Temporal properties of colour opponent receptive fields in the cat lateral geniculate nucleus.

Abstract

The primordial form of mammalian colour vision relies on opponent interactions between inputs from just two cone types, 'blue' (S-) and 'green' (ML-) cones. We recently described the spatial receptive field structure of colour opponent blue-ON cells from the lateral geniculate nucleus of cats. Functional inputs from the opponent cone types were spatially coextensive and equally weighted, supporting their high chromatic and low achromatic sensitivity. Here, we studied relative cone weights, temporal frequency tuning and visual latency of cat blue-ON cells and non-opponent achromatic cells to temporally modulated cone-isolating and achromatic stimuli. We confirmed that blue-ON cells receive equally weighted antagonistic inputs from S- and ML-cones whereas achromatic cells receive exclusive ML-cone input. The temporal frequency tuning curves of S- and ML-cone inputs to blue-ON cells were tightly correlated between 1 and 48Hz. Optimal temporal frequencies of blue-ON cells were around 3Hz, whereas the frequency optimum of achromatic cells was close to 10Hz. Most blue-ON cells showed negligible response to achromatic flicker across all frequencies tested. Latency to visual stimulation was significantly greater in blue-ON than in achromatic cells. The S- and ML-cone responses of blue-ON cells had on average, similar latencies to each other. Altogether, cat blue-ON cells showed remarkable balance of opponent cone inputs. Our results also confirm similarities to primate blue-ON cells suggesting that colour vision in mammals evolved on the basis of a sluggish pathway that is optimized for chromatic sensitivity at a wide range of spatial and temporal frequencies.