Soon after the discovery, in the mid-1950’s, that it was possible via averaging to extract a time series of changes in electrical brain activity recorded at the human scalp before, during, and after an event of interest, it was demonstrated that measurable parameters of these evoked potentials – their amplitudes, latencies, and scalp topographies – systematically varied with stimulus or response features (e.g., pitch, color, intensity). Within five years, the field of cognitive electrophysiology was born from various demonstrations that scalp ERP waveforms indexed not only objective stimulus characteristics (often within the first 200 ms), but also endogenous influences related to people’s reactions or attitudes to the stimuli and experimenters’ instructions (between ~200–1500 ms post-stimulus onset). By 1978, cognitive electrophysiologists had identified ERP markers of stimulus evaluation processes distinct from response preparation and execution. In particular, the P300 (P3b) is an endogenous, mostly modality-independent response observed over central-parietal scalp locations whose latency (300–800 ms) varies systematically with the duration of stimulus categorization. P3b amplitudes are inversely correlated with the eliciting item’s subjective probability of occurrence: the less probable an event, the larger the P3b elicited (reviewed in Hillyard & Kutas 1983).
