Larger Auditory Cortical Area and Broader Frequency Tuning Underlie Absolute Pitch

Abstract

Absolute pitch (AP), the ability of some musicians to precisely identify and name musical tones in isolation, is associated with a number of gross morphological changes in the brain, but the fundamental neural mechanisms underlying this ability have not been clear. We presented a series of logarithmic frequency sweeps to age- and sex-matched groups of musicians with or without AP and controls without musical training. We used fMRI and population receptive field (pRF) modeling to measure the responses in the auditory cortex in 61 human subjects. The tuning response of each fMRI voxel was characterized as Gaussian, with independent center frequency and bandwidth parameters. We identified three distinct tonotopic maps, corresponding to primary (A1), rostral (R), and rostral-temporal (RT) regions of auditory cortex. We initially hypothesized that AP abilities might manifest in sharper tuning in the auditory cortex. However, we observed that AP subjects had larger cortical area, with the increased area primarily devoted to broader frequency tuning. We observed anatomically that A1, R and RT were significantly larger in AP musicians than in non-AP musicians or control subjects, which did not differ significantly from each other. The increased cortical area in AP in areas A1 and R were primarily low frequency and broadly tuned, whereas the distribution of responses in area RT did not differ significantly. We conclude that AP abilities are associated with increased early auditory cortical area devoted to broad-frequency tuning and likely exploit increased ensemble encoding.SIGNIFICANCE STATEMENT Absolute pitch (AP), the ability of some musicians to precisely identify and name musical tones in isolation, is associated with a number of gross morphological changes in the brain, but the fundamental neural mechanisms have not been clear. Our study shows that AP musicians have significantly larger volume in early auditory cortex than non-AP musicians and non-musician controls and that this increased volume is primarily devoted to broad-frequency tuning. We conclude that AP musicians are likely able to exploit increased ensemble representations to encode and identify frequency.

Keywords: Heschl's gyrus; absolute pitch; auditory cortex; music; tonotopy; tuning sharpness.

Figure 1.

Behavioral test scores. AP: open black circles, MUS: open triangles, CON: open squares. n = 20 per group. A, Absolute pitch test scores. “X” represents the quasi-AP subject who scored 67% correct on the AP test. B, JND thresholds. C, Melody mistuning detection (amusia) test results. Error bars indicate SEM. **p < 0.01; ***p < 0.001.

Figure 2.

Volumes of A1, R, and RT, collapsed across hemispheres. AP: open black circles, MUS: open black triangles, CON: open black squares. n = 20 per group. Error bars indicate SEM. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 3.

pRF maps of tonotopy (center frequency) and Q in auditory cortex in representative subjects from each group. A, Left surface inflated hemispheres. B, Left zoomed in view of the pRF map for A1, R, RT and belt regions. Solid black lines indicate boundaries between tonotopic maps and black arrows indicate direction of tonotopic gradient (low-high) consistent with previous interpretations (Kaas and Hackett, 2000; Moerel et al., 2014). C, Left magnified view of the Q maps.

Figure 4.

Center frequency and Q distributions for A1, R, and RT in the AP group (red), the MUS group (green), and the CON group (blue). Each distribution is across all subjects in that group. The red dots indicate points at which the AP distribution differs significantly from the MUS distribution and the green dots indicate points at which the MUS distribution differs significantly from CON.