Differential sensitivity to changes in pitch acceleration in the auditory brainstem and cortex

Abstract

The cortical pitch-specific response (CPR) is differentially sensitive to pitch contours varying in rate of acceleration-time-variant Mandarin Tone2 (T2) versus constant, linear rising ramp (Linear)-as a function of language experience (Krishnan, Gandour, & Suresh, 2014). CPR and brainstem frequency following response (FFR) data were recorded concurrently from native Mandarin listeners using the same stimuli. Results showed that T2 elicited larger responses than Linear at both cortical and brainstem levels (CPR: Na-Pb, Pb-Nb; FFR). However, Pb-Nb exhibited a larger difference in magnitude between T2 and Linear than either Na-Pb or FFR. This finding highlights differential weighting of brain responses elicited by a specific temporal attribute of pitch. Consistent with the notion of a distributed, integrated hierarchical pitch processing network, temporal attributes of pitch are differentially weighted by subcortical and cortical level processing.

Keywords: Cortical pitch response; Experience-dependent plasticity; Functional asymmetry; Fundamental frequency response; Iterated rippled noise; Lexical tone; Mandarin Chinese; Pitch; Pitch acceleration; Pitch encoding.

Figure 1

Grand average CPR waveforms (A) and FFR waveforms (B) elicited by T2 (red) and Linear (blue) pitch contours. The cortical pitch-relevant components (Na, Pb, Nb) are highlighted in gray. The waveform amplitude (A) waveform periodicity (B) and normalized mean response magnitude (C) appear to be more robust for T2 as compared to Linear. (D) Normalized magnitude difference between T2 and Linear per component (Na–Pb, Pb–Nb, FFR) show that Pb–Nb is larger than either Na–Pb or FFR. These data emphasize that Pb–Nb—a pitch-specific neural generator that indexes temporal regularity in a relatively later time window—exhibits heightened sensitivity to changes in acceleration relative to a fixed rate.

Figure 2

Waveform (T2) and spectrograms of each stimulus condition (T2, Linear) illustrate the experimental paradigm used to acquire cortical and brainstem responses. The vertical dashed line at 500 ms demarcates the transition from the initial noise segment to the final pitch segment. CPRs and FFRs were extracted from evoked responses beginning with the onset of the pitch. F0 contours (white) are superimposed on their respective pitch segments. Within the pitch segment (top), the waveform shows robust periodicity at a high IRN iteration step (n=32); the spectrograms (middle) show clear resolution of dynamic, rising spectral bands corresponding to the harmonics of the fundamental frequency. Corresponding acceleration trajectories (bottom panel) are displayed for the two stimuli. T2 (solid), exemplary of Mandarin Tone 2, and Linear (dashed) both represent time-varying rising pitch contours. Linear exhibits a fixed rate of acceleration. T2 is the only pitch pattern that occurs in natural speech and the only one to exhibit a changing acceleration rate.