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The acuities of cells in the primary visual cortex of five tenotomized strabismic cats were measured. Previous behavioural studies have shown such animals to possess a severe amblyopia of approximately 1.5 octaves of spatial frequency, yet the acuities of both retinal ganglion and lateral geniculate X-cells are normal. The receptive fields of the cortical cells sampled were within 5 degrees of the area centralis projection. On average, the acuities of cortical cells driven by the amblyopic eye were nearly 1 octave less than those for the non-deviating eye. However, the best cell acuities for each eye were nearly the same. The relationship between ocular dominance and cell acuity was found to be different for the two eyes despite a symmetrical ocular dominance distribution. The acuity deficit for cells driven through the amblyopic eye was present at all depths along the electrode tracks. We conclude that in this model amblyopia, the initial spatial processing deficit lies in the visual cortex, and most probably in the cells of layer IV. Furthermore, the presence of a few cells driven by the amblyopic eye which can perform nearly as well as those from the fellow eye in processing high spatial frequencies gives new insight into the way in which strabismic and deprivation amblyopias differ.

The acuities of X-cells from the dorsal lateral geniculate nucleus (LGN) were measured in five cats raised with a convergent strabismus, surgically induced by tenotomy. The acuities of cells driven by the strabismic eye were not significantly different from the acuities of cells driven by the non-deviating eye over the range of eccentricities in the visual field studied (from the area centralis to over 20 degrees). The data were also similar to X-cell acuities in the LGN of 3 normal cats. Lowered acuities of LGN X-cells driven by the deviating eye of an esotropic cat in which the strabismus was created by myectomy confirm a previous finding of a retinal locus of amblyopia associated with that preparation. In contrast, the results here implicate the visual cortex as the initial site of the deficit in spatial processing in amblyopia arising from tenotomy strabismus.

Tone bursts produced bands of selective 2-[14C]-deoxyglucose labelling in the inferior colliculus (IC) of the awake monkey. Low tone frequencies produced labelling in dorsal regions and high tone frequencies produced labelling in ventral regions. The position of the bands coincided with the position of a single unit with a characteristic frequency, which was the same as the frequency producing the labelling. These findings indicate that the bands of labelling represent iso-frequency contours in IC. The iso-frequency contours extended across most of the nucleus and were oriented from dorsomedially to ventro-laterally at 20-30 degrees from the horizontal and became more vertical anteriorly. The width of the contours was as narrow as 200 micron, suggesting that the contours might represent 2 or 3 overlapping cellular laminae.

The relative efficacy of distributed versus massed reverse occlusion therapy in promoting recovery from the anatomical and physiological effects of monocular deprivation was studied in two experiments performed on kittens raised with one eye occluded from eye-opening until 5 weeks of age. The first experiment explored the effects of different periods (ranging from 0.5 to 4 h) of reverse occlusion imposed daily for 20 days. The second, involving a fixed period of reverse occlusion (20 h), examined recovery with respect to the distribution of that period over a varied number of brief daily sessions. Recovery was assessed in terms of changes in cortical ocular dominance and lateral geniculate cell morphology. Although recovery of both cortical ocular dominance and geniculate cell morphology showed the same overall progression with increasing periods of reverse occlusion, changes were apparent in the lateral geniculate nucleus before changes were evident in cortical ocular dominance. A given period of reverse occlusion was found to be far more effective in promoting recovery when distributed over a number of different exposure sessions than when massed together in just one or two sessions. The data suggest that there is a maximal rate of cortical recovery which is achieved with surprisingly brief daily periods of forced visual exposure of the initially deprived eye.

Two preparations were used to study the developmental effects of prolonged blurring of retinal images on the acuities of retinal ganglion cells. Five kittens were raised from three weeks to six months of age with daily administration of atropine to one eye. Another two kittens were raised from three weeks to 16 weeks with a contact lens of high refractive power fitted to one eye. Behavioural estimates of the visual acuity were made for two animals from each group. Animals of both groups demonstrated an amblyopia in the experimental eye: visual acuity varied from 1.8 to 2.5 cycles per degree compared with 6.0 to 7.5 cycles per degree when using the normal eye. The spatial resolving properties were measured for retinal ganglion cells within the amblyopic eyes of two lens-reared cats and three atropinized cats. Brisk-sustained (X) cells were recorded from along the naso-temporal division. The acuities of ganglion cells from the lens-reared cats were indistinguishable from those from normal cats at comparable eccentricities. However, for the cats raised with atropine administration, sub-normal acuities were determined for retinal ganglion cells from all regions that were studied in the experimental eye. We conclude that blur of retinal images produced by external means has no effect on the resolving power of retinal ganglion cells. The lowered ganglion cell acuities encountered with the atropinized cats must be attributable to a secondary effect of the atropine administration. Organic changes in the retinal blood vessel pattern support this contention.

Experiments were carried out to determine the effects of different types of experimental strabismus on the acuities of retinal ganglion cells. Six kittens were raised from twenty-one days of age with an esotropia surgically induced by myectomy of the lateral rectus muscle and a large portion of the superior oblique muscle. The results are compared with those, previously reported, from five other cats also made esotropic, but by tenotomy of the lateral rectus. All animals tested behaviourally were amblyopic in the strabismic eye. For square wave gratings, the visual acuities were 1.0 to 2.5 cyc/deg through the strabismic eye compared with 6.0 to 7.5 cyc/deg through the non-deviating eye. The cut-off spatial frequencies were determined for 132 brisk sustained cells from five of the myectomized strabismic cats. There was a loss of approximately 20% in cut-off spatial frequency when compared with both normal and tenotomized cats. A correlate of the physiologically observed difference between the tenotomized cats and the myectomized cats was also found in the morphology of cells in the lateral geniculate nucleus. The tenotomized cats showed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye whereas in the myectomized cats large differences were observed in cell cross-sectional areas between laminae receiving input from the amblyopic eye and those receiving input from the non-deviating eye. Together, these findings indicate that the presence of a neural deficit in the retina of strabismic cats is associated with the actual removal of extra-ocular muscle and probably has little to do with the optical quality of images arriving at the retina.