The responses of 51 neurons in the lateral geniculate nucleus of the rabbit to substitution of colored stimuli different brightness and stimuli differing only in intensity were studied. Neurons in the geniculate nucleus, like neurons in the visual cortex, were found to respond with initial phasic discharges at 50-90 msec after stimulus substitution, the magnitudes of these responses correlating with the interstimulus differences; neurons also showed prolonged tonic responses in which the spike frequency depended on the intensity of the stimulus presented. Analysis of phasic responses allowed two groups of neurons to be identified: some were specialized to discriminate stimulus intensity only, while others were specialized to discriminate both the intensity and the color tone of the stimulus. Use of the magnitude of the early phasic discharge as a measure of the difference between stimuli yielded a sensory space for lateral geniculate nucleus neurons. The responses of neurons in the first group (44 cells, 86%) produced a two-dimensional achromatic space with two axes--brightness and darkness; this structure appeared independently of whether stimuli were of the same or different color tones. The phasic responses of neurons in the second group (seven of 51, 14%) generated a four-dimensional space with two color and two achromatic axes. The color and achromatic spaces of lateral geniculate nucleus neurons were analogous to the spaces previously identified for neurons in the rabbit visual cortex using the same stimulation conditions. The sensory spaces reconstructed on the basis of neuron phasic discharges essentially coincided with the spaces obtained from analysis of the N85 component of visual evoked potentials in rabbits, which provides support for the vector information coding principle in the visual analyzer. The tonic discharges of most lateral geniculate nucleus neurons correlated linearly with changes in stimulus intensity and can be regarded as reflecting a pre-detector function for the visual cortex detector neurons.
The neuronal activity in the rabbit's visual cortex, lateral geniculate nucleus and superior colliculus was investigated in responses to 8 color stimuli changes in pairs. This activity consisted of phasic responses (50-90 and 130-300 Ms after stimuli changes) and tonic response (after 300 Ms). The phasic responses used as a basis for the matrices (8 × 8) constructed for each neuron included the average of spikes/sec in responses to all stimuli changes. All matrices were treated by factor analysis and the basic axes of sensory spaces were revealed. Sensory spaces reconstructed from neuronal spike discharges had a two-dimensional (with brightness and darkness axes) or four-dimensional (with two color and two achromatic axes) structure. Thus it allowed us to split neurons into groups measuring only brightness differences and the measuring of color and brightness differences between stimuli. The tonic component of most of the neurons in the lateral geniculate nucleus showed linear correlation with changes in intensities; therefore, these neurons could be characterized as pre-detectors for cortical selective detectors. The neuronal spaces demonstrated a coincidence with spaces revealed by other methods. This fact may reflect the general principle of vector coding (Sokolov, 2000) of sensory information in the visual system.
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.
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