On Quantifying Surprise: The Variation of Event‐Related Potentials With Subjective Probability

TLDR

ERP waveforms are shaped by both the a priori probability of stimuli and the sequence of immediately preceding stimuli. The study investigated how these two factors contribute to ERP waveforms across nine probability levels ranging from 0.10 to 0.90. Ten male participants listened to random high‑ and low‑frequency tones at each probability level while EEG was recorded from five midline electrodes during both task‑relevant and irrelevant conditions. P300 and Slow Wave amplitudes decreased with higher stimulus probability, were reduced when the tone repeated the preceding tone and increased when preceded by the alternate tone, showing that stimulus probability and sequential structure independently influence the P300 complex.

Abstract

ABSTRACT Two factors are known to determine the waveform of event‐related potentials (ERP) elicited by task‐relevant stimuli: the a priori probability of the stimuli and the sequence of immediately preceding stimuli. The relative contribution of these factors to the ERP waveform was assessed at nine levels of a priori probability (from .10 to .90). Random sequences of high (1500 Hz) and low (1000 Hz) tones were presented to 10 male subjects at each level of probability, both when the events were task‐relevant and when the subjects were performing an alternate task to which the tones were irrelevant. The EEG was recorded from five midline electrode sites referred to linked mastoids. The amplitude of the P300 and Slow Wave components was inversely proportional to the a priori probability of task‐relevant events. At every level of a priori probability, the magnitude of the P300 complex (N200‐P300‐Slow Wave) was diminished when the eliciting tone repeated the preceding tone, and was enhanced when it was preceded by the other tone. Thus, a priori probability and sequential structure appear to be independent determinants of the P300 complex.