G. Sclar

Neurophysiological evaluation of the differential response model for orientation and spatial-frequency discrimination

Recent models have attempted to reconcile low psychophysical orientation and spatial-frequency discrimination thresholds with relatively broad orientation and spatial-frequency tuning of cortical neurons. These models have relied on the ability of the neurons to convert small stimulus changes into reliable response changes. We have examined this ability in a sample of neurons from the cats striate cortex. We present here data from two cells that reliably signaled the smallest orientation and spatial-frequency differences. Using receiver operating characteristic analysis, we find that these cells could reliably signal orientation differences of 1.84 deg and spatial-frequency differences of 0.073 octave. We compare these single-cell results to cat and human behavioral discrimination thresholds.

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

SummaryWe 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.

Regional variations in the effects of monocular deprivation on area 18 of the cat

SummaryWe have studied the effects of monocular deprivation (MD) on area 18 in the cat. Our intention was to determine whether the degree of vulnerability to MD varies regionally within area 18. Single units were recorded in animals used as normal controls (77 cells) and in MD cats (467 cells) from anterior and posterior regions of area 18 representing peripheral and central-paracentral fields, respectively. Recordings from the deprived cats were made from both hemispheres. Overall, around 27% of the cells from the MD group retained input from the deprived eye. Most of these cells (77%) were located in the hemisphere contralateral to the deprived eye. Furthermore, the majority of these cells (96%) were located in the anterior region of area 18 and had receptive fields at eccentric locations (greater than 10 deg). About one third of the cells driven by the deprived eye had diffuse receptive field borders, and most of these units were binocular. In addition, these cells were generally non-oriented. Our main result, that susceptibility to MD decreases with increasing receptive field eccentricity, may be accounted for on the basis of corresponding changes of spatial frequency selectivity of cortical cells.

Cortical binocularity is disrupted by strabismus more slowly than by monocular deprivation.

Abstract Two groups of normally reared kittens were exposed on postnatal day 28 to brief periods of monocular deprivation or experimental strabismus caused by optical prisms. Single-unit analysis was then carried out in striate cortex and ocular dominance distributions were determined. Both procedures were found to disrupt binocular connections, but effects from monocular deprivation occurred more rapidly and were more extensive. We conclude that in the short term, monocular deprivation is a more potent procedure and it is possible that, at least initially, it may involve different mechanisms.