Bharath Chandrasekaran

Music training for the development of auditory skills

The effects of music training in relation to brain plasticity have caused excitement, evident from the popularity of books on this topic among scientists and the general public. Neuroscience research has shown that music training leads to changes throughout the auditory system that prime musicians for listening challenges beyond music processing. This effect of music training suggests that, akin to physical exercise and its impact on body fitness, music is a resource that tones the brain for auditory fitness. Therefore, the role of music in shaping individual development deserves consideration.

The scalp-recorded brainstem response to speech: Neural origins and plasticity

Considerable progress has been made in our understanding of the remarkable fidelity with which the human auditory brainstem represents key acoustic features of the speech signal. The brainstem response to speech can be assessed noninvasively by examining scalp-recorded evoked potentials. Morphologically, two main components of the scalp-recorded brainstem response can be differentiated, a transient onset response and a sustained frequency-following response (FFR). Together, these two components are capable of conveying important segmental and suprasegmental information inherent in the typical speech syllable. Here we examine the putative neural sources of the scalp-recorded brainstem response and review recent evidence that demonstrates that the brainstem response to speech is dynamic in nature and malleable by experience. Finally, we propose a putative mechanism for experience-dependent plasticity at the level of the brainstem.

Neural Timing Is Linked to Speech Perception in Noise

Understanding speech in background noise is challenging for every listener, including those with normal peripheral hearing. This difficulty is attributable in part to the disruptive effects of noise on neural synchrony, resulting in degraded representation of speech at cortical and subcortical levels as reflected by electrophysiological responses. These problems are especially pronounced in clinical populations such as children with learning impairments. Given the established effects of noise on evoked responses, we hypothesized that listening-in-noise problems are associated with degraded processing of timing information at the brainstem level. Participants (66 children; ages, 8–14 years; 22 females) were divided into groups based on their performance on clinical measures of speech-in-noise (SIN) perception and reading. We compared brainstem responses to speech syllables between top and bottom SIN and reading groups in the presence and absence of competing multitalker babble. In the quiet condition, neural response timing was equivalent between groups. In noise, however, the bottom groups exhibited greater neural delays relative to the top groups. Group-specific timing delays occurred exclusively in response to the noise-vulnerable formant transition, not to the more perceptually robust, steady-state portion of the stimulus. These results demonstrate that neural timing is disrupted by background noise and that greater disruptions are associated with the inability to perceive speech in challenging listening conditions.

Mismatch negativity to pitch contours is influenced by language experience.

A cross-language study utilizing the mismatch negativity (MMN) evoked response was conducted to explore the influence of language experience on the preattentive cortical processing of linguistically relevant pitch contours. Chinese and English subjects were presented with Mandarin Chinese tones while the mismatch negativity (MMN) response was elicited using a passive oddball paradigm. Two oddball conditions were constructed with a common deviant, a low falling rising contour tone (T3). One condition consisted of two tones that are acoustically similar to one another (T2/T3: T2, high rising contour=standard). The other condition consisted of two tones that are acoustically dissimilar to one another (T1/T3: T1, high level=standard). These tonal pairs enabled us to assess whether different degrees of similarity between pitch movements exert a differential influence on preattentive pitch processing. Results showed that the mean MMN amplitude of the Chinese group was larger than that of the English group for the T1/T3 condition. No group differences were found for the T2/T3 condition. The mean MMN amplitude was larger for the T1/T3 relative to the T2/T3 condition for the Chinese group only. By virtue of these language group differences, we infer that early cortical processing of pitch contours may be shaped by the relative saliency of acoustic dimensions underlying the pitch patterns of a particular language.

Relative influence of musical and linguistic experience on early cortical processing of pitch contours

To assess domain specificity of experience-dependent pitch representation we evaluated the mismatch negativity (MMN) and discrimination judgments of English musicians, English nonmusicians, and native Chinese for pitch contours presented in a nonspeech context using a passive oddball paradigm. Stimuli consisted of homologues of Mandarin high rising (T2) and high level (T1) tones, and a linear rising ramp (T2L). One condition involved a between-category contrast (T1/T2), the other, a within-category contrast (T2L/T2). Irrespective of condition, musicians and Chinese showed larger MMN responses than nonmusicians; Chinese larger than musicians. Chinese, however, were less accurate than nonnatives in overt discrimination of T2L and T2. Taken together, these findings suggest that experience-dependent effects to pitch contours are domain-general and not driven by linguistic categories. Yet specific differences in long-term experience in pitch processing between domains (music vs. language) may lead to gradations in cortical plasticity to pitch contours.

