Intracerebral event-related potentials to subthreshold target stimuli

Objectives: Event-related potentials (ERPs) elicited by subthreshold visual stimuli were recorded directly from human frontal and temporal lobe struct ures to study unconscious perception. Methods: Thirteen intractable epileptic patients undergoing depth electrode recordings prior to their surgical treatment participated in the study. An original method of modified visual oddball paradigm with supraliminal and subliminal stimuli was applied, and the averaged responses to both kinds of stimuli were subsequently compared. Results: The results clearly prove that, at least from an electrophysiologi cal viewpoint, the mechanism of unaware processing of visual stimuli in the human brain does not differ substantially from the aware processing. Findi ng the subliminal P3 waveform in a number of cortical structures (hippocamp us and parahippocampal gyrus bilaterally, and left-sided mesiofrontal, orbi tofrontal and lateral temporal cortex) indicates their involvement in uncon scious processing, in spite of the fact that typical large-scale neurocogni tive networks are not completely activated. The absence of activation consi stently observed bilaterally in dorsolateral prefrontal cortices, in connec tion with right-sided cortical frontal lobe structures and right-sided late ral temporal neocortex in unconscious perception, supports the importance o f these structures for the awareness of visual stimuli. The proof of the si gnificantly faster unaware information processing represents another distin ctive feature of implicit visual perception. Conclusions: Based on the presented findings and comparisons with the resul ts of previous ERP, functional magnetic resonance imaging, positron emissio n tomography, and clinical neuropsychological studies, a crucial role of th e large-scale neural system for conscious experience of perception is sugge sted, which is distributed extensively among the dorsal posterior associati on areas and the prefrontal cortex, with the dominant part being that of th e right hemisphere. (C) 2001 Elsevier Science Ireland Ltd. All rights reser ved.