Neural correlates of consonance, dissonance, and the hierarchy of musical pitch in the human brainstem

Abstract

Consonant and dissonant pitch relationships in music provide the foundation of melody and harmony, the building blocks of Western tonal music. We hypothesized that phase-locked neural activity within the brainstem may preserve information relevant to these important perceptual attributes of music. To this end, we measured brainstem frequency-following responses (FFRs) from nonmusicians in response to the dichotic presentation of nine musical intervals that varied in their degree of consonance and dissonance. Neural pitch salience was computed for each response using temporally based autocorrelation and harmonic pitch sieve analyses. Brainstem responses to consonant intervals were more robust and yielded stronger pitch salience than those to dissonant intervals. In addition, the ordering of neural pitch salience across musical intervals followed the hierarchical arrangement of pitch stipulated by Western music theory. Finally, pitch salience derived from neural data showed high correspondence with behavioral consonance judgments (r = 0.81). These results suggest that brainstem neural mechanisms mediating pitch processing show preferential encoding of consonant musical relationships and, furthermore, preserve the hierarchical pitch relationships found in music, even for individuals without formal musical training. We infer that the basic pitch relationships governing music may be rooted in low-level sensory processing and that an encoding scheme that favors consonant pitch relationships may be one reason why such intervals are preferred behaviorally.

Figure 1.

Procedure for computing neural pitch salience from FFR responses to musical intervals [unison, perfect fifth (consonant), and minor second (dissonant) shown here]. Dichotic presentation of a musical dyad elicits the scalp-recorded FFR response (top row). From each FFR waveform, the autocorrelation function (ACF) is computed and time weighted with a decaying exponential (solid gray line) to calculate the behaviorally relevant periodicities present in the response (second row). Each ACF is then passed through a series of harmonic interval pitch sieves consisting of “windows” centered at f₀ and its integer harmonics (third row). Each sieve template represents a single pitch and the magnitude of the output of each individual sieve represents a measure of neural pitch salience at that pitch. Analyzing the outputs across all possible pitches (25–1000 Hz) results in a running pitch salience for the stimulus (fourth row). As the arrows indicate, the magnitude of pitch salience for a consonant musical interval is more robust than that of a dissonant musical interval (e.g., compare perfect fifth to minor second). Yet, neither interval produces stronger neural pitch salience than the unison.

Figure 2.

Grand-average FFR waveforms (A) and their corresponding frequency spectra (B) elicited from the dichotic presentation of four representative musical intervals. Consonant intervals are shown in black, dissonant intervals in gray. A, Time waveforms reveal clearer periodicity and more robust amplitudes for consonant intervals than dissonant intervals. In addition, dissonant dyads (e.g., minor second and major seventh) show significant interaction of frequency components as evident from the modulated nature of their waveforms. Insets show the musical notation for the input stimulus. B, Frequency spectra reveal that FFRs faithfully preserve the harmonic constituents of both musical notes even though they were presented separately between the two ears (compare response spectrum, filled area, to stimulus spectrum, harmonic locations denoted by dots). Consonant intervals have higher spectral magnitudes across harmonics than dissonant intervals. All amplitudes are normalized relative to the unison.

Figure 3.

Perceptual consonant ratings of musical intervals and estimates of neural pitch salience derived from their respective FFRs. Solid bars, Consonant intervals; hatched bars, dissonant intervals. A, Mean behavioral consonance ratings for dichotic presentation of nine musical intervals. Dyads considered consonant according to music theory (solid bars) are preferred over those considered dissonant [minor second (m2), tritone (TT), major seventh (M7)]. For comparison, the solid line shows predictions from a mathematical model of consonance and dissonance (Sethares, 1993) in which local maxima denote higher degrees of consonance than minima, which denote dissonance. B, Mean neural pitch salience derived from FFR responses to dichotic musical intervals. Consonant intervals produce greater pitch salience than dissonant intervals. Even among intervals common to a single class (e.g., all consonant intervals) FFRs show differential encoding resulting in the hierarchical arrangement of pitch described by Western music theory. Error bars indicate one SEM.

Figure 4.

Neural pitch salience derived from FFRs versus behavioral consonance ratings. Consonant intervals elicit a larger neural pitch salience than dissonant intervals and are judged more pleasant by the listener. Note the systematic clustering of consonant and dissonant intervals and the maximal separation of the unison (most consonant interval) from the minor second (most dissonant interval). Error bars indicate one SEM in either the behavioral or neural dimension, respectively.