All ETDs from UAB

Advisory Committee Chair

Allan C Dobbins

Advisory Committee Members

Norberto M Grzywacz

Kent T Keyser

Thomas T Norton

Document Type

Dissertation

Date of Award

2012

Degree Name by School

Doctor of Philosophy (PhD) School of Optometry

Abstract

The retina is the first stage of the visual system, responsible for transducing light into the neural signal and for subsequent processing of this signal before sending it to the higher visual centers in the brain. The output to the higher visual centers is mediated by retinal ganglion cells, which not only relay the signal, but also substantially contribute to its processing. Depending on what computations they perform, they are subdivided into different types. These cells have been extensively studied in various spices, one prominent example being the rabbit. In rabbits, one characteristic type comprises On-Off directionally selective retinal ganglion cells (On-Off DS RGCs), whose response is modulated by the direction of motion of a moving visual stimulus. These cells respond most strongly to a stimulus moving in their preferred direction and very weakly to a stimulus moving in the opposite direction, whereas intermediate directions evoke intermediate responses. However, the response is also affected by nondirectional parameters, such as speed and contrast, which confounds information on the stimulus direction and makes the response ambiguous. This raises a question of what information is actually conveyed by these cells and in what form. To address this problem, the precise relationship between various stimulus parameters and the cell response must be determined. This dissertation shows that responses of On-Off DS RGCs are multiplicatively separable: response measured as the mean spike count is a product of the effects of direction, speed, and luminance. Moreover, speed and luminance tuning curves are similar in On-Off DS RGCs across the retina. Furthermore, it demonstrates that responses of On-Off DS RGCs involve characteristic bursts, which allow these cells to encode direction and nondirectional parameters in different aspects of the spike train. These results suggest that direction could possibly be decoded from mean spike count responses of local populations of On-Off DS RGCs, whereas the combined effect of the nondirectional parameters could be estimated from interburst intervals.

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