Effect of subthreshold target stimuli on event-related potentials

Objectives: Event-related potentials (ERPs) elicited by subthreshold visual stimuli were studied to assess the relationship between unconscious cognitive processing and the electrical activity of the brain. Methods: A new method of modified visual oddball paradigm with supraliminal and subliminal stimuli was applied. prior to the experiment, the individual 'subjective' threshold for the conscious discrimination between frequent and target stimuli was established for each subject. Supraliminal and subliminal, frequent and target visual stimuli were then alternatively presented in random order to each subject. Results: Both the individual and the grand average ERPs revealed a typical response (P3) in the parietal region after supraliminally presented target stimuli. In subliminal conditions an analogous positive deflection in the central-parietal region was observed, which was elicited by the target stimulus, but not the frequent stimulus. Its latency could be clearly distinguished from the latency of the classical P3, the time difference between the two waveforms was approximately 100 ms. Results: The results of this study revealed the impact of unconscious processing to target stimuli on the configuration of event-related responses. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.