural channels in the parvocellular pathway and to be encoded in an opponent manner, while other, non-parvocellular, spectrally non-opp onent channels are thought to play no part in colour vision. But is th e parvocellular pathway the only way that colours can be discriminated in human vision? We studied two patients with cerebral achromatopsia, who lack conscious colour perception but are nevertheless able to mak e use of colour information. In particular, we investigated whether, i n these patients, colour discrimination is mediated by the parvocellul ar pathway. Results: The achromatopsic patients carried out a forced-c hoice colour- and luminance-discrimination task, and showed clear evid ence of unconscious colour processing, consistent with previous studie s. We added different types of luminance noise to see when this uncons cious colour information could be masked. The results of the colour-di scrimination-with-noise and the brightness-non-additivity experiments showed a double-dissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally non-opponent mechanism, which does not have the characteris tics of the parvocellular pathway and which is responsive to fast flic ker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvo cellular system. The second achromatopsic patient showed evidence of a residual parvocellular system. Conclusions: Our results show that chr omatic discrimination need not be mediated by neural mechanisms, the p arvocellular system in particular, normally assumed to subserve consci ous colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally non-opponent, and supports normal motion perception.