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We provide evidence that callosal projections within the primary somatosensory cortex of the rat are distributed in a detailed pattern which is complementary to the pattern of specific thalamocortical projections to this cortical region.

Summary The asymmetry of monocular OKN was assessed in normal, monocularly deprived (non-deprived eye) and strabismic cats. No significant differences were observed between the responses of these 3 groups. In young kittens, the gain of OKN for nasalward stimulus was larger than that for temporalward. This asymmetry declined to a low residual level by about one year of age. Neurophysiological recordings indicated that the symmetry of OKN is not related to cortical binocularity.

Following single injections of horseradish peroxidase (HRP) in the superior colliculus (SC) and [3H]proline in the striate cortex of rats, a close correspondence was observed in the topographical arrangements of extrastriate cortical fields of HRP retrograde label and of isotope anterograde label. These results support the notion that extrastriate cortex is divided into multiple physiologically and anatomically defined areas, and they suggest that these areas project separately to SC.

We have examined recovery from the acute effects of unilateral and bilateral visual cortex lesions on horizontal optokinetic nystagmus (OKN) in the cat. A single bilateral cortical lesion virtually abolishes monocular OKN in response to temporalward stimulation, and severely reduces OKN in response to nasalward stimulation at higher drum velocities. A unilateral cortical lesion causes deficits in OKN toward the side of the lesion, but these changes are less substantial than those observed after a bilateral lesion. The nature of the acute changes in OKN caused by cortical lesions supports the notion that the visual cortex has two important roles in the OKN system of the cat: first, it mediates OKN in response to temporalward motion of the visual environment during monocular viewing; second, it expands the response range of the OKN system to include higher stimulus velocities. Finally, the results from unilaterally lesioned cats suggest that a single hemisphere can mediate OKN in both directions, with an emphasis on the direction ipsilateral to that hemisphere. The patterns of recovery of OKN in lesioned cats follow two major trends: a small, gradual increase in the overall gain of OKN from its severely depressed level immediately after a large bilateral lesion, and a 'balancing' process that reduces the marked asymmetry between rightward and leftward OKN caused by a unilateral lesion. This balancing of the reflex is accomplished by an improvement in performance in one direction while performance in the opposite direction actually declines. Recovery is faster and more pronounced in lesioned cats with one cortical hemisphere intact, suggesting that the surviving visual cortex compensates for the effects of the lesion by modifying activity in subcortical pathways. These experiments indicate that the visual system of the cat has the capacity to recover at least partially from the deficits in OKN behavior caused by cortical lesions, and that this recovery is the outcome of an interaction between cortical and subcortical pathways mediating this important visuomotor reflex.

Experiments using double-label anterograde tracers in normally reared and monocularly deprived cats reveal that the overall pattern of retino-pretectal projections appears similar for the deprived and non-deprived eyes of monocularly deprived cats, and no different from that observed in normally reared cats, in terms of both overall extent and amount of binocular overlap. These findings indicate that this pathway is unaffected by prolonged periods of monocular deprivation, suggesting that the presence of binocular overlap per se is not a sufficient condition for binocular competition to occur.

It is known that development of the interhemispheric pathway is accompanied by a dramatic reduction in the number of callosal fibers. By injecting neuroanatomical tracers into the cortex of newborn and adult rats, we examined the distribution of fibers within the corpus callosum both before and after this loss takes place. We found that in both newborn and mature rats, axons from restricted regions of the cortex are grouped into relatively tight bundles within the corpus callosum and that the location of these bundles depends upon the region of cortex from which they originate. These results indicate that callosal fibers that are destined to disappear are not distributed diffusely throughout the corpus callosum of the neonate, but rather that they traverse this commissure through the sector that is appropriate for the area of the cortex from which they originate. This finding suggests that loss of axons does not play a significant role in establishing the overall topographic pattern of the fibers within the callosal commissure.

The ocular dominance of single units in the striate cortex was assessed in normal kittens and in kittens exposed to either 2 days or 2 weeks of divergent strabismus at various times during rearing. The data indicate that the sensitive period for disrupting cortical binocularity by means of visual axis misalignment is similar to that for monocular deprivation. This result suggests that there is just one sensitive period for the development of cortical binocularity.

Following multiple injections of HRP into the posterior cortex of one hemisphere of adult rats, dense and overlapping distributions of retrogradely labeled cells and anterogradely labeled terminations are observed throughout the depth of the cortex in the region of the border between the lateral portion of area 17 and area 18 in the opposite hemisphere. In contrast to previous studies of the visual callosal pathway, we also find large numbers of labeled callosal cells extending throughout areas 17 and 18 in cortical layers Vc and VIa.