Effects of neonatally induced strabismus on binocular responses in cat area 18

Abstract

Responses to binocular visual stimulation were compared in cortical area 18 of normal cats and in cats in which one eye was exodeviated by surgery early in postnatal life. In contrast to normal cats where most units (58%) were binocularly activated, relatively few units (10%) in strabismic cats were activated well by stimulation of either eye. Rather individual units were driven mainly via one eye or the other but not both. In addition, there was a tendency for more units to be driven well via the unoperated eye than via the exodeviated eye. Fewer cells preferring vertically oriented than horizontally oriented stimuli were found in area 18 of strabismic cats. This trend was observed for cells driven by either the normal or deviated eye and was especially marked among the small number of binocularly activated cells. Generally binocular responses and binocular interactions were found with stimulation at corresponding retinal points. In a few striking instances, however, the receptive fields of binocular neurons were located on noncorresponding retinal points at loci which would enable the cat to correlate the two images of an external object despite the large divergent strabismus. Quantitative responses to binocular stimuli presented at varied disparities and to stimuli with varied directions of motion in depth were compared in normal and strabismic cats. Despite the large strabismus, a reduced fraction of cortical neurons displayed substantial binocular interactions. In fact, binocular facilitation was as marked in the population of cells studied in strabismic cats as it was in normal animals. The major effect of strabismus was a reduction in the strength of binocular inhibition when units were tested with sideways motion. Disparity-specific responses to motion toward or away from the organism were little affected by strabismus. The degree of binocular facilitation and binocular inhibition among the cell population was similar in normal and strabismic cats. A subpopulation of units encountered in strabismic cats showed strong disparity-specific interactions for motion toward or away from the animal without equivalent modulation for sideways moving stimuli. Units with these properties were not found in normal animals and may, therefore, represent a special adaptation of the strabismic animals.

Processing Deficits in Primary Visual Cortex of Amblyopic Cats

“…Although optical imaging captures activity of many neurons, we did not observe any reduction of neurons preferring vertical contours (Chino et al 1991;Cynader et al 1984;Singer et al 1979). However, it is possible that in our study, not all orientation compartments were sampled sufficiently to allow for a pixel-based angle statistic.…”

“…Since the number of neurons preferring vertical contours is reduced in amblyopic cats (Chino et al 1983(Chino et al , 1991Cynader et al 1984;Singer et al 1979), we investigated the relative size of the cortical territory devoted to the vertical orientation domains (0°). To this end, we split interocular vector strength differences at 0.5 cycles/°of all cats into four compartments (ct) according to the preference angle ␣ per pixel: ␣ ϩ 45°Ͼ ct Ն ␣ Ϫ 45°.…”

Visual interhemispheric transfer to areas 17 and 18 in cats with convergent strabismus

“…1). It is important to point out that the nondeviated (left) eye was chosen to analyse interhemispheric transfer because it has a better acuity than the deviated eye (Ikeda & Tremain, 1979;Chino et al, 1980) and it does not display any nasotemporal asymmetry (Ikeda & Jacobson, 1977;Cynader et al, 1984;Kalil et al, 1984;Sireteanu & Best, 1992). (3) Orientation selectivity was assessed by comparing responses to light bars of different orientations.…”

Interocular suppression in the visual cortex of strabismic cats

“…After rearing with optically induced vertical or rotational misalignment of the eyes, Shlaer (197 l), Shinkman and Bruce (1977), and Dtirsteler and von der Heydt (1983) reported some degree of compensation in the positional or orientational disparity of the receptive fields of surviving binocular cells in kitten striate cortex. In area 18 (Cynader et al, 1984) and in the lateral suprasylvian cortex of strabismic cats (Sireteanu and Best, 1992) some binocular neurons also appear to have receptive fields in anomalously corresponding retinal positions, which might allow registration of the images seen by the two eyes.…”

“…which results in virtual blanking of vision in the nonfixating eye, although psychophysical evidence points to a cortical origin (Blake and Lehmkuhle, 1976;Hess, 1991). Analysis of visual evoked potentials in cats (Sclar et al, 1986) and of the responses of striate neurons (Xue et al, 1987) suggests that binocular summation is generally lost in long-term esotropic cats, while some binocular interactions are retained in the apparently monocularly driven cells found in exotropes (see also Cynader et al, 1984). Interocular inhibition, as assessed by visually stimulating cells through one eye while electrically stimulating the optic nerve of the other side, showed asymmetries in esotropes but not in exotropes (Freeman and Tsumoto, 1983): responses through the deviating eye were more efficiently suppressed by the nondeviating eye than vice versa.…”