Neuronal basis of contrast discrimination.

Psychophysical contrast increment thresholds were compared with neuronal responses, inferred from functional magnetic resonance imaging (fMRI) to test the hypothesis that contrast discrimination judgements are limited by neuronal signals in early visual cortical areas. FMRI was used to measure human brain activity as a function of stimulus contrast, in each of several identifiable visual cortical areas. Contrast increment thresholds were measured for the same stimuli across a range of baseline contrasts using a temporal 2AFC paradigm. FMRI responses and psychophysical measurements were compared by assuming that: (1) fMRI responses are proportional to local average neuronal activity; (2) subjects choose the stimulus interval that evoked the greater average neuronal activity; and (3) variability in the observer's psychophysical judgements was due to additive (IID) noise. With these assumptions, FMRI responses in visual areas V1, V2d, V3d and V3A were found to be consistent with the psychophysical judgements, i.e. a contrast increment was detected when the fMRI responses in each of these brain areas increased by a criterion amount. Thus, the pooled activity of large numbers of neurons can reasonably well predict behavioral performance. The data also suggest that contrast gain in early visual cortex depends systematically on spatial frequency.