White matter anisotropy in the ventral language pathway predicts sound-to-word learning success

According to the dual stream model of auditory language processing, the dorsal stream is responsible for mapping sound to articulation and the ventral stream plays the role of mapping sound to meaning. Most researchers agree that the arcuate fasciculus (AF) is the neuroanatomical correlate of the dorsal steam; however, less is known about what constitutes the ventral one. Nevertheless, two hypotheses exist: one suggests that the segment of the AF that terminates in middle temporal gyrus corresponds to the ventral stream, and the other suggests that it is the extreme capsule that underlies this sound-to-meaning pathway. The goal of this study was to evaluate these two competing hypotheses. We trained participants with a sound-to-word learning paradigm in which they learned to use a foreign phonetic contrast for signaling word meaning. Using diffusion tensor imaging, a brain-imaging tool to investigate white matter connectivity in humans, we found that fractional anisotropy in the left parietal–temporal region positively correlated with the performance in sound-to-word learning. In addition, fiber tracking revealed a ventral pathway, composed of the extreme capsule and the inferior longitudinal fasciculus, that mediated auditory comprehension. Our findings provide converging evidence supporting the importance of the ventral steam, an extreme capsule system, in the frontal–temporal language network. Implications for current models of speech processing are also discussed.

Individual variability in cue-weighting and lexical tone learning

Speech sound patterns can be discerned using multiple acoustic cues. The relative weighting of these cues is known to be language-specific. Speech-sound training in adults induces changes in cue-weighting such that relevant acoustic cues are emphasized. In the current study, the extent to which individual variability in cue weighting contributes to differential success in learning to use foreign sound patterns was examined. Sixteen English-speaking adult participants underwent a sound-to-meaning training paradigm, during which they learned to incorporate Mandarin linguistic pitch contours into words. In addition to cognitive tests, measures of pitch pattern discrimination and identification were collected from all participants. Reaction time data from the discrimination task was subjected to 3-way multidimensional scaling to extract dimensions underlying tone perception. Two dimensions relating to pitch height and pitch direction were found to underlie non-native tone space. Good learners attended more to pitch direction relative to poor learners, before and after training. Training increased the ability to identify and label pitch direction. The results demonstrate that variability in the ability to successfully learn to use pitch in lexical contexts can be explained by pre-training differences in cue-weighting.

Brainstem correlates of speech-in-noise perception in children.

Children often have difficulty understanding speech in challenging listening environments. In the absence of peripheral hearing loss, these speech perception difficulties may arise from dysfunction at more central levels in the auditory system, including subcortical structures. We examined brainstem encoding of pitch in a speech syllable in 38 school-age children. In children with poor speech-in-noise perception, we find impaired encoding of the fundamental frequency and the second harmonic, two important cues for pitch perception. Pitch, an essential factor in speaker identification, aids the listener in tracking a specific voice from a background of voices. These results suggest that the robustness of subcortical neural encoding of pitch features in time-varying signals is a key factor in determining success with perceiving speech in noise.

Auditory brainstem measures predict reading and speech-in-noise perception in school-aged children.

Reading and speech-in-noise perception, fundamental aspects of human communication, have been linked to neural indices of auditory brainstem function. However, how these factors interact is currently unclear. Multivariate analysis methods (structural equation modeling) were employed to delineate and quantify the relationships among factors that relate to successful reading and speech in noise perception in children. Neural measures of subcortical speech encoding that reflect the utilization of stimulus regularities, differentiation of stop consonants, and robustness of neural synchrony predicted 73% of the variance in reading scores. A different combination of neural measures, specifically, utilization of stimulus regularities, strength of encoding of lower harmonics, and the extent of noise-induced timing delays uniquely predicted 56% of the variance in speech-in-noise perception measures. The neural measures relating to reading and speech-in-noise perception were substantially non-overlapping and resulted in poor fitting models when substituted for each other, thereby suggesting distinct neural signatures for the two skills. When phonological processing and working memory measures were added to the models, brainstem measures still uniquely predicted variance in reading ability and speech-in-noise perception, highlighting the robustness of the relationship between subcortical auditory function and these skills. The current study suggests that objective neural markers may prove valuable in the assessment of reading or speech-in-noise abilities in children.

Experience-dependent neural plasticity is sensitive to shape of pitch contours

Language experience is known to modulate the preattentive processing of linguistically relevant pitch contours when presented in the speech domain. To assess if experience-dependent effects are specific to speech, we evaluated the mismatch negativity response to nonspeech homologs (iterated rippled noise) of such curvilinear pitch contours (Mandarin: Tone 1, ‘high level’; Tone 2, ‘high rising’) by Chinese and English listeners as well as to a pitch contour that was a linear approximation of Tone 2 (‘linear ascending ramp’). Mandarin speakers showed larger mismatch negativity responses than English to the curvilinear pitch contours only. These results suggest that experience-dependent neural plasticity in early cortical processing of linguistically relevant pitch contours is sensitive to naturally occurring pitch dimensions but not specific to speech per se.