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The frequency-following response, or FFR, is a neurophysiological response to sound that precisely reflects the ongoing dynamics of sound. It can be used to study the integrity and malleability of neural encoding of sound across the lifespan. Sound processing in the brain can be impaired with pathology and enhanced through expertise. The FFR can index linguistic deprivation, autism, concussion, and reading impairment, and can reflect the impact of enrichment with short-term training, bilingualism, and musicianship. Because of this vast potential, interest in the FFR has grown considerably in the decade since our first tutorial. Despite its widespread adoption, there remains a gap in the current knowledge of its analytical potential. This tutorial aims to bridge this gap. Using recording methods we have employed for the last 20 + years, we have explored many analysis strategies. In this tutorial, we review what we have learned and what we think constitutes the most effective ways of capturing what the FFR can tell us. The tutorial covers FFR components (timing, fundamental frequency, harmonics) and factors that influence FFR (stimulus polarity, response averaging, and stimulus presentation/recording jitter). The spotlight is on FFR analyses, including ways to analyze FFR timing (peaks, autocorrelation, phase consistency, cross-phaseogram), magnitude (RMS, SNR, FFT), and fidelity (stimulus-response correlations, response-to-response correlations and response consistency). The wealth of information contained within an FFR recording brings us closer to understanding how the brain reconstructs our sonic world., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Human subcortical auditory processing is sexually dimorphic. The prevailing view - that sex differences arise from cochlear differences - remains unproven, and the extent to which these differences reflect distinct auditory processes is unknown. To determine the origin of subcortical sex differences, we mapped their emergence onto the peripheral-to-central maturation of the auditory system in 516 participants (250 female) across three age groups: 3-5, 14-15, and 22-26 years. To examine whether these sex differences arise from distinct processes, we compared developmental trajectories of each evoked-response component and tested their ability to predict a participant's sex and age. We find that some subcortical sex differences emerge well after the cochlea is mature and that each measure uniquely contributes to predicting participant demographics, indicating that sex differences arise from multiple central auditory processes., (Copyright © 2019. Published by Elsevier B.V.)

From bustling classrooms to unruly lunchrooms, school settings are noisy. To learn effectively in the unwelcome company of numerous distractions, children must clearly perceive speech in noise. In older children and adults, speech-in-noise perception is supported by sensory and cognitive processes, but the correlates underlying this critical listening skill in young children (3-5 year olds) remain undetermined. Employing a longitudinal design (two evaluations separated by ∼12 months), we followed a cohort of 59 preschoolers, ages 3.0-4.9, assessing word-in-noise perception, cognitive abilities (intelligence, short-term memory, attention), and neural responses to speech. Results reveal changes in word-in-noise perception parallel changes in processing of the fundamental frequency (F0), an acoustic cue known for playing a role central to speaker identification and auditory scene analysis. Four unique developmental trajectories (speech-in-noise perception groups) confirm this relationship, in that improvements and declines in word-in-noise perception couple with enhancements and diminishments of F0 encoding, respectively. Improvements in word-in-noise perception also pair with gains in attention. Word-in-noise perception does not relate to strength of neural harmonic representation or short-term memory. These findings reinforce previously-reported roles of F0 and attention in hearing speech in noise in older children and adults, and extend this relationship to preschool children., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The ability to synchronize motor movements along with an auditory beat places stringent demands on the temporal processing and sensorimotor integration capabilities of the nervous system. Links between millisecond-level precision of auditory processing and the consistency of sensorimotor beat synchronization implicate fine auditory neural timing as a mechanism for forming stable internal representations of, and behavioral reactions to, sound. Here, for the first time, we demonstrate a systematic relationship between consistency of beat synchronization and trial-by-trial stability of subcortical speech processing in preschoolers (ages 3 and 4 years old). We conclude that beat synchronization might provide a useful window into millisecond-level neural precision for encoding sound in early childhood, when speech processing is especially important for language acquisition and development., (Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The auditory system is stunning in its capacity for change: a single neuron can modulate its tuning in minutes. Here we articulate a conceptual framework to understand the biology of auditory learning where an animal must engage cognitive, sensorimotor, and reward systems to spark neural remodeling. Central to our framework is a consideration of the auditory system as an integrated whole that interacts with other circuits to guide and refine life in sound. Despite our emphasis on the auditory system, these principles may apply across the nervous system. Understanding neuroplastic changes in both normal and impaired sensory systems guides strategies to improve everyday communication., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Previous studies have evaluated representation of the fundamental frequency (F0) in the frequency following response (FFR) of infants, but the development of other aspects of the FFR, such as timing and harmonics, has not yet been examined. Here, FFRs were recorded to a speech syllable in 28 infants, ages three to ten months. The F0 amplitude of the response was variable among individuals but was strongly represented in some infants as young as three months of age. The harmonics, however, showed a systematic increase in amplitude with age. In the time domain, onset, offset, and inter-peak latencies decreased with age. These results are consistent with neurophysiological studies indicating that (1) phase locking to lower frequency sounds emerges earlier in life than phase locking to higher frequency sounds and (2) myelination continues to increase in the first year of life. Early representation of low frequencies may reflect greater exposure to low frequency stimulation in utero. The improvement in temporal precision likely parallels an increase in the efficiency of neural transmission accompanied by exposure to speech during the first year of life.

