Brainstem Frequency-following Responses Research Articles

Short- and long-term neuroplasticity interact during the perceptual learning of concurrent speech.

Plasticity from auditory experience shapes the brain's encoding and perception of sound. However, whether such long-term plasticity alters the trajectory of short-term plasticity during speech processing has yet to be investigated. Here, we explored the neural mechanisms and interplay between short- and long-term neuroplasticity for rapid auditory perceptual learning of concurrent speech sounds in young, normal-hearing musicians and nonmusicians. Participants learned to identify double-vowel mixtures during ~ 45min training sessions recorded simultaneously with high-density electroencephalography (EEG). We analyzed frequency-following responses (FFRs) and event-related potentials (ERPs) to investigate neural correlates of learning at subcortical and cortical levels, respectively. Although both groups showed rapid perceptual learning, musicians showed faster behavioral decisions than nonmusicians overall. Learning-related changes were not apparent in brainstem FFRs. However, plasticity was highly evident in cortex, where ERPs revealed unique hemispheric asymmetries between groups suggestive of different neural strategies (musicians: right hemisphere bias; nonmusicians: left hemisphere). Source reconstruction and the early (150-200ms) time course of these effects localized learning-induced cortical plasticity to auditory-sensory brain areas. Our findings reinforce the domain-general benefits of musicianship but reveal that successful speech sound learning is driven by a critical interplay between long- and short-term mechanisms of auditory plasticity, which first emerge at a cortical level.

Cortical-brainstem interplay during speech perception in older adults with and without hearing loss.

Real time modulation of brainstem frequency-following responses (FFRs) by online changes in cortical arousal state via the corticofugal (top-down) pathway has been demonstrated previously in young adults and is more prominent in the presence of background noise. FFRs during high cortical arousal states also have a stronger relationship with speech perception. Aging is associated with increased auditory brain responses, which might reflect degraded inhibitory processing within the peripheral and ascending pathways, or changes in attentional control regulation via descending auditory pathways. Here, we tested the hypothesis that online corticofugal interplay is impacted by age-related hearing loss. We measured EEG in older adults with normal-hearing (NH) and mild to moderate hearing-loss (HL) while they performed speech identification tasks in different noise backgrounds. We measured α power to index online cortical arousal states during task engagement. Subsequently, we split brainstem speech-FFRs, on a trial-by-trial basis, according to fluctuations in concomitant cortical α power into low or high α FFRs to index cortical-brainstem modulation. We found cortical α power was smaller in the HL than the NH group. In NH listeners, α-FFRs modulation for clear speech (i.e., without noise) also resembled that previously observed in younger adults for speech in noise. Cortical-brainstem modulation was further diminished in HL older adults in the clear condition and by noise in NH older adults. Machine learning classification showed low α FFR frequency spectra yielded higher accuracy for classifying listeners' perceptual performance in both NH and HL participants. Moreover, low α FFRs decreased with increased hearing thresholds at 0.5-2 kHz for clear speech but noise generally reduced low α FFRs in the HL group. Collectively, our study reveals cortical arousal state actively shapes brainstem speech representations and provides a potential new mechanism for older listeners' difficulties perceiving speech in cocktail party-like listening situations in the form of a miss-coordination between cortical and subcortical levels of auditory processing.

Brainstem speech encoding is dynamically shaped online by fluctuations in cortical [formula omitted] state

Experimental evidence in animals demonstrates cortical neurons innervate subcortex bilaterally to tune brainstem auditory coding. Yet, the role of the descending (corticofugal) auditory system in modulating earlier sound processing in humans during speech perception remains unclear. Here, we measured EEG activity as listeners performed speech identification tasks in different noise backgrounds designed to tax perceptual and attentional processing. We hypothesized brainstem speech coding might be tied to attention and arousal states (indexed by cortical α power) that actively modulate the interplay of brainstem-cortical signal processing. When speech-evoked brainstem frequency-following responses (FFRs) were categorized according to cortical α states, we found low α FFRs in noise were weaker, correlated positively with behavioral response times, and were more ”decodable” via neural classifiers. Our data provide new evidence for online corticofugal interplay in humans and establish that brainstem sensory representations are continuously yoked to (i.e., modulated by) the ebb and flow of cortical states to dynamically update perceptual processing.

