P300, PROBABILITY, AND THE 3-TONE PARADIGM

The effects of stimulus probability on P300 from a 3-tone paradigm wer e examined in two experiments. Experiment 1 manipulated the probabilit y of the non-target tone as 0.10, 0.45, or 0.80, while the target tone probability was always 0.10. Experiment 2 manipulated the probability of 3 tones as 0.10, 0.30, or 0.60, with one of the infrequent tones a ssigned as the target in each condition. Subjects were required to pre ss a button in response to the target stimulus in both experiments. Th e results indicated that the P300 to the target and the non-target wer e both affected by the probability of the eliciting stimulus, such tha t component amplitude was inversely related to probability; no reliabl e P300 latency effects were found. Target tones elicited larger P300 a mplitude than the non-target tones at the same probability. The findin gs suggest that probability effects on P300 amplitude are independent of responding to a specific target stimulus and are discussed with ref erence to the clinical utility of the 3-tone paradigm.