EEG oscillations induced by contour closure in a noisy visual field

This study was designed to separate the neural response associated with awareness of a visual object from the neural responses to the visual input without awareness. Two classes of stimuli were presented to subjects: (1) a field of randomly positioned and oriented Gabor patches (no contour) or (2) the same field of Gabor patches in which a subset of the Gabor patches were oriented in such a way as to give rise to the percept of an ellipse (contour). To measure the electroencephalograph (EEG) responses to the contour without awareness, trials during which a contour was present but not perceived were compared to trials where no contour was present and its absence was correctly indicated. At beta frequencies, the presence of the contour reduced the evoked potential (EP) power approximately 130-230 ms post-stimulus. Coherence in the theta band were also reduced when a contour was present but not perceived. Thus, modulations of the EP take place when a contour is presented even without the subjects' awareness. To explore the correlates of awareness, trials during which the subject correctly indicated the presence of a contour were compared to trials in which the object was not detected. Consistent across subjects, awareness caused an increase in the EP power in the beta range around 130-230 ms post-stimulus. These results suggest that the neural response to an object and awareness of that object are dissociable in EEG recordings.