Attention reinforces human corticofugal system to aid speech perception in noise

Perceiving speech-in-noise (SIN) demands precise neural coding between brainstem and cortical levels of the hearing system. Attentional processes can then select and prioritize task-relevant cues over competing background noise for successful speech perception. In animal models, brainstem-cortical interplay is achieved via descending corticofugal projections from cortex that shape midbrain responses to behaviorally-relevant sounds. Attentional engagement of corticofugal feedback may assist SIN understanding but has never been confirmed and remains highly controversial in humans. To resolve these issues, we recorded source-level, anatomically constrained brainstem frequency-following responses (FFRs) and cortical event-related potentials (ERPs) to speech via high-density EEG while listeners performed rapid SIN identification tasks. We varied attention with active vs. passive listening scenarios whereas task difficulty was manipulated with additive noise interference. Active listening (but not arousal-control tasks) exaggerated both ERPs and FFRs, confirming attentional gain extends to lower subcortical levels of speech processing. We used functional connectivity to measure the directed strength of coupling between levels and characterize “bottom-up” vs. “top-down” (corticofugal) signaling within the auditory brainstem-cortical pathway. While attention strengthened connectivity bidirectionally, corticofugal transmission disengaged under passive (but not active) SIN listening. Our findings (i) show attention enhances the brain's transcription of speech even prior to cortex and (ii) establish a direct role of the human corticofugal feedback system as an aid to cocktail party speech perception.

Tone language experience-dependent advantage in pitch representation in brainstem and auditory cortex is maintained under reverberation

Long-term language and music experience enhances neural representation of temporal attributes of pitch in the brainstem and auditory cortex in favorable listening conditions. Herein we examine whether brainstem and cortical pitch mechanisms—shaped by long-term language experience—maintain this advantage in the presence of reverberation-induced degradation in pitch representation. Brainstem frequency following responses (FFR) and cortical pitch responses (CPR) were recorded concurrently from Chinese and English-speaking natives, using a Mandarin word exhibiting a high rising pitch (/yi2/). Stimuli were presented diotically in quiet (Dry), and in the presence of Slight, Mild, and Moderate reverberation conditions. Regardless of language group, the amplitude of both brainstem FFR (F0) and cortical CPR (NaPb) responses decreased with increases in reverberation. Response amplitude for Chinese, however, was larger than English in all reverberant conditions. The Chinese group also exhibited a robust rightward asymmetry at temporal electrode sites (T8 > T7) across stimulus conditions. Regardless of language group, direct comparison of brainstem and cortical responses revealed similar magnitude of change in response amplitude with increasing reverberation. These findings suggest that experience-dependent brainstem and cortical pitch mechanisms provide an enhanced and stable neural representation of pitch-relevant information that is maintained even in the presence of reverberation. Relatively greater degradative effects of reverberation on brainstem (FFR) compared to cortical (NaPb) responses suggest relatively stronger top-down influences on CPRs.

Brainstem-cortical functional connectivity for speech is differentially challenged by noise and reverberation

Everyday speech perception is challenged by external acoustic interferences that hinder verbal communication. Here, we directly compared how different levels of the auditory system (brainstem vs. cortex) code speech and how their neural representations are affected by two acoustic stressors: noise and reverberation. We recorded multichannel (64 ch) brainstem frequency-following responses (FFRs) and cortical event-related potentials (ERPs) simultaneously in normal hearing individuals to speech sounds presented in mild and moderate levels of noise and reverb. We matched signal-to-noise and direct-to-reverberant ratios to equate the severity between classes of interference. Electrode recordings were parsed into source waveforms to assess the relative contribution of region-specific brain areas [i.e., brainstem (BS), primary auditory cortex (A1), inferior frontal gyrus (IFG)]. Results showed that reverberation was less detrimental to (and in some cases facilitated) the neural encoding of speech compared to additive noise. Inter-regional correlations revealed associations between BS and A1 responses, suggesting subcortical speech representations influence higher auditory-cortical areas. Functional connectivity analyses further showed that directed signaling toward A1 in both feedforward cortico-collicular (BS→A1) and feedback cortico-cortical (IFG→A1) pathways were strong predictors of degraded speech perception and differentiated “good” vs. “poor” perceivers. Our findings demonstrate a functional interplay within the brain's speech network that depends on the form and severity of acoustic interference. We infer that in addition to the quality of neural representations within individual brain regions, listeners' success at the “cocktail party” is modulated based on how information is transferred among subcortical and cortical hubs of the auditory-linguistic network.

