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Irfa Nisar, James H. Elder; Cortical magnification analysis of shape adaptation reveals early curvature coding mechanisms. Journal of Vision 2021;21(9):1830. https://doi.org/10.1167/jov.21.9.1830.
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© ARVO (1962-2015); The Authors (2016-present)
Background. Sustained stimulation of the retina with a circular stimulus produces the impression of a polygon (Khuu et al 2002, Ito 2012). What could explain this surprising phenomenon? A low-level account posits an opponency between localized curvature-tuned neurons in early visual cortex, while a high-level account assumes an opponency between neurons in higher visual areas coding whole shapes. To test these competing accounts, we measure how the illusion varies with the size and eccentricity of the stimulus. While a high-level account predicts invariance to size and position, a low-level account predicts that the order of the perceived polygon will increase with the size and decrease with the eccentricity of the stimulus, due to cortical magnification. Method. We employed the method of Sakurai (2014) to rapidly induce the circle-polygon illusion. The stimulus was a static dark outline circle augmented with a 2 Hz pulsed luminance gradient around the inner border. Both the radius and eccentricity of the stimulus were varied in a crossed design over 1-8 deg. Observers indicated their percept by selecting from a palette of shapes that included a circle and polygons of orders 3-10, and also reported the strength of the polygon percept on a 0-10 scale. Results. As previously reported, the strength of the percept increased with eccentricity. Importantly, the mean order of the perceived polygon systematically increased with stimulus size and decreased with eccentricity. To relate this more clearly to the underlying neural substrate, we computed the circumference of each stimulus in both retinal and cortical coordinates, taking cortical magnification into account. Linear regression analysis reveals that the cortical size of the stimulus is a significantly better predictor of perceived polygon order. Implications. These results suggest that the circle-polygon illusion arises from and reveals the architecture of an early visual opponent mechanism for curvature coding.
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