An electrophysiological comparison of convergent and divergent strabismus in the cat: visual evoked potentials.

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Abstract
Ocular misalignment in humans is often associated with amblyopia of 1 eye, but the prevalence of this relationship is significantly higher in convergent than in divergent strabismus. Although animal models of strabismus have been studied, little attention has been paid to direct comparisons of convergent and divergent strabismus. Results of electrophysiological experiments intended to compare these conditions in cats reared with surgically created ocular misalignment are presented. Measurements of visual evoked potentials using as stimuli large, bright, high-contrast gratings are given. The 17 cats studied included 4 each with convergent and divergent squint and 4 normals. Data from 5 monocularly deprived (MD) cats were obtained for comparison. Results from normal cats show typical positive and negative deflections, which decrease in amplitude at both high and low spatial frequencies. Spatial frequency response functions, computed by harmonic analysis of averaged raw signals, illustrate these amplitude changes as high- and low-frequency falloffs on both sides of peak responses. Results from cats with divergent strabismus (exotropia) are closely similar to those from the normal controls. In particular, there are no apparent differences between deviated and nondeviated eyes. Abnormal spatial frequency response functions were found from measurements of the deviated eye of the convergent (esotropic) strabismus group. In comparison with the nondeviated eye, amplitudes were reduced over a broad range of spatial frequencies. These reductions are modest when considered in relation to the MD group from which severe deficits were found, especially at the low-frequency end. Although responses from esotropic eyes were abnormal, extrapolated high-frequency cutoff values were nearly identical for both eyes. The means for deviated and nondeviated eyes, in cycles per degree are, 2.0 and 2.0, respectively. These values compare with values of 2.6 and 2.2, respectively, for deviated and nondeviated eyes of the exotropic group and a mean of 2.0 for the normal cats. These visually evoked-potential (VEP) results demonstrate clear differences between experimentally created convergent and divergent strabismus in cats. Deficits are apparent from measurements of the former but not from the latter group. Estimated cutoff values, which may be related to acuity, are similar in both types of strabismus and equal to those for normal cats.