Auditory event-related potentials as a function of abstract change magnitude.

Event-related potentials were recorded in healthy volunteers to test the accuracy of the human brain to extract, preattentively, auditory abstract rules. The abstract rule was determined by the frequency relationship between two pure tones forming a pair. The standard pairs had identical tone frequency, whereas the deviant pairs had the second tone two, four, six or eight musical steps higher or lower in frequency than the first one. All abstract changes elicited mismatch negativity, which was not affected by the magnitude of change. However, the subsequent P3a increased as a function of the magnitude of the abstract change. These results suggest that mismatch negativity detects violations of abstract rules, and the amount of violation is analyzed in subsequent stages of auditory processing.