Date of Award

Spring 2003

Project Type

Dissertation

Program or Major

Psychology

Degree Name

Doctor of Philosophy

First Advisor

Ken Fuld

Abstract

The squint response corresponding to photophobia (light-induced discomfort) was assessed via electromyography as a function of stimulus size, retinal locus (superior, inferior, temporal, nasal), and retinal eccentricity in three subjects. These results were compared to the same subjects' psychophysically-determined spatial distributions of macular pigment, to examine in detail the attenuation of photophobia by macular pigment. Results indicate that the photophobia response is preferentially biased to prevent the fovea from exposure to potentially damaging short-wavelength light. In this regard, macular pigment appears to act as a spatially-integrated filter, serving to attenuate photophobia to a great extent. Photophobia and macular pigment therefore appear to serve a "tandem" function of foveal photoprotection.

Spatial summation for photophobia to long-wavelength "orange" light was found to be linear and somewhat greater than that dictated by Piper's law. The spatial summation functions derived from broadband xenon-white lights are more complicated, with two or perhaps three different branches. It is suggested that xenon-white light, which contains much short-wave energy, interacts with macular pigment (which absorbs short-wave energy) to give rise to the different branches of summation found in this function.

The present study provides data that are suggestive of the pathways that gives rise to the photophobia response. A close correspondence was found between the degree of spatial summation determined for photophobia and the dendritic area of two kinds of retinal ganglion cell, parasol and small bistratified. Moreover, the spatial densities of parasol and small bistratified ganglion cells were found to decline in a fashion commensurate with the photophobia response, as a function of retinal eccentricity. These findings suggest that photophobia is mediated via a combination of parasol and small bistratified ganglion cells; hence the magnocellular and koniocellular visual pathways are implicated as channeling the visual signals that eventually give rise to photophobia.

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