Responses of Rabbit Visual Cortex Neurons to Changes in the Orientation and Intensity of Visual Stimuli

We report here studies of changes in the numbers of spikes in the early phasic discharges (50–90 msec from the moment of stimulus substitution) of neurons in the primary visual cortex of conscious rabbits in response to substitution of lines of different orientations (0–90°) but flashing at constant intensity on a screen, to substitution of lines of constant orientation but different intensities, and to substitutions of complex stimuli in which simultaneous changes were made to the orientation and intensity. Factor analysis of the results showed that the number of spikes in the early phasic discharges of some neurons allowed the two-dimensional sensory space of orientations to be reconstructed. This space was identified in 13 of the 43 neurons studied (30%). Five of the 30 cells studied (16.7%) showed both two-dimensional orientation sensory spaces and two-dimensional intensity spaces. Achromatic spaces were reconstructed by substituting lines of different intensity but constant orientation. On substitution of complex stimuli (intensity + orientation), four stimuli with initial orientations of 0–38.58° (0° corresponding to a vertical line) had an intensity of 5 cd/m², while the other four stimuli (with orientations of 51.44–90°) were presented at an intensity of 15 cd/m². On the plane of the sensory space formed by the first two significant factors, the two groups of stimuli with different intensities were located in opposite quadrants of a circle, while within the groups the stimuli were ordered in a sequence close to the order of increases in their slope angles, from smaller angles to greater. It is suggested that in this version, a single sensory plane space reflects the interaction between the orientation and intensity attributes of the visual stimulus, the intensity factor being predominant. A total of seven such cells were found among the 57 studied (12%).