Music and language provide an important context in which to understand the human auditory system. While they perform distinct and complementary communicative functions, music and language are both rooted in the human desire to connect with others. Since sensory function is ultimately shaped by what is biologically important to the organism, the human urge to communicate has been a powerful driving force in both the evolution of auditory function and the ways in which it can be changed by experience within an individual lifetime. This chapter emphasizes the highly interactive nature of the auditory system as well as the depth of its integration with other sensory and cognitive systems. From the origins of music and language to the effects of auditory expertise on the neural encoding of sound, we consider key themes in auditory processing, learning, and plasticity. We emphasize the unique role of the auditory system as the temporal processing "expert" in the brain, and explore relationships between communication and cognition. We demonstrate how experience with music and language can have a significant impact on underlying neural function, and that auditory expertise strengthens some of the very same aspects of sound encoding that are deficient in impaired populations., (© 2015 Elsevier B.V. All rights reserved.)

Understanding speech in noise is one of the most complex activities encountered in everyday life, relying on peripheral hearing, central auditory processing, and cognition. These abilities decline with age, and so older adults are often frustrated by a reduced ability to communicate effectively in noisy environments. Many studies have examined these factors independently; in the last decade, however, the idea of an auditory-cognitive system has emerged, recognizing the need to consider the processing of complex sounds in the context of dynamic neural circuits. Here, we used structural equation modeling to evaluate the interacting contributions of peripheral hearing, central processing, cognitive ability, and life experiences to understanding speech in noise. We recruited 120 older adults (ages 55-79) and evaluated their peripheral hearing status, cognitive skills, and central processing. We also collected demographic measures of life experiences, such as physical activity, intellectual engagement, and musical training. In our model, central processing and cognitive function predicted a significant proportion of variance in the ability to understand speech in noise. To a lesser extent, life experience predicted hearing-in-noise ability through modulation of brainstem function. Peripheral hearing levels did not significantly contribute to the model. Previous musical experience modulated the relative contributions of cognitive ability and lifestyle factors to hearing in noise. Our models demonstrate the complex interactions required to hear in noise and the importance of targeting cognitive function, lifestyle, and central auditory processing in the management of individuals who are having difficulty hearing in noise., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Aging disrupts neural timing, reducing the nervous system's ability to precisely encode sound. Given that the neural representation of temporal features is strengthened with musical training in young adults, can musical training offset the negative impact of aging on neural processing? By comparing auditory brainstem timing in younger and older musicians and nonmusicians to a consonant-vowel speech sound /da/. we document a musician's resilience to age-related delays in neural timing., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Objective: Sex differences have been demonstrated in the peripheral auditory system as well as in higher-level cognitive processing. Here, we aimed to determine if the subcortical response to a complex auditory stimulus is encoded differently between the sexes., Methods: Using electrophysiological techniques, we assessed the auditory brainstem response to a synthesized stop-consonant speech syllable [da] in 76 native-English speaking, young adults (38 female). Timing and frequency components of the response were compared between males and females to determine which aspects of the response are affected by sex., Results: A dissimilarity between males and females was seen in the neural response to the components of the speech stimulus that change rapidly over time; but not in the slower changing, lower frequency information in the stimulus. We demonstrate that, in agreement with the click-evoked brainstem response, females have earlier peaks relative to males in the subcomponents of the response representing the onset of the speech sound. In contrast, the response peaks comprising the frequency-following response, which encode the fundamental frequency (F(0)) of the stimulus, as well as the spectral amplitude of the response to the F(0), is not affected by sex. Notably, the higher-frequency elements of the speech syllable are encoded differently between males and females, with females having greater representation of spectrotemporal information for frequencies above the F(0)., Conclusions: Our results provide a baseline for interpreting the higher incidence of language impairment (e.g. dyslexia, autism, specific language impairment) in males, and the subcortical deficits associated with these disorders., Significance: These results parallel the subcortical encoding patterns that are documented for good and poor readers in that poor readers differ from good readers on encoding fast but not slow components of speech. This parallel may thus help to explain the higher incidence of reading impairment in males compared to females., (Copyright © 2011 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.)

