[Estimation of color and brightness differences by neurons in the rabbit superior colliculus]

Changes in activity of 83 neurons in the rabbit colliculus superior evoked by the replacement of eight color and eight achromatic stimuli in pairs were analyzed. It was found out that neurons displayed the early and late phasic responses (within 50-90 and 120-300 ms respectively, after the replacement) and long-term tonic response component, which depended on stimuli intensity. Analysis of phasic component revealed three neuronal groups. The first group (n=25, 30%) selected on the basis of the earliest component, was specialized to differentiate stimuli only by intensities. The perceptual spaces of these neurons reconstructed on the basis of spike discharge in the earliest response were two-dimensional. The second group of neurons (n=16, 19%) selected on the basis of the late phasic component demonstrated four-dimensional structure of perceptual space. Neurons of the third group (n=4, 5%) possessed a two-dimensional structure of perceptual space reconstructed by the analysis of the early component, whereas analysis of the late response revealed a four-dimensional structure. We suggest that information about differences between stimuli in color and intensity coming from cortical neurons is necessary for the reconstruction of four-dimensional space. The structure of perceptual spaces reconstructed on the basis of phasic responses of neurons in the colliculus superior was similar to the spaces of neurons in the primary visual cortex and lateral geniculate nucleus. The structure of perceptual space reconstructed on the basis of neuronal spikes was also similar to the space calculated from the N85 component of the visual evoked potential recorded under similar conditions. This finding confirms the general principle of vector coding in the visual system.