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Neural mechanisms of the octave illusion: electrophysiological evidence for central origin

THE octave illusion is experienced when two simultaneous tones, separated by one octave and presented to the opposite ears, are continuously reversed between the two ears. Subjects consistently report a sequence of alternating single tones: the high tone in the right ear and the low in the left. We wished to determine whether such a complex tone sequence is encoded as it is presented or as it is perceived. This was accomplished by making the tone sequence infrequently correspond to how it is perceived, and recording event-related potentials (ERPs) to these perceptually equivalent but physically different events. The illusion-mimicking tones elicited the mismatch negativity (MMN), a change-specific ERP component with origin in the auditory cortex. This indicates that the stimuli giving rise to the octave illusion are encoded according to their physical rather than perceptual properties. Consequently, the generator of the octave illusion is located beyond the level of the auditory cortex.