Abstract
Accurate perception of time-variant pitch is important for speech recognition, particularly for tonal languages with different lexical tones such as Mandarin, in which different tones convey different semantic information. Previous studies reported that the auditory nerve and cochlear nucleus can encode different pitches through phase-locked neural activities. However, little is known about how the inferior colliculus (IC) encodes the time-variant periodicity pitch of natural speech. In this study, the Mandarin syllable /ba/ pronounced with four lexical tones (flat, rising, falling then rising and falling) were used as stimuli. Local field potentials (LFPs) and single neuron activity were simultaneously recorded from 90 sites within contralateral IC of six urethane-anesthetized and decerebrate guinea pigs in response to the four stimuli. Analysis of the temporal information of LFPs showed that 93% of the LFPs exhibited robust encoding of periodicity pitch. Pitch strength of LFPs derived from the autocorrelogram was significantly (p < 0.001) stronger for rising tones than flat and falling tones. Pitch strength are also significantly increased (p < 0.05) with the characteristic frequency (CF). On the other hand, only 47% (42 or 90) of single neuron activities were significantly synchronized to the fundamental frequency of the stimulus suggesting that the temporal spiking pattern of single IC neuron could encode the time variant periodicity pitch of speech robustly. The difference between the number of LFPs and single neurons that encode the time-variant F0 voice pitch supports the notion of a transition at the level of IC from direct temporal coding in the spike trains of individual neurons to other form of neural representation.
Highlights
Voice pitch is a perceptual quality of speech that varies with the fundamental frequency (F0), and is controlled by the rate of vibrations produced by the vocal folds
To test the relationship between the Local field potentials (LFPs) and the stimulus waveform, we compared the waveform of the flat tone stimulus, a low-pass filtered version of the stimulus and a typical trace of the LFPs recorded in the inferior colliculus (IC) (Figure 2A)
The LFP waveforms were morphologically similar to the low pass filtered stimulus waveform, suggesting that the LFPs were phase-locked to the stimulus (Figure 2A right panel in dashed rectangular)
Summary
Voice pitch is a perceptual quality of speech that varies with the fundamental frequency (F0), and is controlled by the rate of vibrations produced by the vocal folds. Natural sounds such as animal vocalizations and human speech are complex, quasi-periodic signals that can be partly characterized by their F0 and its harmonics. Periodicity in speech signals is related to the perception of low pitches (‘periodicity pitch’) It is widely known that pitch conveys essential semantic information in speech and, in tonal languages, changes in voice pitch over time can affect the meaning of words. One syllable /ma/ pronounced with four tones: ‘ma’, ‘má’, ‘ma’ and ‘mà’ has four different meanings: ‘mother,’ ‘fiber,’ ‘horse’ and ‘scold’, respectively
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