Human colour discrimination based on a
non-parvocellular pathway

Tom Troscianko, Jules Davidofft, Glyn Humphreyst, Theodor Landis, Manfred Fahle, Mark Greenlee, Peter Brugger and William Phillips

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, nonparvocellular, spectrally nonopponent channels are thought to play no part in colour vision. But is the 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 make use of colour information. In particular, we investigated whether, in these patients, colour discrimination is mediated by the parvocellular pathway.

Results
The achromatopsic patients carried out a forcedchoice colour and luminancediscrimination task, and showed clear evidence of unconscious colour processing, consistent with previous studies. We added different types of luminance noise to see when this unconscious colour information could be masked. The results of the colourdiscriminationwithnoise and the brightness-nonadditivity experiments showed a doubledissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally nonopponent mechanism, which does not have the characteristics of the parvocellular pathway and which is responsive to fast flicker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvocellular system. The second achromatopsic patient showed evidence of a residual parvocellular system.

Conclusions
Our results show that chromatic discrimination need not be mediated by neural mechanisms, the parvocellular system in particular, normally assumed to subserve conscious colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally nonopponent, and supports normal motion perception.