Emitted and Evoked P300 Potentials and Variation in Stimulus Probability

There have been a number of reports of a cerebral potential occurring at about the time of an expected but absent stimulus when absence provided significant information for the subject. This potential consists primarily of a positive peak occurring with a latency of about 300 msec with respect to the time of stimulus absence and is referred to as an emitted P 300 potential. It has been conjectured that the emitted P 300 is a manifestation of the same process that underlies the evoked P 300 . Evidence supporting this hypothesis is provided by demonstrating that both the evoked and emitted P 300 potentials are similarly affected by variation in event probability. A paradigm was used in which click presence and absence provided information. The relative probability of click presence and absence was experimentally manipulated. Both evoked and emitted p 300 amplitude responded in the same way to event probability, larger for the less frequent event and smaller for the more frequent event.