Electrophysiological correlates of contrast perception

Contrast information, a primary aspect of visual perception, has been usually bound to psychophysical investigation in humans. In the current work we recorded the electroencephalographic activity in healthy participants during a detection and discrimination task where a gabor was presented under six contrast levels. We analyzed how the variation of the contrast modulates the behavioural (i.e, accuracy and reaction times) and physiological responses, such as event related potentials (P100, N2pc) and evoked oscillatory activity. Results showed that both behavioural and electrophysiological indeces increased with a non-linear trend in relation to the contrast modulation, which increased exponentially through the conditions. While changes in the early visual P100 amplitude were more affected by exogenous stimulation, the late N2pc component varied consistently with the performance, appearing to be predictive of the behaviour. Moreover, the frequency analysis revealed a power increase of specific frequency bands such as theta (4-7Hz) and alpha (8–14Hz) in relation to the contrast variation. No modulation of the high frequencies was observed. On the whole, this study indicates that activity recorded over human visual cortex is related more to the subjects’ percept than to the physically presented stimuli.