Rolf Verleger

Abstract P3 is the most prominent of the electrical potentials of the human electroencephalogram that are sensitive to psychological variables. According to the most influential current hypothesis about its psychological significance [E. Donchin's], the “context updating” hypothesis, P3 reflects the updating of working memory. This hypothesis cannot account for relevant portions of the available evidence and it entails some basic contradictions. A more general formulation of this hypothesis is that P3 reflects the updating of expectancies. This version implies that P3-evoking stimuli are initially unexpected but later become expected. This contradiction cannot be resolved within this formulation. The alternative “context closure” hypothesis retains the concept of “strategic information processing” emphasized by the context updating hypothesis. P3s are evoked by events that are awaited when subjects deal with repetitive, highly structured tasks; P3s arise from subjects' combining successive stimuli into larger units. The tasks in which P3s are elicited can accordingly be classified in terms of their respective formal sequences of stimuli. P3 may be a physiological indicator of excess activation being released from perceptual control areas.

Abstract. Hypotheses about the P3 component of the event-related EEG potential have usually assumed that P3b reflects some processing independent from organizing the response. In contrast, the notion that P3b is related to a decision process implies some mediating function between stimulus and response. If P3b does indeed reflect the link between perceptual processing and response preparation (1) amplitudes should be as large in response-locked averages as in stimulus-locked averages, (2) this should be true independent of response speed, for separate subaverages of slow and fast responses, and (3) latencies should vary across response speed both in stimulus-locked and in response-locked averages. These hypotheses were tested in data evoked by visual and auditory stimuli in choice-response tasks. All three predictions were confirmed. In contrast to this balanced relation to perception and responding, fronto-central P3 with auditory stimuli was stimulus-related and, for comparison, the peak amplitudes of both the response-force and of the lateralized readiness potential were response-related. We conclude that P3b reflects a process that mediates between perceptual analysis and response initiation, possibly monitoring whether the decision to classify some stimulus is appropriately transformed into action.

The stimulus evaluation view on P3 latency holds that P3 latency mainly reflects stimulus-processing time, in contrast to response-processing time. A review of the experimental evidence, however, leads to the conclusion that P3 is not a sensitive tool for separating between stimulus- and response-related processes. Rather, it appears that P3 latency is a sensitive index of any response-time changes when response times in the fast condition are brief, with its sensitivity decreasing when response times in the fast condition get longer. This regularity was confirmed by a detailed analysis of the published evidence from Sternberg's task and was not attributable to speed-accuracy trade-off or to different methods of parametrization. The structures generating the scalp P3b are involved both in stimulus processing and in response selection. Response selection may exert its effect on P3 in one of two ways; either directly, fully delaying P3 latency, or affecting a second P3 component (P-CR) only, thus having an attenuated effect on P3 latency.