Separating pitch chroma and pitch height in the human brain

Abstract

Musicians recognize pitch as having two dimensions. On the keyboard, these are illustrated by the octave and the cycle of notes within the octave. In perception, these dimensions are referred to as pitch height and pitch chroma, respectively. Pitch chroma provides a basis for presenting acoustic patterns (melodies) that do not depend on the particular sound source. In contrast, pitch height provides a basis for segregation of notes into streams to separate sound sources. This paper reports a functional magnetic resonance experiment designed to search for distinct mappings of these two types of pitch change in the human brain. The results show that chroma change is specifically represented anterior to primary auditory cortex, whereas height change is specifically represented posterior to primary auditory cortex. We propose that tracking of acoustic information streams occurs in anterior auditory areas, whereas the segregation of sound objects (a crucial aspect of auditory scene analysis) depends on posterior areas.

Fig. 1.

(a) The pitch helix. The musical scale is wrapped around so that each circuit (red) is an octave. The equivalent change in pitch height with fixed chroma is shown (blue). (b) Examples of sounds with changing pitch height. Each of these harmonic complexes (h1, h2, h3) has a flat spectral envelope in the frequency band 0–4 kHz. In h1 (Top), all harmonics of the fundamental, f₀, have equal amplitude; in h2 (Middle), the odd harmonics are attenuated by 10 dB, producing a large increase in pitch height without changing pitch chroma; in h3 (Bottom), the odd harmonics are completely attenuated, producing a one-octave rise in pitch height with the same chroma (2f₀).

Fig. 2.

Psychometric functions for pitch height. The psychometric function shows the dependence of the proportion of correct responses on the size of the change in pitch height. Psychometric functions for three subjects were derived from a two-interval two-alternative forced-choice experiment in which subjects were asked to detect which note is higher. The ordinate shows the proportion of correct subject responses, where 50% is equivalent to performance at chance; the abscissa shows the attenuation of odd harmonics in the test stimulus relative to baseline intensity. The fundamental frequency, f₀, was fixed at 80 Hz throughout the experiment (see Methods). The psychometric functions are based on change in attenuation of odd harmonics relative to standards in which the odd harmonics have fixed attenuation of 0 dB (Left), 6dB(Center), and 12 dB (Right). Each data point is based on at least 60 trials. Weibull functions were fitted by using maximum likelihood estimation implemented in matlab [Mathworks (Natick, MA) (fitting software at http://bootstrap-software.org/psignifit)]. The 75% threshold is defined as the attenuation value at which subjects achieve a score of 75% correct (halfway between chance and ceiling performance); the 75% thresholds and 95% confidence intervals for each threshold shown (+) were derived by using bootstrapping with 999 simulations (17, 18). To obtain 95% confidence intervals, the bootstrapping procedure used a Monte Carlo method for estimating the variability of the fitted psychometric function. All subjects heard an increase in pitch height from the standard when the odd harmonics were attenuated by ≈1 dB in the test stimulus, and the pitch height threshold is uniform along the dimension.

Fig. 3.

Statistical parametric maps for the group. For each contrast (indicated below panels), activated voxels are rendered on the normalized group mean structural MRI in an axial section tilted 0.5 radians to include much of the surface of the superior temporal plane. The statistical criterion was P < 0.05 corrected for multiple comparisons across the whole brain volume. The 90% probability boundaries for PAC (6) are outlined (black). (a) Broadband noise contrasted with silence (noise–silence, green) activates extensive bilateral superior temporal areas including both medial and lateral HG. The pitch-producing stimuli contrasted with noise (pitch–noise, lilac) produce more restricted bilateral activation in lateral HG, PP, and PT. (b) Pitch chroma change contrasted with fixed chroma (all Δ chroma, red) activates bilateral areas in lateral HG, PP, and anterolateral PT. (c) Pitch height change contrasted with fixed height (all Δ height, blue) activates bilateral areas in lateral HG and anterolateral PT. (d) Voxels in b and c activated both by pitch chroma change and pitch height change have been exclusively masked. Pitch chroma change but not height change (Δ chroma only, red) activates bilateral areas anterior to HG in PP; pitch height change but not chroma change (Δ height only, blue) activates bilateral areas in posterior PT. These areas represent distinct brain substrates for processing the two musical dimensions of pitch. The relative magnitude of the blood oxygen-level-dependent signal change in anterior and posterior areas is shown for each of the contrasts of interest (Right). The height of the histogram columns represents the mean size of effect (signal change) relative to global mean signal for the contrasts Δ chroma-only (red) and Δ height-only (blue) at the peak voxels for each contrast in the right hemisphere; vertical bars represent the standard error of the mean size of effect. The histograms demonstrate opposite patterns of pitch chroma and pitch height processing in anterior and posterior auditory areas.

Fig. 4.

Statistical parametric maps for individual subjects. Activated voxels (P < 0.001 uncorrected) are rendered on each individual's structural MRI. The axial section is tilted to run along the superior temporal plane as in Fig. 3, and the contrasts and color key are the same as in Fig. 3 a and d. Bilateral areas including medial HG are activated in the contrast between broadband noise and silence (green). After exclusive masking of voxels activated by both pitch chroma change and pitch height change, the two pitch dimensions show distinct activation patterns in most individuals: pitch chroma change (but not pitch height change) activates mainly areas anterior to HG on the PP (red); pitch height change (but not pitch chroma change) activates mainly PT (blue).