Neural representation of interaural correlation in human auditory brainstem: Comparisons between temporal-fine structure and envelope

Central processing of interaural correlation (IAC), which depends on the precise representation of acoustic signals from the two ears, is essential for both localization and recognition of auditory objects. A complex soundwave is initially filtered by the peripheral auditory system into multiple narrowband waves, which are further decomposed into two functionally distinctive components: the quickly-varying temporal-fine structure (TFS) and the slowly-varying envelope. In rats, a narrowband noise can evoke auditory-midbrain frequency-following responses (FFRs) that contain both the TFS component (FFRTFS) and the envelope component (FFREnv), which represent the TFS and envelope of the narrowband noise, respectively. These two components are different in sensitivity to the interaural time disparity. In human listeners, the present study investigated whether the FFRTFS and FFREnv components of brainstem FFRs to a narrowband noise are different in sensitivity to IAC and whether there are potential brainstem mechanisms underlying the integration of the two components. The results showed that although both the amplitude of FFRTFS and that of FFREnv were significantly affected by shifts of IAC between 1 and 0, the stimulus-to-response correlation for FFRTFS, but not that for FFREnv, was sensitive to the IAC shifts. Moreover, in addition to the correlation between the binaurally evoked FFRTFS and FFREnv, the correlation between the IAC-shift-induced change of FFRTFS and that of FFREnv was significant. Thus, the TFS information is more precisely represented in the human auditory brainstem than the envelope information, and the correlation between FFRTFS and FFREnv for the same narrowband noise suggest a brainstem binding mechanism underlying the perceptual integration of the TFS and envelope signals.

Test–Retest Reliability of Dual-Recorded Brainstem versus Cortical Auditory-Evoked Potentials to Speech

Auditory-evoked potentials have proven useful in the objective evaluation of sound encoding at different stages of the auditory pathway (brainstem and cortex). Yet, their utility for use in clinical assessment and empirical research relies critically on the precision and test-retest repeatability of the measure. To determine how subcortical/cortical classes of auditory neural responses directly compare in terms of their internal consistency and test-retest reliability within and between listeners. A descriptive cohort study describing the dispersion of electrophysiological measures. Eight young, normal-hearing female listeners. We recorded auditory brainstem responses (ABRs), brainstem frequency-following responses (FFRs), and cortical (P1-N1-P2) auditory-evoked potentials elicited by speech sounds in the same set of listeners. We reassessed responses within each of four different test sessions over a period of 1 mo, allowing us to detect possible changes in latency/amplitude characteristics with finer detail than in previous studies. Our findings show that brainstem and cortical amplitude/latency measures are remarkably stable; with the exception of slight prolongation of the P1 wave, we found no significant variation in any response measure. Intraclass correlation analysis revealed that the speech-evoked FFR amplitude and latency measures achieved superior repeatability (intraclass correlation coefficient >0.85) among the more widely used obligatory brainstem (ABR) and cortical (P1-N1-P2) auditory-evoked potentials. Contrasting these intersubject effects, intrasubject variability (i.e., within-subject coefficient of variation) revealed that while latencies were more stable than amplitudes, brainstem and cortical responses did not differ in their variability at the single subject level. We conclude that (1) the variability of auditory neural responses increases with ascending level along the auditory neuroaxis (cortex > brainstem) between subjects but remains highly stable within subjects and (2) speech-FFRs might provide a more stable measure of auditory function than other conventional responses (e.g., click-ABR), given their lower inter- and intrasubject variability.

