Combined orientation and colour information in human V1 for both L-M and S-cone chromatic axes.

Although it is widely held that colour and form are processed separately in early visual cortex, there is growing evidence that primary visual cortex (V1) may show some joint selectivity for orientation and colour. Colour is supplied to V1 via two very different pathways: the parvocellular pathway (which also supports detailed form processing) carries L-M ("red-green") chromatic information, while a koniocellular pathway carries S-cone ("lilac-yellow") information. Therefore on entering V1, S-cone information is segregated from the pathways carrying form information, while L-M information is not. Whether signals from neuronal populations in human V1 reflect combined orientation and S-cone information has not been systematically addressed. We used fMRI in combination with a multivariate data analysis technique to investigate whether BOLD signals recorded from V1 contain information that could directly discriminate between orientations based on different types of chromatic information. We found selectivity in V1 for L-M and luminance-defined orientation signals, and most interestingly, also for S-cone defined orientation. We also found similarly successful orientation discrimination for both colour dimensions in V2 and V3. These results imply that a proportion of cells throughout human visual cortex show joint sensitivity to both colour and orientation. We discuss also the potential role of feedback to V1 from higher visual areas.