Non-mirror-symmetric patterns of callosal linkages in areas 17 and 18 in cat visual cortex

Abstract

In the cat, callosal connections in area 17 are largely confined to a 5-6-mm-wide strip at the 17/18 border. It is commonly thought that callosal fibers extending from between the 17/18 border regions interconnect loci that are mirror-symmetric with respect to the midline of the brain, but this idea has not been tested experimentally. The present study examined the organization of callosal linkages in the 17/18 border region of normal adult cats by analyzing the patterns of connections revealed in one hemisphere after small injections of different fluorescent tracers into the opposite 17/18 callosal region. The location of the injection sites within areas 17 and 18 was assessed by examining architectonic data and by inspecting the labeling pattern in the ipsilateral visual thalamus. Area 17 and 18 were separated by a 1-1.5-mm-wide zone of cytoarchitectonic transition rather than by a sharp border. The results show that, in general, callosal fibers interconnect loci that are not mirror-symmetric with respect to the midline. Thus, area 17 injections placed nearly 3 mm away from the 17/18 transition zone produced discrete labeled areas located preferentially within the contralateral 17/18 transition zone. However, when the injection site was within the 17/18 transition zone, labeled cells were found primarily medial and lateral to, but not within, the 17/18 transition zone in the contralateral hemisphere. Previous studies have indicated that the 17/18 transition zone contains a representation of a strip of the ipsilateral visual field. Comparison of the retinotopy of the 17/18 border region with the mirror-reversed pattern of callosal linkages found in the present study suggests that callosal fibers link points that are in retinotopic correspondence in both hemispheres.

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections

“…However, binocularity seems not to play an important organizing role in this pathway, since strabismus does not affect the pattern or number of callosal cells (49,50). Rather, the outcome of deafferentation is predicted by the effect of the manipulation on retinotopic correspondence between the two hemispheres (18,19,44,49,50). In deaf animals, there may be no information available on which to base elimination of connections, as in the retention of overlapped corticocortical (51) or thalamocortical (52) arbors after visual deprivation.…”

“…In cat visual cortex, bilateral deprivation or enucleation result in a reduced callosal projection (44)(45)(46), whereas monocular manipulations result in expanded callosal projections (16,47,48). However, binocularity seems not to play an important organizing role in this pathway, since strabismus does not affect the pattern or number of callosal cells (49,50). Rather, the outcome of deafferentation is predicted by the effect of the manipulation on retinotopic correspondence between the two hemispheres (18,19,44,49,50).…”

One hundred million years of interhemispheric communication: the history of the corpus callosum

“…Since each hemisphere contains a representation of the contralateral visual hemifield, callosal fibers have been proposed to connect the two hemirepresentations of the whole visual field at the level of the midline. On the border between the primary and secondary visual areas of the cat and other mammals there is a band containing two mirror images (one in V1 and the other in V2) of the central, binocular visual field; each image has an ipsilateral and a contralateral representation of the central visual field (14)(15)(16). In this region, many callosal cells connect points of the contralateral visual representa- In the somatosensory cortex, S1, and the motor cortex, M1, callosal-projecting cells have been described to be restricted to a narrow band representing the body midline (see Ref.…”

Transient Synaptic Silencing of Developing Striate Cortex Has Persistent Effects on Visual Function and Plasticity

“…The callosal pathway matures quite early in development and links retinotopically corresponding loci in the two hemispheres (Lewis and Olavarria, 1995;Olavarria, 1996). Interhemispheric linkages serve important functions, including binding together the separate representations of the two halves of the visual field (Berlucchi and Rizzolatti, 1968;Engel et al, 1991).…”

“…This organization is different from that observed in higher mammals such as cats and primates, in which callosal terminals are mainly concentrated at the border between area 17 and 18 (Olavarria, 1996;Houzel and Milleret, 1999). This raises the issue of whether callosal influences are as important for cortical development in higher mammals as reported here for the rat.…”