The spectral responsiveness and the temporal frequency response (TFR) of cat optic tract and lateral geniculate neurons: sinusoidal stimulation studies

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

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 48 Hz. Optimal temporal frequencies of blue-ON cells were around 3 Hz, whereas the frequency optimum of achromatic cells was close to 10 Hz. 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.

Photopic spectral sensitivity of the cat

1. The psychophysical spectral sensitivity of cats was assessed using a two-choice visual discrimination task by determining increment thresholds and critical flicker frequency on white and chromatic backgrounds. 2. For large increments, on 0.0, 0.3 and 3.0 cd/m2 white backgrounds, the cats were most sensitive to 497 nm indicating that these backgrounds are scotopic. On 30 and 300 cd/m2 white backgrounds, the cats were most sensitive to about 454 and 561 nm indicating that these backgrounds are photopic. Sensitivity to intermediate wave-lengths indicated independent action of 'blue' and 'green' cones. 3. For large increments, thresholds on photopic yellow and magenta backgrounds indicated the additive influence of 'blue' and 'green' cones. 4. Spectral sensitivity functions obtained with a critical flicker frequency criterion of 10 Hz on a 30 cd/m2 white background reflected only the activity of the 'green' cone while at 20 Hz the function reflected an additive contribution of both 'blue' and 'green' cones. 5. For small increments, on a 30 cd/m2 white or 96 cd/m2 orange background, sensitivity reflected only the activity of the 'green' cone. 6. The cat's photopic spectral sensitivity is influenced by the psychophysical test upon which it is based in a manner that is similar to what has been found for other vertebrates. No evidence was found for a 500 nm mechanism active at photopic levels.