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Brightness induction reveals changes in neural response time to changes in stimulus contrast

33.3013, Sunday, 17-May, 8:30 am - 12:30 pm, Banyan Breezeway
Session: Color and light: Adaptation and constancy

Karen Gunther1, Jacob Owens1; 1Psychology Department, Wabash College

We explore whether brightness induction can be used to reveal changes in neural response time to changes in stimulus contrast. Brightness induction is a phenomenon in which the brightness of a stimulus is influenced by its surrounding context (e.g., Chevreul, 1839/1987; Shevell, Holliday & Whittle, 1992). For example, a red dot surrounded by the color black appears brighter than a red dot surrounded by the color white when both red dots are the same intensity. Neurophysiological studies (e.g., Shapley & Victor, 1978; Solomon & Lennie, 2005) have shown that as the contrast of a stimulus decreases, the neuronal response time increases. Subjects performed a brightness induction heterochromatic flicker photometry task (HFP, 5Hz; Gunther & Dobkins, 2005) in three conditions: red intense (bull’s-eye dot stimulus with a red center [3.5° diameter] and black surround [6° diameter] alternating with a bull’s-eye dot with a green center and white surround), green intense (green is paired with black, red with white), and a non-induction control (red and green dots alternating with no inducing annulus). Subjects performed HFP (minimize flicker by adjusting relative red/green luminances) on the red/green central dot for each induction condition. The red/green center dot was presented at seven contrasts (25-60% of the maximum possible on the monitor; 6.99–8.24% root mean square cone contrast), in seven linear steps. The logic of the induction paradigm is that as the contrast of the stimulus decreases, the center should appear to flicker slower, thus becoming perceptually out of phase with the inducing surround. At low enough contrasts, the center may be perceptually 180° out of phase with the inducer, thus appearing as assimilation instead of induction. The HFP settings do show changes in induction magnitude with contrast, reflecting changes in neural response time. Results are not yet statistically significant - data collection continues.

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