The role of corner angle in visual physiology and brightness perception
How do corners of different angles affect visual physiology and brightness processing in the brain? Some visual illusions show that corners can be more salient perceptually than edges, even when their physical luminance is equivalent. Combining several techniques (computational modeling, human psychophysics, and human fMRI) we have studied the relationship between comer angle, brightness perception, and visual physiology. Our psychophysical results show that corners appear quantifiably brighter for sharp than for shallow angles, and that the perceived brightness of the comer is linearly correlated to the corner's angle. Basic linear models of center-surround receptive fields predict the main result from the psychophysical experiments (that is, that sharp corners are brighter/more salient than shallow comers). Thus our data suggest that comers start to be processed from the very first stages of the visual system. Our human fMRI experiments furthermore show that BOLD signal response to corners increases parametrically with angle sharpness in all the retinotopic areas of the visual cortex, suggesting a general principle for comer processing throughout the visual hierarchy.