Curvature formation in the visual cortex: How do we sample?

Background. The circle-polygon illusion produces a polygon percept when a static dark outline circle was pulsed at 2 Hz with a luminance gradient around the inner border. We modify the method of Sakurai (2014) and display arclengths that are 1/8, ¼ , 3/8, ½ , 5/8 , ¾ , 7/8 and 1 (whole) of a circle. We test if different arc lengths, that are fractions of the circumference of the same circle, change the edge length reported. If the edge length does not change with arc length, this implies neurophysiological design that codes for curvature at specific eccentricities in the same manner. Method. Arc lengths (1/8, ¼ , 3/8, ½ , 5/8 , ¾ , 7/8 and 1 (whole) ) of a circle of 4 and 8 deg and eccentricity (0,1,2,4 and 8 deg) were varied in a cross design. Observers (30 online participants; experiment was hosted on Pavlovia) indicated the edge length formed as part of the percept by selecting a edge length from an array of edge lengths shown between 0.105 deg and 2.107 deg. The perceived edge lengths were displayed as a series of twenty line segments built from the equation log(a*b c) such that a=1 and b=0.9. c belonged to the range [1,20] and each value of c gave rise to a unique line segment. Observers reported strength on a scale 1-10. Result. Edge length remained approximately uniform for all arc lengths for a fixed circle size. Strength was strongest for the shortest arc length, gradually decreasing as the arc length increased. Lowest strength was reported for whole circles. This may indicate that the early visual cortex has an affinity for closure and prefers to see shapes as composited closed curves or circles, reporting smaller edge lengths and lower strength for polygonal percepts in the process.