Recording the human brainstem frequency-following-response in the free-field

BackgroundThe human auditory brainstem frequency-following response (FFR) is an objective measure used to investigate the brainstem's encoding ability of sounds. Traditionally, FFRs are recorded under close-field conditions (earphones), but free-field stimulations (loudspeaker) have yet to be attempted, which would increase the applications of FFRs by making this technique accessible to those who cannot wear inserted transducers. Here we test the feasibility and reliability of measuring speech ABRs across free and close-field. New methodThe FFR was evoked by a 40-ms consonant–vowel (cv) /da/ syllable which was presented in the standard close-field conditions with insert earphones, and in a novel free-field condition via a loudspeaker. ResultsA well-defined FFR was observed for each stimulating method (free or close-field). We show that it is possible and reliable to elicit FFRs from a speaker and that these do not systematically differ from those elicited by conventional earphones. Comparison with existing methodNeural responses were subjected to a comparative within-subjects analysis, using standard measures found in the literature in order to quantify and compare the intrinsic (amplitude, noise, consistency), acoustic (latency, spectral amplitude) and reliability properties (intraclass correlation coefficients and Bland and Altman limits of agreement) of the neural signal. ConclusionsReliable FFRs can be elicited using free-field presentation with comparable to acoustical, intrinsic and reliability properties as those elicited by standard close-field presentations.

Relative contribution of envelope and fine structure to the subcortical encoding of noise-degraded speech.

Brainstem frequency-following responses (FFR) were elicited to the speech token /ama/ in noise containing only envelope (ENV) or fine structure (TFS) cues to assess the relative contribution of these temporal features to the neural encoding of degraded speech. Successive cue removal weakened FFRs with noise having the most deleterious effect on TFS coding. Neuro-acoustic and response-to-response correlations revealed speech-FFRs are dominated by stimulus ENV for clean speech, with TFS making a stronger contribution in moderate noise levels. Results suggest that the relative weighting of temporal ENV and TFS cues to the neural transcription of speech depends critically on the degree of noise in the soundscape.

Comparison of brainstem frequency-following responses associated with auditory streaming based on spectral and temporal cues

With the ABA streaming paradigm (van Noorden, 1975), Yamagishi et al. (2016) used spectrally differing A and B tones and showed that the auditory brainstem frequency-following response (FFR) to the second A tone co-varied with the perceived auditory stream. This study examined whether this effect is specific to the case in which the streaming is based on the spectral cues. We compared stream-perception-related FFRs between ABA stimuli based on spectral and temporal cues. In the spectral condition, the A and B tones differed in frequency (315 and 400 Hz, respectively). In the temporal condition, the stimulus consisted of sinusoidally amplitude-modulated 4-kHz tones that had identical long-term excitation patterns but differed in modulation frequency (60 and 170 Hz, respectively). We analyzed the spectral amplitudes of the FFRs at the stimulus frequencies for the spectral condition and those at the stimulus modulation frequencies for the temporal condition. We found that the FFR amplitudes to the second A tone generally tended to be larger in the two-stream percept than in the one-stream one for both the spectral and temporal conditions. This result implies that the mechanism responsible for the stream-perception-related FFR is common across cue domains.

Tracing the neural basis of auditory entrainment

Neurons in the auditory cortex synchronize their responses to temporal regularities in sound input. This coupling or “entrainment” is thought to facilitate beat extraction and rhythm perception in temporally structured sounds, such as music. As a consequence of such entrainment, the auditory cortex responds to an omitted (silent) sound in a regular sequence. Although previous studies suggest that the auditory brainstem frequency-following response (FFR) exhibits some of the beat-related effects found in the cortex, it is unknown whether omissions of sounds evoke a brainstem response. We simultaneously recorded cortical and brainstem responses to isochronous and irregular sequences of consonant–vowel syllable /da/ that contained sporadic omissions. The auditory cortex responded strongly to omissions, but we found no evidence of evoked responses to omitted stimuli from the auditory brainstem. However, auditory brainstem responses in the isochronous sound sequence were more consistent across trials than in the irregular sequence. These results indicate that the auditory brainstem faithfully encodes short-term acoustic properties of a stimulus and is sensitive to sequence regularity, but does not entrain to isochronous sequences sufficiently to generate overt omission responses, even for sequences that evoke such responses in the cortex. These findings add to our understanding of the processing of sound regularities, which is an important aspect of human cognitive abilities like rhythm, music and speech perception.

