Towards an electroretinographic assay for studying colour vision in human observers

It has been argued that parvocellular cells could also be the physiological basis for luminance vision and thus has a “double-duty”. In our opinion, there are two arguments against this proposition: First, it has been found that the psychophysical sensitivities to isoluminant chromatic stimuli decrease for frequencies above about 4 Hz although the opponent cells respond well to these stimuli at much higher frequencies. It therefore has been suggested that there is a cortical filter in the opponent channel. It is difficult to imagine how such a filter would not involve luminance signals from opponent cells, particularly because these luminance signals manifest themselves at very high temporal frequencies (typically above about 40 Hz as it is caused by the small centre-surround latency difference in opponent cells). Second, The L/M ratio in the luminance channel is larger than one, can (dependent on subject, stimulus size, position, etc.) be as large 10:1 and is correlated with the ratio of L- to M-cone numbers. The L/M ratio in the chromatic channel is, however, about unity. It is difficult to conceive how different psychophysical L/M ratios can result from one underlying physiological mechanism.