Abstract
Accurate perception of voice pitch plays a vital role in speech understanding, especially for tonal languages such as Mandarin. Lexical tones are primarily distinguished by the fundamental frequency (F0) contour of the acoustic waveform. It has been shown that the auditory system could extract the F0 from the resolved and unresolved harmonics, and the tone identification performance of resolved harmonics was better than unresolved harmonics. To evaluate the neural response to the resolved and unresolved components of Mandarin tones in quiet and in speech-shaped noise, we recorded the frequency-following response. In this study, four types of stimuli were used: speech with either only-resolved harmonics or only-unresolved harmonics, both in quiet and in speech-shaped noise. Frequency-following responses (FFRs) were recorded to alternating-polarity stimuli and were added or subtracted to enhance the neural response to the envelope (FFRENV) or fine structure (FFRTFS), respectively. The neural representation of the F0 strength reflected by the FFRENV was evaluated by the peak autocorrelation value in the temporal domain and the peak phase-locking value (PLV) at F0 in the spectral domain. Both evaluation methods showed that the FFRENV F0 strength in quiet was significantly stronger than in noise for speech including unresolved harmonics, but not for speech including resolved harmonics. The neural representation of the temporal fine structure reflected by the FFRTFS was assessed by the PLV at the harmonic near to F1 (4th of F0). The PLV at harmonic near to F1 (4th of F0) of FFRTFS to resolved harmonics was significantly larger than to unresolved harmonics. Spearman's correlation showed that the FFRENV F0 strength to unresolved harmonics was correlated with tone identification performance in noise (0 dB SNR). These results showed that the FFRENV F0 strength to speech sounds with resolved harmonics was not affected by noise. In contrast, the response to speech sounds with unresolved harmonics, which were significantly smaller in noise compared to quiet. Our results suggest that coding resolved harmonics was more important than coding envelope for tone identification performance in noise.
Highlights
Mandarin is a popular tonal language and has four lexical tones: flat tone, rising tone, falling rising tone, and falling tone
We found no significant phase-locking value (PLV) in FFRTFS to the unresolved stimuli, in agreement with the idea that the FFRTFS only reflects neural phase-locking to frequencies lower than 1,500 Hz (Aiken and Picton, 2008)
We found that the moderate correlation between tone identification performance and FFRENV F0 strength was significant in noise (0 dB SNR), but not in quiet
Summary
Mandarin is a popular tonal language and has four lexical tones: flat tone, rising tone, falling rising tone, and falling tone. The resolvability of harmonics in this work was defined according to the theoretically derived properties of the bandwidth of the auditory filter (Glasberg and Moore, 1990): The low-order harmonics of a complex tone can be separated out by a single auditory filter and are called resolved harmonics. The resolved harmonics evoke distinct patterns of excitation on the basilar membrane. The unresolved harmonics evoke a complex temporal pattern of activation on the basilar membrane whose envelope repeats at the fundamental period of the waveform. The auditory nerve fibers respond to the different patterns of basilar membrane activation evoked by stimuli with resolved and unresolved harmonics. For stimuli with only unresolved harmonics, the activation pattern in the auditory nerve corresponds to the envelope of the signal. For stimuli with only unresolved harmonics, the temporal discharge patterns in auditory nerve fibers will reflect amplitude modulations (beating, interference patterns) produced through the interactions of nearby harmonics
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