Visual colour processing is completed in the retina

Analysis of mammalian visual pathways is limited by difficulty in recording from single cells earlier than the third-order ganglion cells, but the electroretinogram (ERG) has separable components for each of the three afferent orders. The flash ERG a-wave is generated by the photoreceptors (Tomita and Yanagida, 1981; Heynen and van Norren, 1985), the b-wave by bipolar cells (Heynen and van Norren, 1985; Dick et al., 1985) and the pattern ERG by the ganglion cells (Fiorentini et al., 1981; Maffei and Fiorentini, 1981). We have obtained action spectra from each of these components and behavioural thresholds from the final common path. At the photoreceptor stage the data are fit by the sum of middle- and long-wave cone response. At the bipolar stage some inhibitory interaction between the red and green responses begins. At the ganglion stage there is greatly enhanced red-green inhibition and amplification of the blue response. There is no significant change between the ganglion cells and the final common path, so it may be concluded that colour processing is complete in the retina.