Awareness of P300-related cognitive processes: a signal detection approach.

Although the P300 component of the human event-related potential is purported to indicate consciousness of an event, it is unclear which of the P300-related processes, if any, are accessible to awareness. This study explored awareness of such processes in Bernoulli series of auditory stimuli using signal detection theory and selective averaging of ERPs according to the subjects' classifications. Following targets eliciting appropriate P300 amplitudes, the subject was requested to classify his brain response as small, medium, or large, whereupon visual feedback was given about the correctness of the response. Averaging the ERPs according to the subjects' classifications showed P300 amplitudes to increase with the classification category, whereas the prestimulus and post-P300 event-related potentials were unrelated to the category. Signal detection analysis revealed non-zero sensitivity measures for the P300 amplitude categories. Although no conclusive suggestion can be made at present as to the basis of the P300 discrimination ability found here, it was possible to rule out electro-ocular and electromyographic activity, the preceding auditory stimulus, and the prior feedback given, as cues for discrimination.