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.