The addition of background noise to an auditory signal delays brainstem response timing. This effect has been extensively documented using manual peak selection. Peak picking, however, is impractical for large-scale studies of spectrotemporally complex stimuli, and leaves open the question of whether noise-induced delays are frequency-dependent or occur across the frequency spectrum. Here we use an automated, objective method to examine phase shifts between auditory brainstem responses to a speech sound (/da/) presented with and without background noise. We predicted that shifts in neural response timing would also be reflected in frequency-specific phase shifts. Our results indicate that the addition of background noise causes phase shifts across the subcortical response spectrum (70-1000 Hz). However, this noise-induced delay is not uniform such that some frequency bands show greater shifts than others: low-frequency phase shifts (300-500 Hz) are largest during the response to the consonant-vowel formant transition (/d/), while high-frequency shifts (720-1000 Hz) predominate during the response to the steady-state vowel (/a/). Most importantly, phase shifts occurring in specific frequency bands correlate strongly with shifts in the latencies of the predominant peaks in the auditory brainstem response, while phase shifts in other frequency bands do not. This finding confirms the validity of phase shift detection as an objective measure of timing differences and reveals that this method detects noise-induced shifts in timing that may not be captured by traditional peak latency measurements., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Much of our daily communication occurs in the presence of background noise, compromising our ability to hear. While understanding speech in noise is a challenge for everyone, it becomes increasingly difficult as we age. Although aging is generally accompanied by hearing loss, this perceptual decline cannot fully account for the difficulties experienced by older adults for hearing in noise. Decreased cognitive skills concurrent with reduced perceptual acuity are thought to contribute to the difficulty older adults experience understanding speech in noise. Given that musical experience positively impacts speech perception in noise in young adults (ages 18-30), we asked whether musical experience benefits an older cohort of musicians (ages 45-65), potentially offsetting the age-related decline in speech-in-noise perceptual abilities and associated cognitive function (i.e., working memory). Consistent with performance in young adults, older musicians demonstrated enhanced speech-in-noise perception relative to nonmusicians along with greater auditory, but not visual, working memory capacity. By demonstrating that speech-in-noise perception and related cognitive function are enhanced in older musicians, our results imply that musical training may reduce the impact of age-related auditory decline.

Low-frequency temporal information present in speech is critical for normal perception, however the neural mechanism underlying the differentiation of slow rates in acoustic signals is not known. Data from the rat trigeminal system suggest that the paralemniscal pathway may be specifically tuned to code low-frequency temporal information. We tested whether this phenomenon occurs in the auditory system by measuring the representation of temporal rate in lemniscal and paralemniscal auditory thalamus and cortex in guinea pig. Similar to the trigeminal system, responses measured in auditory thalamus indicate that slow rates are differentially represented in a paralemniscal pathway. In cortex, both lemniscal and paralemniscal neurons indicated sensitivity to slow rates. We speculate that a paralemniscal pathway in the auditory system may be specifically tuned to code low-frequency temporal information present in acoustic signals. These data suggest that somatosensory and auditory modalities have parallel sub-cortical pathways that separately process slow rates and the spatial representation of the sensory periphery., (Copyright © 2010 Elsevier B.V. All rights reserved.)

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., (Copyright © 2010 Elsevier B.V. All rights reserved.)

