Simulation of responses of spectrally-opponent neurones in the macaque lateral geniculate nucleus to chromatic and achromatic light stimuli.

The responses of spectrally opponent, parvocellular-layer cells in the lateral geniculate nucleus of the macaque vary with stimulation along all three dimensions of colour space (luminance, dominant wavelength and purity). Here, the responses of different cell types to light stimuli successively replacing a white adaptation field are simulated mathematically by a simple model using known cone spectral sensitivities. Hyperbolic response functions with two adjustable parameters relate cone excitation with the output signals of each cone mechanism. For the majority of cells, differences of outputs of two cone mechanisms, receiving inputs from L- and M-cones, describe responses to chromatic and achromatic stimuli. For each opponent combination of cone inputs, either L-M or M-L, two cell types were found. Off-centre cells had lower maximum firing rates of their on-component, but were the more sensitive of the two groups, preferring dark colours, whereas on-centre cells showed better responsiveness to brighter stimuli. For all four cell types, the predictions of the equations replicated well the responses measured to stimuli varying along all three dimensions of colour space. The quantification of cell responses presented here provides possibilities for studying suprathreshold coding of colour and lightness, and the model can help to link physiology with the psychophysics of colour vision.