Nonlinear properties of visual perception: electrophysiological evidence in humans

Visual perception in humans has been investigated mainly through a psychophysical approach, which allows investigating perceptual experience as a function of physical features variations. However, relatively little is known about the neuronal correlates of stimulus contrast perception in the human brain. In the current work we recorded the electroencephalographic (EEG) activity in healthy participants during a detection and discrimination task where a visual stimulus (i.e., gabor) was presented under six contrast levels. We analyzed how the contrast levels modulate behavioral and physiological responses. Six healthy volunteers (age 24-34 years; 4 female) participated in the study. The stimuli could have six different contrast levels, presented in a random order. Only the first value was adjusted at the individual threshold level, the others were an exponential increase of the first (~3%, 6%, 12%, 25%, 50% and 100%; Michelson contrast index). In each trial, the participants had to perform two tasks, first a gabor detection task (yes/no) and then a orientation discrimination task (leftward/rightward). EEG was recorded continuously during all the experiment from 32 electrodes mounted on an elastic cap according to the 10-20 international system. Analyses were focused both on the behavioral responses (i.e., accuracy and reaction times) and physiological responses, such as evoked oscillatory activity in the 4-80Hz frequency range and event related potentials (P100, N2pc). Results showed that both behavioural and electrophysiological indexes increased in a non-linear trend in relation to contrast modulation, which increased exponentially through the conditions. The frequency analysis revealed a modulation of specific frequency bands such as theta (4–7Hz) and alpha (8–14Hz) which showed an increase of power as the contrast increased. No modulation of the high frequencies was observed. Concerning event related potentials, changes in the early visual P100 amplitude were more affected by exogenous stimulation while the late N2pc component varied consistently with the performance, appearing to be predictive of the behaviour. The correlation analysis applied to the estimated spline functions of the behavioral and electrophysiological indexes revealed that theta frequency and N2pc component were highly correlated to response accuracy, more than alpha band and P100. These data suggest that activity in visual areas corresponds to subjects’ percept and both can be similarly modulated by physical variations.