Binocular summation in normal, monocularly deprived, and strabismic cats: visual evoked potentials

We have studied visual evoked potentials (VEP) in the cat using dichoptically presented sinusoidal gratings. Our goals were to determine if binocular disparity causes differential responses in the VEP, and to examine the effects of monocular deprivation and convergent or divergent strabismus on the degree of binocular summation. Binocular disparity in stimuli causes no regular alterations of visual evoked responses, except at very low spatial frequencies. However, this apparent selectivity is probably due to luminance modulation in the central retina at low frequencies. The insensitivity to binocular disparity establishes that binocular summation in the VEP may be estimated without regard to the relative phase of gratings presented to the two eyes. Binocular summation of the VEP was examined in normal animals. We found that the ratio of the binocularly evoked response to the largest monocular response (averaged across spatial frequency) ranged from 1.27 to 2.12 (4 animals) and had a mean of 1.48. These values fall within the range which has been reported for human subjects. The degree of summation might be expected to be greatly reduced in strabismic and monocularly deprived animals, in which the majority of the cells are functionally monocular. While summation was found to be reduced in 5 esotropic (convergent) animals (range = 1.13-1.24; mean = 1.18) it was approximately normal in three exotropic (divergent) animals (range = 1.29-2.12; mean = 1.61). However, single unit recordings carried out on the same animals show similar reductions of cells that can be driven through either eye for both groups of animals. Recordings from three monocularly deprived animals, on the other hand, show evidence of binocular interaction in the form of suppression. In this case, response amplitudes obtained using binocular stimulation were consistently and substantially smaller than those obtained from the normal eye alone (range = 0.76-0.85; mean = 0.80). We conclude that convergent and divergent strabismus differ substantially in the degree to which binocular summation is exhibited in the VEP, which in the latter condition, is indistinguishable from the normal cat. Monocular deprivation causes an effect which is markedly different from either form of strabismus in that the deprived eye suppresses the response of the normal eye.