Interrelation and plasticity of inter- and intrahemispheric corticocortical connections, and the relation between ocular dominance columns and callosal connections in rat visual cortex
Laing, Robyn Jessica
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Interhemispheric callosal connections are present throughout areas 17 (V1; striate cortex) and 18a (lateral extrastriate cortex) of the rodent visual cortex, are topographically organized, and form a variety of patterns including strips, patches, and bridges as they share relationships to the underlying structures in cortex. Previous studies have shown that the retina plays a role in the development of topographically organized areas in the cortex. Here, we investigated the relationship between callosal connections and the underlying structures in the rat visual cortex, as well as the influence that the retina plays on these connections. In Chapter 1, we examine both the structural and functional properties of ocular dominance territories and interhemispheric callosal connections in primary visual cortex. We demonstrate a novel finding that rats have patchy callosal connections and segregated ocular dominance columns. Furthermore, in the central portion of V1, callosal connections overlap with ipsilateral ocular dominance columns and at the lateral border of V1, callosal connections overlap with contralateral domains, as has previously been described in the cat. These results suggest that development of cortical modular architecture is more conserved among rodents, carnivores and primates than previously thought. In Chapter 2, we investigated the role that retinal input plays in the development of interhemispheric callosal connections and ipsilateral striate-extrastriate connections. We found that after bilateral enucleation, the patterns of striate-extrastriate connections and interhemispheric callosal connections were abnormal. While striate-extrastriate projections consisted of multiple well-defined fields that were largely confined to acallosal regions as they do in normal rats, these projections were highly irregular and variable, as were the acallosal regions observed in extrastriate cortex. In Chapter 3, we investigated the role that the eyes play in the topography of striate-extrastriate connections. Using multiple, single injections of different tracers placed at different locations in striate cortex, we were able to map out the patterns of projections to extrastriate cortex. In lateral extrastriate cortex of normal rats, the representation of the naso-temporal axis of visual loci mirrors the representation of this axis in V1. We found that the correlation between the distance from injection sites to the lateral border of V1 and the distance from this border to the labeling patterns in area 18a was strong in controls and much weaker in neonatally enucleated rats, suggesting that the eyes play a role in the development of mediolateral topography during neonatal life.
- Psychology