Curvature coding in early visual system revealed by scale invariance during adaptation to flashing circles

How is curvature coded in the early human visual system? Humans are successful in recognizing objects and by extension, the shape representing the object, under varying scale conditions (Biederman & Cooper, 1992; Lindeberg, 2013). How do we neuro-physiologically code the invariance (or variance) in curvature and does the curvature coding change with scale? The circle-polygon illusion produces polygonal percepts during adaptation when a static dark outline circle is pulsed at 2 Hz alternating with a gradient luminance circle. We use the circle-polygon to study curvature processing with respect to size and scale. Both the radius and eccentricity of the stimulus were varied in a crossed design over 1-8 deg. Observers reported a circle or the polygon order and the strength of the percept. We test a lower level account that argues for curvature opponency between neurons against a higher level account that codes for whole shapes. This higher level account supports scale invariance, a property through which we recognize objects regardless of the objects size on the retina. We show the following: (1) Scale invariance is not obeyed during adaptation. The mean order of the perceived polygon increased with stimulus size and decreased with eccentricity. This also demonstrates that curvature coding occurs in the early visual system. (2) Linear regression analysis reveals that the cortical size of the stimulus is a better predictor of perceived polygon order. We quantify the relationship parametrically between cortical size and polygon order. Using integration and regression, we identify the region of the cortex, V1, where the shape, a regular ordered polygon, is being computationally constructed.