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

Area of Science:

• Neuroscience
• Psychophysics
• Cognitive Science

Background:

• Stevens' power law accurately describes perceived brightness, with a typical exponent of .33.
• Individual variations in this power exponent exist, but their neural basis is poorly understood.
• Understanding these neural substrates is crucial for a comprehensive model of visual perception.

Purpose of the Study:

• To investigate the neural substrates underlying individual differences in perceived brightness.
• To examine how neural activation levels in the visual cortex scale subjective brightness.
• To correlate brain activity with psychophysical measurements of brightness perception.

Main Methods:

• Functional magnetic resonance imaging (fMRI) was employed to measure brain activity.
• Participants rated the perceived brightness of visual stimuli with varying luminance.
• Analysis focused on activity in the primary visual cortex (V1/Brodmann's area 17).

Main Results:

• Subjective brightness ratings exhibited a log-linear relationship with luminance, averaging an exponent of .32.
• fMRI data revealed log-linear increases in V1 activation with luminance (exponents of .32 and .27 in left and right hemispheres).
• Individual variations in power exponents for both subjective ratings and V1 activation were substantial and significantly correlated (r = .62).

Conclusions:

• Primary visual cortex (V1) activation levels scale with physical luminance, mirroring subjective brightness perception.
• V1 activation is closely linked to stimulus luminance, rather than contrast at the stimulus border.
• These findings suggest that V1 plays a key role in scaling subjective brightness according to Stevens' power law, accounting for individual differences.