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David J. Heeger; Neuronal correlates of contrast detection and discrimination. Journal of Vision 2002;2(10):13. doi: https://doi.org/10.1167/2.10.13.
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We performed two pattern detection/discrimination experiments, using fMRI to study how sensory signals in visual cortex evoke percepts and guide behavior.
Results from the first experiment demonstrated that V1 activity was correlated with contrast discrimination thresholds. Observers performed a contrast discrimination task on a sinusoidal target that was either presented in isolation, or embedded in a high-contrast surround. We found that the surround substantially increased the psychophysical thresholds while reducing fMRI responses. The psychophysical and fMRI data sets were compared, based on the assumption that a fixed response difference is required for correct discrimination, and we found that the psychophysics accounted for 96.6% of the variance in the measured fMRI responses. The good quantitative agreement suggests that contrast discrimination judgments are limited by neuronal signals in early visual areas (at or before V1).
Results from the second experiment demonstrated that activity in early visual areas (including V1) was correlated with variability in perception. Subjects performed a rapid sequence of trials in which allow contrast grating target was to be detected in the presence of a high-contrast background pattern. We found that activity was larger on average for false alarm trials than for misses. In other words, the activity in early visual cortex corresponded to the subjects' percepts even when that percept was opposite to what was physically presented in the stimulus, consistent with observers making perceptual decisions based on activity in these early visual areas. These results were retinotopically specific, suggesting that they are related to the percepts themselves as opposed to a non-specific effect (e.g., arousal) associated with “yes” responses.
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