Anatomical and Functional Organization of the Visual Cortex, and the Effect of Visual Deprivation in Animal Models
Andelin, Adrian Keith
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Despite the fact that the visual system has been the most well-studied of all sensory systems, many questions remain in regard to its structure and function in both human and animal models. While a basic blueprint of the visual system exists across all animal species that sets in place the basic structure prior to the onset of visual experience, it has been well established that this system becomes fine-tuned through experience early in life. Using a variety of techniques and animal models, this dissertation addresses some questions regarding the functional organization and the effect of visual deprivation on the visual cortex of several animal models. Rodents offer several advantages for studying various aspects of the development, organization and plasticity of the visual system. An important model for studies of visual cortex plasticity is the system of ocular dominance columns (ODC, aggregates of cells with the same eye preference), which have been extensively studied in many carnivores and primates, but have been thought not to exist in rodents. Our lab recently reported the existence of ODCs in pigmented, Long Evans rats (Laing et al. 2015), but previous reports in albino rats (Diao et al., 1983) point to differences in the binocularity of certain regions of primary visual cortex (V1) and in the role that callosal connections may have in these differences. To explain these strain differences, we hypothesized that albino rats, unlike Long Evans rats, do not have ODCs, and that callosal connections in V1 of albino rats are not patchy, as they are in Long Evans rats. In the first chapter of this dissertation, we present anatomical and electrophysiological experiments supporting our prediction that input from both eyes intermix in the binocular region of V1 in albino rats, without segregating into ODCs, and that callosal connections in albino rats are homogeneously distributed in V1. In the second chapter, we explore the effect of loss of vision during early development on the surface area of V1. Using histological methods as well as MRI techniques, we examined how the reduction in mature brain surface area varies with age when blindness occurs in rats, ferrets and humans. To compare data across species, we translated the post-conception ages of each species to a common neurodevelopmental event-time scale. We predicted that the critical period for the effect of blindness on the area of V1 ends at a common developmental event-time across species. Our results support our prediction, and also show that the critical period for the effect of blindness on V1 surface area ends well before the visual cortex reaches its normal, mature size. Much of the research on the organization and function of visual cortex is presently carried out in mice. While a present advantage of mice is the possibility of using genetic tools, a disadvantage is the small size of their brain and visual cortex. In the third chapter, we use multiple anatomical tracers to explore the number, arrangement and internal topographic organization of extrastriate visual areas in the rabbit, whose brain is about 60 times larger than the mouse brain. Our results show that the visual cortical plan in rabbits closely resembles the plan in mice and rats, suggesting that the rodent plan may be more general, encompassing Lagomorphs and possibly other orders. Our study also underscores the usefulness of the rabbit as an alternative model to rats and mice for projects benefiting from a larger brain.
- Psychology