Binocular interactions and visual acuity loss in esotropic cats.

Visual acuity was measured behaviorally in various groups of cats by using a two-choice discrimination procedure. Cats in group 1 were rendered strabismic soon after birth by sectioning the tendon of the lateral rectus muscle (unilateral esotropia); at adulthood, their visual acuity (VA) was evaluated, after which the optic chiasm was sectioned and VA reassessed. Cats in group 2 were not only tenotomized but also chiasmatomized neonatally, while cats in group 3 underwent a neonatal section of the optic chiasm only. VA was measured at adulthood in the two latter groups. Group 4 consisted of adult cats whose VA was evaluated before and after an optic chiasm section. Stimuli consisted of square-wave gratings of various spatial frequencies. Results showed that in normal cats, the average threshold values under monocular viewing were identical for each eye (4.76 cycles/degree); however, following optic chiasm section, monocular VA was reduced to 1.23 cycles/degree. VA in early optic chiasm section cats was lower than that of the normal cats but higher than that of late-lesioned animals (2.33 cycles/degree). In strabismic cats, mean VA was 1.25 cycles/degree for the deviated eye and 2.8 cycles/degree for the normal eye. Following the optic chiasm section at adulthood, VA was lower not only for the deviated eye (< 0.17 cycles/degree) but also for the normal eye (1.14 cycles/degree). Similar results were found when both the deviation and chiasmatomy were performed neonatally. The elimination of interocular interactions through chiasm transection failed to improve VA in the strabismic eye.