Effects of Light Intensity and Edge Density on Visually Evoked Potentials in Rhesus Monkeys

Visually evoked potentials were recorded from bipolar electrodes implanted in the foveal and parafoveal striate, prestriate and inferotemporal cortices of mature rhesus monkeys (Macaca mulatta). The stimuli used were a blank field and three checkerboard patterns containing checks whose sizes were 2° 24′, 36′, and 9′ of visual arc. In addition to the size of the checks within the patterns, the intensity at which the stimuli were presented was also varied. In all four regions studied, check size had a significant effect on the amplitude of the average evoked potential, but intensity showed an effect only in the two striate regions. Certain regions of the visual cortex can apparently process two dimensions of a stimulus concurrently, and the same mass of neuronal tissue can participate in the processing of different dimensions of a stimulus at different times. The results are discussed in the context of existing literature concerning the evoked potential as an indicator of sensory information processing.