The effect of pre-stimulus alpha activity on the auditory P300 paradigm: a prospective study.

A method is described for carrying out Evoked Response Potential (ERP) testing in which the background EEG is itself treated as an independent variable, and the application of a stimulus made contingent upon this variable. As an illustration of the method, an Auditory Oddball paradigm was implemented, in association with real time processing and testing of the background EEG, and an investigation was carried out into the effect of background alpha activity in 35 normal subjects. In the novel methodology presented here, stimulus presentation was conditional on a "high" level of alpha activity in the pre-stimulus EEG as evaluated by real-time spectral analysis. The dependency of presentation of the stimuli on the state of the pre-stimulus EEG suggests the term "interactive" ERP for the methodology. The resulting ERP was compared with that of a matched recording without pre-stimulus EEG testing. The results show that the parietal P300 peak is reduced by the use of alpha dependent recording. This result indicates that the background EEG activity at the time of application of a stimulus can have an effect on the resulting ERP.