Spatial frequency tuning of brightness polarity identification.

Recent studies have shown that cells in the primary visual cortex can, in addition to borders, also encode surface brightness. Whether the brightness is encoded by a large extraclassical receptive field or by a filling-in type mechanism activated by the luminance border is not known. These explanations imply different spatial frequency tunings for the underlying mechanism. In a psychophysical masking paradigm we measured spatial frequency tuning functions for identification of both luminance polarity (bright/dark) and luminance border orientation of oval and circular luminance patches with variable diameters (0.2-10 deg). For both tasks we obtained nearly overlapping narrow (1.5 octave) bandpass masking tuning functions centered at 1.5-5.0 c/deg. Stimulus size and shape had only minimal effect on the tuning functions. The results favor the idea of brightness filling-in and suggest that the cells activated by the luminance border modulate the activity of the cells signaling surface brightness. Further, the brightness processing mechanism is spatial frequency selective.