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 structures 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 electrophysiological viewpoint, the mechanism of unaware processing of visual stimuli in the human brain does not differ substantially from the aware processing. Finding the subliminal P3 waveform in a number of cortical structures (hippocampus and parahippocampal gyrus bilaterally, and left-sided mesiofrontal, orbitofrontal and lateral temporal cortex) indicates their involvement in unconscious processing, in spite of the fact that typical large-scale neurocognitive networks are not completely activated. The absence of activation consistently observed bilaterally in dorsolateral prefrontal cortices, in connection with right-sided cortical frontal lobe structures and right-sided lateral temporal neocortex in unconscious perception, supports the importance of these structures for the awareness of visual stimuli. The proof of the significantly faster unaware information processing represents another distinctive feature of implicit visual perception.
CONCLUSIONS: Based on the presented findings and comparisons with the results of previous ERP, functional magnetic resonance imaging, positron emission tomography, and clinical neuropsychological studies, a crucial role of the large-scale neural system for conscious experience of perception is suggested, which is distributed extensively among the dorsal posterior association areas and the prefrontal cortex, with the dominant part being that of the right hemisphere.