Assessment of brightness and color differences by neurons in the superior colliculus of the rabbit.

The responses of 83 neurons in the rabbit superior colliculus to substitution of color stimuli of different brightnesses and black-and-white stimuli of different intensities were studied. Superior colliculus neurons were found to respond with initial and late phasic discharges (over the periods 50-90 msec and 120-300 msec from the moment of stimulus substitution respectively), along with prolonged tonic discharges whose spike frequencies depended on the intensity of the stimulus. Analysis of the phasic responses of the neurons allowed three groups of cells to be identified. One group of cells (25 of the cells studied, 30%), identified on the basis of early neuron responses, were specialized for detecting brightness differences between black-and-white and color stimuli of different intensities. The sensory spaces reconstructed on the basis of spike discharge frequencies in the early discharges of these neurons were achromatic and two-dimensional. Another group of neurons (16 of the cells studied, 19%) were mainly identified on analysis of late phasic discharges and had four-dimensional spaces with two color and two achromatic axes. The third group of cells (four neurons, 5%) had early discharges with two-dimensional achromatic sensory spaces and late discharges with four-dimensional spaces. It is suggested that reconstruction of the four-dimensional space requires processing of information from the visual cortex on color and intensity differences between stimuli. The sensory spaces of superior colliculus neurons reconstructed on the basis of phasic discharges essentially coincided with the sensory spaces of neurons in the visual cortex and lateral geniculate body and spaces obtained by analysis of the N85 component of visual evoked potentials in rabbits recorded using similar stimulation. This may support the vector coding principle in the visual analyzer.