Human Frequency Following Response

Listeners with sensorineural hearing loss (SNHL) typically experience reduced speech perception, which is not completely restored with amplification. This likely occurs because cochlear damage, in addition to elevating audiometric thresholds, alters the neural representation of speech transmitted to higher centers along the auditory neuroaxis. While the deleterious effects of SNHL on speech perception in humans have been well-documented using behavioral paradigms, our understanding of the neural correlates underlying these perceptual deficits remains limited. Using the scalp-recorded frequency following response (FFR), the authors examine the effects of SNHL and aging on subcortical neural representation of acoustic features important for pitch and speech perception, namely the periodicity envelope (F0) and temporal fine structure (TFS; formant structure), as reflected in the phase-locked neural activity generating the FFR. FFRs were obtained from 10 listeners with normal hearing (NH) and 9 listeners with mild-moderate SNHL in response to a steady-state English back vowel /u/ presented at multiple intensity levels. Use of multiple presentation levels facilitated comparisons at equal sound pressure level (SPL) and equal sensation level. In a second follow-up experiment to address the effect of age on envelope and TFS representation, FFRs were obtained from 25 NH and 19 listeners with mild to moderately severe SNHL to the same vowel stimulus presented at 80 dB SPL. Temporal waveforms, Fast Fourier Transform and spectrograms were used to evaluate the magnitude of the phase-locked activity at F0 (periodicity envelope) and F1 (TFS). Neural representation of both envelope (F0) and TFS (F1) at equal SPLs was stronger in NH listeners compared with listeners with SNHL. Also, comparison of neural representation of F0 and F1 across stimulus levels expressed in SPL and sensation level (accounting for audibility) revealed that level-related changes in F0 and F1 magnitude were different for listeners with SNHL compared with listeners with NH. Furthermore, the degradation in subcortical neural representation was observed to persist in listeners with SNHL even when the effects of age were controlled for. Overall, our results suggest a relatively greater degradation in the neural representation of TFS compared with periodicity envelope in individuals with SNHL. This degraded neural representation of TFS in SNHL, as reflected in the brainstem FFR, may reflect a disruption in the temporal pattern of phase-locked neural activity arising from altered tonotopic maps and/or wider filters causing poor frequency selectivity in these listeners. Finally, while preliminary results indicate that the deleterious effects of SNHL may be greater than age-related degradation in subcortical neural representation, the lack of a balanced age-matched control group in this study does not permit us to completely rule out the effects of age on subcortical neural representation.

Multichannel recordings of the human brainstem frequency-following response: Scalp topography, source generators, and distinctions from the transient ABR

Brainstem frequency-following responses (FFRs) probe the neural transcription of speech/music, auditory disorders, and plasticity in subcortical auditory function. Despite clinical and empirical interest, the response's neural basis remains poorly understood. The current study aimed to more fully characterize functional properties of the human FFR (topography, source locations, generation). Speech-evoked FFRs were recorded using a high-density (64 channel) electrode montage. Source dipole modeling and 3-channel Lissajous analysis was used to localize the most likely FFR generators and their orientation trajectories. Additionally, transient auditory brainstem responses (ABRs), recorded in the same listeners, were used to predict FFRs and test the long-held assumption that the sustained potential reflects a series of overlapping onset responses. Results showed that FFRs were maximal at frontocentral scalp locations with obliquely oriented sources from putative generators in the midbrain (i.e., upper brainstem). Comparisons between derived and actual recordings revealed the FFR is not a series of repeated ABR wavelets and thus, represents a functionally distinct brainstem response. FFRs recorded at temporal electrode sites showed larger amplitudes and contained higher frequency components than vertex channels (Fz, Cz) suggesting that FFRs measured near the mastoid are generated more peripherally (auditory nerve) than measurements at frontocentral scalp locations. Furthermore, this reveals the importance of choice in reference electrode location for FFR interpretation. Our findings provide non-invasive evidence that (i) FFRs reflect sustained neural activity whose sources are consistent with rostral brainstem generators and (ii) FFRs are functionally distinct from the onset ABR response.

