Jaime Olavarria

Evidence for the complementary organization of callosal and thalamic connections within rat somatosensory cortex

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.

Unfolding and flattening the cortex of gyrencephalic brains

The present paper describes a procedure for unfolding and flattening gyrencephalic brains that makes it possible to cut single tangential sections through extensive regions of originally convoluted cortex. This procedure involves incising the arachnoid, removing the white matter, and then opening up the gyri and sulci in tissue that has been fixed in such a way as to maintain much of its natural flexibility. A technique is also described for preserving the complete gyral pattern of the intact brain on the surface of a flattened cerebral hemisphere. Finally, examples are presented in which the gyral pattern in the cat brain is related to the location of identified cortical areas in myelin-stained tangential sections from flattened material.

Widespread callosal connections in infragranular visual cortex of the rat

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.

Reciprocal connections between the striate cortex and extrastriate cortical visual areas in the rat

Following injections of horseradish peroxidase (HRP) in the striate cortex of rats, a precise topographical correspondence between extrastriate cortical fields of anterograde and retrograde label was observed. The arrangement of these extrastriate labeled fields corresponds closely to the previously reported division of the peristriate cortex into multiple visual areas, suggesting that each of these areas is reciprocally connected to striate cortex. Cortical layers II--VI participate in this reciprocal connection.

The projection from striate and extrastriate cortical areas to the superior colliculus in the rat

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.

Axons from restricted regions of the cortex pass through restricted portions of the corpus callosum in adult and neonatal rats

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.

Pattern of extrastriate visual areas connecting reciprocally with striate cortex in the mouse.

Abstract Several areas reciprocally connected to striate cortex were found in the extrastriate cortex of the mouse after small single injections of horseradish peroxidase into the striate cortex. By showing that the arrangement of these labeled extrastriate areas resembles closely the physiologic and anatomic subdivision of the extrastriate cortex reported previously in several rodent species, this study supports the hypothesis that there exists a common pattern of visual cortical organization in rodents.

Retino-pretectal projections in monocularly deprived cats

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.