Auditory Evoked Potentials to Unpredictable Shifts in Pitch

ABSTRACT Auditory evoked potentials (EPs) were recorded from subjects presented with a regular series of tone pips at one of three rates (1/sec, 2/sec, or 4/sec). Occasionally one of the pips was 5%, 25%, or 100% (an octave) different in pitch from the repetitive background pips. These “mismatch” tones occurred at random with a mean interval of 12 sec (range 1–24 sec) for all of the repetition rates. This determined an average sequential probability = 0.083, 0.042, 0.021 that a tone would be a mismatch. On different runs, subjects either responded with a button press each time they heard a mismatch or they read a book. The amplitude of a large negative component (N2 at 154 msec) evoked after mismatch tones was unrelated to degree of mismatch but was larger to all the mismatch tones than to the background pip. Instructions to attend did not affect N2. A subsequent positive peak at 277 msec (P3), recorded during reading, did increase in amplitude with increasing mismatch, as did a P3 at 330 msec recorded during responding. Sequential probability had no main effect on N2 or P3. This lack of an effect on P3 suggests that the well‐known influence of stimulus uncertainty on P3 may be determined by the temporal rather than sequential uncertainty of events, or that our range of probabilities was too narrow to obtain the effect.