Relation of GABA Levels in Visual Cortex to Strength of Interocular Suppression

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DESCRIPTION (provided by applicant): This R21 proposal seeks to develop analytic procedures for relating results obtained using magnetic resonance spectroscopy (MRS) to behavioral results obtained using multiple measures of visual function within a large group of healthy, adult participants. The study focuses specifically on levels of the inhibitory neurotransmitter substance gamma amino butyric acid (GABA) within occipital lobe and the relation of those levels to multiple psychophysical measures indexing depth of interocular suppression/binocular rivalry in human vision. The novel, untested aspect of this project is the attempt to identify whether individual differences in perceptual suppression can be identified that are stable across time and consistent across measures and whether GABA levels predict such differences. The primary analytic approach used to address these questions will be structural equation modeling (SEM), which will allow us to address three inter-related questions: (1) Are individual differences in behavioral task performance evident on a measure-specific basis? (2) Does one higher-order dimension of suppression underlie the inter-correlations among these different psychophysical measures? (3) Do individual differences in GABA levels within visual cortex, which we have good reason to believe exist, account for a significant proportion of the variance of the higher-order suppression dimension and/or measure-specific components of variance? SEM itself is a well-established statistical technique for testing hypotheses concerning individual differences in personality and temperament, but it has not been used often to relate biological and perceptual markers. If successful, this project could provide a breakthrough technique for relating neurochemical data and behavioral-cognitive data in human subjects. Being an expert on binocular vision, the PI chose to explore this untested application within the context of interocular suppression. To maximize the chances of success the project includes two co-investigators who are both Vanderbilt University faculty members, Dr. Kevin Waddell (an expert on MR spectroscopy) and Dr. Andrew Tomarken (an expert on SEM). The proposal, while exploratory, is built around plausible linking hypotheses and established individual differences in aspects of interocular suppression. In addition, we present pilot data confirming our ability to measure and quantify GABA levels in visual cortex of humans scanned using the facilities of the Vanderbilt University Institute of Imaging Sciences.