Why should the eyes be open during P300 testing - just to ensure alertness?
Closing the eyes during Cortical Event-Related Potentials (CERPs) has the effect of producing large alpha EEG waves, and as the patient drifts into even light sleep, the cortex becomes less active and thalamo-cortical rhythms associated with sleep begin to emerge. This, in effect, would reduce or even eliminate the evoked potential of interest, particularly if the goal is to discriminate between two acoustic events.
Secondly, P300 tests are generally quite boring and long, and if the patient has eyed closed, the likelihood they will fall asleep during an active button-pressing or counting task, will be quite high.
For an in-depth discussion of this topic, please view Dr. Atcherson’s recorded course on AudiologyOnline, Neurodiagnostic Auditory Evoked Responses Applications.