Student Theses and Dissertations

Date of Award

1994

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Knight Laboratory

Abstract

Contrast forms the substrate for human pattern vision. The retina is the first stage in the processing of complex spatial and temporal contrast signals. The responses of retinal ganglion cells to such signals provide the means to explore such processing in the retina. Most primate retinal ganglion cells fall into two broad classes, M and P, which have many anatomical and physiological differences. The responses of M and P cells to luminance and chromatic signals were studied using nonlinear systems analysis to determine the dynamics and the functional organization of the cells' receptive fields. Previously tried methods as well as a new method described here were used to study the interactions of contrast signals within the receptive fields of M and P cells. The first-order responses of P cells to spots and annuli isolated the dynamics of the center and surround. The new multiple m-sequence method also allowed analysis of the second-order responses. Although traditionally viewed as "linear" cells, P cells display characteristic second-order kernels from their center and surround regions as well as a mixed center-surround cross-kernel. This nontrivial nonlinearity was explored further using spots and steady annuli which were set at several levels of illumination. These experiments demonstrated that the level of ambient illumination in the surround could affect the gain and dynamics of the P cell center. The responses of P cells to chromatic and achromatic gratings were used to assess the interactions of different cone-driven inputs to P cells. While P cells appear to be driven by opponent color mechanisms, responses to luminance contrast demonstrate a nonlinearity in the summation of cone signals. The responses of M cells to chromatic and achromatic contrast signals differ substantially from those of P cells. The first-order responses of M cells show the contrast gain control phenomenon found in cat retinal ganglion cells (Shapley 'and Victor, 1978), a nonlinear contrast-response relationship. This phenomenon is recognized as an increasing amount of low frequency attenuation in response to increasing contrast stimulation. Most M cells have second-order responses similar to those from cat retinal ganglion cells. These nonlinearities in M and P cell responses lead to several conjectures about the anatomical organization and the functional roles of M and P cells. The nonlinearity found in P cells can lead to dynamic regulation of contrast sensitivity and an enhanced selectivity to fine patterns. M cells, on the other hand, perform like an adaptive filter that is optimized to extract temporal information. The data suggest that the anatomical substrates of these M and P cell nonlinearities must be different.

Comments

A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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