Auditory attention in a dynamic scene: Behavioral and electrophysiological correlates

The ability to direct and redirect selective auditory attention varies substantially across individuals with normal hearing thresholds, even when sounds are clearly audible. We hypothesized that these differences can come from both differences in the spectrotemporal fidelity of subcortical sound representations and in the efficacy of cortical attentional networks that modulate neural representations of the auditory scene. Here, subjects were presented with an initial stream from straight ahead and a second stream (from either left or right), each comprised of four monotonized consonant-vowel syllables. Listeners were instructed to report the contents of either the first stream (holding attentional focus) or the second stream (switching attentional focus). Critically, the direction of the second stream informed subjects whether to hold or to switch attention. Pilot results suggest that when the lateral angle of the second stream is small, task performance is linked to subcortical encoding fidelity of suprathreshold sound (as measured using brainstem frequency-following responses obtained separately in the same subjects). Using a paradigm that allows simultaneous collection of behavioral measures, FFRs, and cortical responses, here we test whether differences in top-down attentional control explain subject variability when the second stream’s lateral angle is large and coding fidelity does not limit performance.

Aging Degrades the Neural Encoding of Simple and Complex Sounds in the Human Brainstem

Older adults, with or without normal peripheral hearing sensitivity, have difficulty understanding speech. This impaired speech perception may, in part, be due to desynchronization affecting the neural representation of acoustic features. Here we determine if phase-locked neural activity generating the brainstem frequency-following response (FFR) exhibits age-related desynchronization and how this degradation affects the neural representation of simple and complex sounds. The objectives of this study were to (1) characterize the effects of age on the neural representation of simple tones and complex consonant-vowel stimuli, (2) determine if sustained and transient components of the FFR are differentially affected by age, and (3) determine if the inability to encode a simple signal predicts degradation in representation for complex speech signals. Correlational. Thirty four adults (aged 22-77 yr) with hearing thresholds falling within normal limits. Stimuli used to evoke FFRs were 1000 Hz tone bursts as well as a consonant-vowel /da/ sound. The neural representation of simple (tone) and complex (/da/) stimuli declines with advancing age. Tone-FFR phase coherence decreased as chronological age increased. For the consonant-vowel FFRs, transient onset and offset response amplitudes were smaller, and offset responses were delayed with age. Sustained responses at the onset of vowel periodicity were prolonged in latency and smaller in amplitude as age increased. FFT amplitude of the consonant-vowel FFR fundamental frequency did not significantly decline with increasing age. The ability to encode a simple signal was related to degradation in the neural representation of a complex, speechlike sound. Tone-FFR phase coherence was significantly related to the later vowel response components but not the earlier vowel components. FFR components representing the tone and consonant-vowel /da/ stimulus were negatively affected by age, showing age-related reductions in response synchrony and amplitude, as well as prolonged latencies. These aging effects were evident in middle age, even in the absence of significant hearing loss.

Relationship between brainstem, cortical and behavioral measures relevant to pitch salience in humans

Neural representation of pitch-relevant information at both the brainstem and cortical levels of processing is influenced by language or music experience. However, the functional roles of brainstem and cortical neural mechanisms in the hierarchical network for language processing, and how they drive and maintain experience-dependent reorganization are not known. In an effort to evaluate the possible interplay between these two levels of pitch processing, we introduce a novel electrophysiological approach to evaluate pitch-relevant neural activity at the brainstem and auditory cortex concurrently. Brainstem frequency-following responses and cortical pitch responses were recorded from participants in response to iterated rippled noise stimuli that varied in stimulus periodicity (pitch salience). A control condition using iterated rippled noise devoid of pitch was employed to ensure pitch specificity of the cortical pitch response. Neural data were compared with behavioral pitch discrimination thresholds. Results showed that magnitudes of neural responses increase systematically and that behavioral pitch discrimination improves with increasing stimulus periodicity, indicating more robust encoding for salient pitch. Absence of cortical pitch response in the control condition confirms that the cortical pitch response is specific to pitch. Behavioral pitch discrimination was better predicted by brainstem and cortical responses together as compared to each separately. The close correspondence between neural and behavioral data suggest that neural correlates of pitch salience that emerge in early, preattentive stages of processing in the brainstem may drive and maintain with high fidelity the early cortical representations of pitch. These neural representations together contain adequate information for the development of perceptual pitch salience.

