Computation of color and brightness differences by neurons in the rabbit visual cortex

Changes in activity of 54 neurons in the rabbit visual cortex evoked by the replacement of eight color and eight achromatic stimuli in pairs were analyzed. The diffused stimuli generated by color SVGA monitor were used in the experiments. The earliest response of phasic neurons (50-90 ms after the replacement) was strongly correlated with differences between stimuli in color or intensity. This response ("the signal of differences") was used as a basis of a matrix (8 x 8) constructed for each neuron. Such matrices included mean numbers of spikes per second in responses to changes of different stimuli pairs. All matrices were subjected to factor analysis, and the basic axes (the main factors) of sensory spaces were revealed. It was found that 16 neurons (30%) detected only achromatic differences between stimuli. Perceptual spaces of these neurons were two-dimensional with brightness and darkness orthogonal axes. The spaces of 12 neurons (22%) were four-dimensional with two chromatic and two achromatic axes. The structure of the perceptual space reconstructed from neuronal spikes was similar to the space calculated from the early VEP components recorded under similar conditions and to another space reconstructed on the basis of rabbit's instrumental learning. The fundamental coincidence of color spaces revealed by different methods may reflect the general principle of vector coding in the visual system and suggests the coexistence of two independent cortical mechanisms of the detection of chromatic and achromatic differences.