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Consonance and dissonance of musical chords: neural correlates in auditory cortex of monkeys and humans Fishman YI, Volkov IO, Noh MD, Garell PC, Bakken H, Arezzo JC, Howard
MA, Steinschneider M J Neurophysiol 2001 Dec 86:2761-88 Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461, USA. Abstract: Some musical chords sound pleasant, or consonant, while others sound
unpleasant, or dissonant. Helmholtz's psychoacoustic theory of consonance
and dissonance attributes the perception of dissonance to the sensation
of "beats" and "roughness" caused by interactions
in the auditory periphery between adjacent partials of complex tones comprising
a musical chord. Conversely, consonance 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 dissonance were recorded in A1 of awake macaques
and evaluated using auditory-evoked potential (AEP), multiunit activity
(MUA), and current-source density (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 epilepsy. Chords were composed of two simultaneous
harmonic complex tones. The magnitude of oscillatory phase-locked activity
in A1 of the monkey correlates with the perceived dissonance of the musical
chords. Responses evoked by dissonant chords, such as minor and major
seconds, display oscillations phase-locked to the predicted difference
frequencies, whereas responses evoked by consonant chords, such as
octaves and perfect fifths, display little or no phase-locked activity.
AEPs recorded in Heschl's gyrus display strikingly similar oscillatory
patterns to those observed in monkey A1, with dissonant chords eliciting
greater phase-locked activity than consonant chords. In contrast to recordings
in Heschl's gyrus, AEPs recorded in the planum temporale do not display
significant phase-locked activity, suggesting functional differentiation
of auditory cortical regions in humans. These findings support the relevance
of synchronous phase-locked neural ensemble activity in A1 for the physiological
representation of sensory dissonance in humans and highlight the merits
of complementary monkey/human studies in the investigation of neural substrates
underlying auditory perception. Comment: Phase-locking to the beat frequencies of dissonant musical chords, e.g., C4 = 256 Hz plus C#4 = 269.7 Hz resulting in beat of 13.7 Hz, in the compound activity of large neuron populations of the primary auditory cortex is an interesting new finding. It remains to be seen if this effect has a causal relationship to the perception of roughness or unpleasantness when hearing dissonant chords. A more plausible location for the neuronal origin of dissonance perception may be the auditory midbrain, where the frequencies of dissonance beats are represented in a much more complete way and, more importantly, even in the firing activity of single neurons (Sinex et al., 2002). Further, the auditory midbrain is situated very close to major reward-related centers in the upper brainstem, which may respond specifically to temporal coherence in consonant chords and to "roughness" beats in dissonant chords. The most interesting detail in Fishman's study may be that neuronal beats related to the pitch frequencies (f0) of two tones are in most cases much stronger than the neuronal beats related to the frequencies of single harmonics of the tones, despite identical sound levels for fundamentals and single harmonics of the tones (Figs. 3-7, 10, and 11). Most compelling are the results shown in Figs. 3 and 4, where the chords of "minor second" and "major second" produce by far the strongest phase-locking to the beat between the pitch frequencies, even though these frequencies are clearly below the frequency response range of the recording site and two resp. three octaves below the best-frequency of the recording site. Because the input to the recording sites came from subcortical auditory centers, we have to conclude that the pitch frequencies had already been extracted and amplified below the auditory cortex. The present findings therefore elegantly support earlier evidence that the pitch of speech vowels and musical tones is extracted in the auditory midbrain. The new results actually exclude the alternative assumption that pitch may first be extracted in the auditory cortex. (Comment Martin Braun) |