Tsubomi,H., Ikeda, T., & Osaka, N.


Primary visual cortex scales individual’s perceived brightness with

power function: Inner psychophysics with fMRI.


Journal of Experimental Psychology: Human Perception & Performance





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/m(2) 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 VI) 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.

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