Primary visual cortex scales individual's perceived brightness with power function: Inner psychophysics with fMRI.

Abstract

Perceived brightness is well described by Stevens' power function (S. S. Stevens, 1957, On the psychophysical law, Psychological Review, Vol. 64, pp. 153-181), with a power exponent of .33 (the cubic-root function of luminance). The power exponent actually varies across individuals, yet little is known about neural substrates underlying this individual difference. The present functional MRI study investigated how neural activation levels in the visual cortex serve to scale individual's subjective brightness. Participants rated brightness of a disk ranging from 1- to 100-cd/m² luminance. Subjective brightness ratings showed an almost perfect log-linear dependence on luminance intensity, with the power exponent averaging .32. The fMRI results showed that activity in the bilateral primary visual cortex along with the calcarine sulcus (also known as Brodmann's area 17 and V1) increased log-linearly with physical luminance, showing average power exponents of .32 and .27 in the left and right hemispheres, respectively. There were substantial individual variations in the power function exponents for both subjective brightness ratings (.14 to .46) and primary visual cortex activation (.12 to .55). An important finding was that 2 power exponents were closely correlated (r = .62). Subjective brightness ratings and primary visual cortex activation were both better correlated with stimulus luminance than stimulus contrast (at the border of the stimulus). These results suggest that primary visual cortex activation can scale individual's subjective brightness in accordance with Stevens' power law.