SUSPENSE AND SURPRISE - ON THE RELATIONSHIP BETWEEN EXPECTANCIES AND P3

Few theories of the P3 component have emphasized the distinction betwe en its parietal and frontocentral parts. This study used a new paradig m for testing the predictions that the parietal P3 is evoked by awaite d stimuli (suspense) and the frontal P3 by unexpected stimuli (surpris e). Subjects had to make simple responses whenever a yellow ring appea red. This signal appeared on the screen within a clock, most frequentl y when the pointer was at 12 o'clock (every 6 s) but sometimes also at other times. The suspense process should therefore have its minimum s hortly after 12 o'clock and then steadily increase until 12 o'clock, a nd the parietal P3 should accordingly be smallest with stimuli shortly after 12 o'clock, then gradually increase and be largest with 12 o'cl ock stimuli. Further, the stimuli presented at times other than 12 o'c lock should evoke large frontal P3s because they were unexpected. The results confirmed parts of these predictions. A frontocentral and a pa rietocentral component could indeed be discerned. The frontal P3 was l argest with non-12 o'clock stimuli, whereas the parietal P3 was large with all stimuli. The parietal result was not predicted, but these res ults taken together pose more problems for the usual view, which assum es that the parietal P3 is evoked by unexpected stimuli, than for our assumption that the parietal P3 reflects suspense, and the frontocentr al P3 reflects surprise. Generalizing to other paradigms, we assume th at different topographies of P3 in different paradigms or in different groups of subjects might be due to different mixtures of these two co mponents.