Novelty Detection in the Human Auditory Brainstem

Auditory deviance detection has been associated with a human auditory-evoked potential (AEP), the mismatch negativity, generated in the auditory cortex 100-200 ms from sound change onset. Yet, single-unit recordings in animals suggest much earlier (∼20-40 ms), and anatomically lower (i.e., thalamus and midbrain) deviance detection. In humans, recordings of the scalp middle-latency AEPs have confirmed early (∼30-40 ms) deviance detection. However, involvement of the human auditory brainstem in deviance detection has not yet been demonstrated. Here we recorded the auditory brainstem frequency-following response (FFR) to consonant-vowel stimuli (/ba/, /wa/) in young adults, with stimuli arranged in oddball and reversed oddball blocks (deviant probability, p=0.2), allowing for the comparison of FFRs to the same physical stimuli presented in different contextual roles. Whereas no effect was observed for the /wa/ syllable, we found for the /ba/ syllable a reduction in the brainstem FFR to deviant stimuli compared with standard ones and to similar stimuli arranged in a control block, with five equiprobable, rarely occurring sounds. These findings demonstrate that the human auditory brainstem is able to encode regularities in the recent auditory past to detect novel events, and confirm the multiple anatomical and temporal scales of human deviance detection.

Musicians demonstrate experience-dependent brainstem enhancement of musical scale features within continuously gliding pitch

In contrast to language, where pitch patterns consist of continuous and curvilinear contours, musical pitch consists of relatively discrete, stair-stepped sequences of notes. Behavioral and neurophysiological studies suggest that both tone-language and music experience enhance the representation of pitch cues associated with a listener's domain of expertise, e.g., curvilinear pitch in language, discrete scale steps in music. We compared brainstem frequency-following responses (FFRs) of English-speaking musicians (musical pitch experience) and native speakers of Mandarin Chinese (linguistic pitch experience) elicited by rising and falling tonal sweeps that are exemplary of Mandarin tonal contours but uncharacteristic of the pitch patterns typically found in music. In spite of musicians’ unfamiliarity with such glides, we find that their brainstem FFRs show enhancement of the stimulus where the curvilinear sweep traverses discrete notes along the diatonic musical scale. This enhancement was note specific in that it was not observed immediately preceding or following the scale tone of interest (passing note). No such enhancements were observed in Chinese listeners. These findings suggest that the musician's brainstem may be differentially tuned by long-term exposure to the pitch patterns inherent to music, extracting pitch in relation to a fixed, hierarchical scale.

Functional ear (a)symmetry in brainstem neural activity relevant to encoding of voice pitch: A precursor for hemispheric specialization?

Pitch processing is lateralized to the right hemisphere; linguistic pitch is further mediated by left cortical areas. This experiment investigates whether ear asymmetries vary in brainstem representation of pitch depending on linguistic status. Brainstem frequency-following responses (FFRs) were elicited by monaural stimulation of the left and right ear of 15 native speakers of Mandarin Chinese using two synthetic speech stimuli that differ in linguistic status of tone. One represented a native lexical tone (Tone 2: T2); the other, T2′, a nonnative variant in which the pitch contour was a mirror image of T2 with the same starting and ending frequencies. Two 40-ms portions of f 0 contours were selected in order to compare two regions (R1, early; R2 late) differing in pitch acceleration rate and perceptual saliency. In R2, linguistic status effects revealed that T2 exhibited a larger degree of FFR rightward ear asymmetry as reflected in f 0 amplitude relative to T2′. Relative to midline (ear asymmetry = 0), the only ear asymmetry reaching significance was that favoring left ear stimulation elicited by T2′. By left- and right-ear stimulation separately, FFRs elicited by T2 were larger than T2′ in the right ear only. Within T2′, FFRs elicited by the earlier region were larger than the later in both ears. Within T2, no significant differences in FFRS were observed between regions in either ear. Collectively, these findings support the idea that origins of cortical processing preferences for perceptually-salient portions of pitch are rooted in early, preattentive stages of processing in the brainstem.