Suspense and surprise: on the relationship between expectancies and P3.

Few theories of the P3 component have emphasized the distinction between its parietal and frontocentral parts. This study used a new paradigm for testing the predictions that the parietal P3 is evoked by awaited stimuli (suspense) and the frontal P3 by unexpected stimuli (surprise). Subjects had to make simple responses whenever a yellow ring appeared. This signal appeared on the screen within a clock, most frequently 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 shortly after 12 o'clock and then steadily increase until 12 o'clock, and the parietal P3 should accordingly be smallest with stimuli shortly after 12 o'clock, then gradually increase and be largest with 12 o'clock stimuli. Further, the stimuli presented at times other than 12 o'clock should evoke large frontal P3s because they were unexpected. The results confirmed parts of these predictions. A frontocentral and a parietocentral component could indeed be discerned. The frontal P3 was largest with non-12 o'clock stimuli, whereas the parietal P3 was large with all stimuli. The parietal result was not predicted, but these results taken together pose more problems for the usual view, which assumes that the parietal P3 is evoked by unexpected stimuli, than for our assumption that the parietal P3 reflects suspense, and the frontocentral P3 reflects surprise. Generalizing to other paradigms, we assume that different topographies of P3 in different paradigms or in different groups of subjects might be due to different mixtures of these two components.