Consonance and dissonance of musical chords: Neural correlates in auditorycortex of monkeys and humans

Some musical chords sound pleasant, or consonant, while others sound unplea sant, or dissonant. Helmholtz's psychoacoustic theory of consonance and dis sonance attributes the perception of dissonance to the sensation of "beats" and "roughness" caused by interactions in the auditory periphery between a djacent partials of complex tones comprising a musical chord. Conversely, c onsonance is characterized by the relative absence of beats and roughness. Physiological studies in monkeys suggest that roughness may be represented in primary auditory cortex (A1) by oscillatory neuronal ensemble responses phase-locked to the amplitude-modulated temporal envelope of complex sounds . However, it remains unknown whether phase-locked responses also underlie the representation of dissonance in auditory cortex. In the present study, responses evoked by musical chords with varying degrees of consonance and d issonance were recorded in A1 of awake macaques and evaluated using auditor y-evoked potential (AEP), multiunit activity (MUA), and current-source dens ity (CSD) techniques. In parallel studies, intracranial AEPs evoked by the same musical chords were recorded directly from the auditory cortex of two human subjects undergoing surgical evaluation for medically intractable epi lepsy. Chords were composed of two simultaneous harmonic complex tones. The magnitude of oscillatory phase-locked activity in A1 of the monkey correla tes with the perceived dissonance of the musical chords. Responses evoked b y dissonant chords, such as minor and major seconds, display oscillations p hase-locked to the predicted difference frequencies, whereas responses evok ed by consonant chords, such as octaves and perfect fifths, display little or no phase-locked activity. AEPs recorded in Heschl's gyrus display striki ngly similar oscillatory patterns to those observed in monkey A1, with diss onant chords eliciting greater phase-locked activity than consonant chords. In contrast to recordings in Heschl's gyrus, AEPs recorded in the planum t emporale do not display significant phase-locked activity, suggesting funct ional differentiation of auditory cortical regions in humans. These finding s support the relevance of synchronous phase-locked neural ensemble activit y in A1 for the physiological representation of sensory dissonance in human s and highlight the merits of complementary monkey/human studies in the inv estigation of neural substrates underlying auditory perception.