This tutorial provides a comprehensive overview of the methodological approach to collecting and analyzing auditory brain stem responses to complex sounds (cABRs). cABRs provide a window into how behaviorally relevant sounds such as speech and music are processed in the brain. Because temporal and spectral characteristics of sounds are preserved in this subcortical response, cABRs can be used to assess specific impairments and enhancements in auditory processing. Notably, subcortical auditory function is neither passive nor hardwired but dynamically interacts with higher-level cognitive processes to refine how sounds are transcribed into neural code. This experience-dependent plasticity, which can occur on a number of time scales (e.g., life-long experience with speech or music, short-term auditory training, on-line auditory processing), helps shape sensory perception. Thus, by being an objective and noninvasive means for examining cognitive function and experience-dependent processes in sensory activity, cABRs have considerable utility in the study of populations where auditory function is of interest (e.g., auditory experts such as musicians, and persons with hearing loss, auditory processing, and language disorders). This tutorial is intended for clinicians and researchers seeking to integrate cABRs into their clinical or research programs.

Numerous factors contribute to understanding speech in noisy listening environments. There is a clinical need for objective biological assessment of auditory factors that contribute to the ability to hear speech in noise, factors that are free from the demands of attention and memory. Subcortical processing of complex sounds such as speech (auditory brainstem responses to speech and other complex stimuli [cABRs]) reflects the integrity of auditory function. Because cABRs physically resemble the evoking acoustic stimulus, they can provide objective indices of the neural transcription of specific acoustic elements (e.g., temporal, spectral) important for hearing speech. As with brainstem responses to clicks and tones, cABRs are clinically viable in individual subjects. Subcortical transcription of complex sounds is also clinically viable because of its known experience-dependence and role in auditory learning. Together with other clinical measures, cABRs can inform the underlying biological nature of listening and language disorders, inform treatment strategies, and provide an objective index of therapeutic outcomes. In this article, the authors review recent studies demonstrating the role of subcortical speech encoding in successful speech-in-noise perception.

A growing body of research suggests that cognitive functions, such as attention and memory, drive perception by tuning sensory mechanisms to relevant acoustic features. Long-term musical experience also modulates lower-level auditory function, although the mechanisms by which this occurs remain uncertain. In order to tease apart the mechanisms that drive perceptual enhancements in musicians, we posed the question: do well-developed cognitive abilities fine-tune auditory perception in a top-down fashion? We administered a standardized battery of perceptual and cognitive tests to adult musicians and non-musicians, including tasks either more or less susceptible to cognitive control (e.g., backward versus simultaneous masking) and more or less dependent on auditory or visual processing (e.g., auditory versus visual attention). Outcomes indicate lower perceptual thresholds in musicians specifically for auditory tasks that relate with cognitive abilities, such as backward masking and auditory attention. These enhancements were observed in the absence of group differences for the simultaneous masking and visual attention tasks. Our results suggest that long-term musical practice strengthens cognitive functions and that these functions benefit auditory skills. Musical training bolsters higher-level mechanisms that, when impaired, relate to language and literacy deficits. Thus, musical training may serve to lessen the impact of these deficits by strengthening the corticofugal system for hearing., (2009 Elsevier B.V. All rights reserved.)

Objective: Otoacoustic emissions and the speech-evoked auditory brainstem response are objective indices of peripheral auditory physiology that are used clinically for assessing hearing function. While each measure has been extensively explored, their interdependence and the relationships between them remain relatively unexplored., Methods: Distortion product otoacoustic emissions (DPOAEs) and speech-evoked auditory brainstem responses (sABRs) were recorded from 28 normal-hearing adults. Through correlational analyses, DPOAE characteristics were compared to measures of sABR timing and frequency encoding. Data were organized into two DPOAE (Strength and Structure) and five brainstem (Onset, Spectrotemporal, Harmonics, Envelope Boundary, and Pitch) composite measures., Results: DPOAE Strength shows significant relationships with sABR Spectrotemporal and Harmonics measures. DPOAE Structure shows significant relationships with sABR Envelope Boundary. Neither DPOAE Strength nor Structure is related to sABR Pitch., Conclusions: The results of the present study show that certain aspects of the speech-evoked auditory brainstem responses are related to, or covary with, cochlear function as measured by distortion product otoacoustic emissions., Significance: These results form a foundation for future work in clinical populations. Analyzing cochlear and brainstem function in parallel in different clinical populations will provide a more sensitive clinical battery for identifying the locus of different disorders (e.g., language based learning impairments, hearing impairment).

It is well established that in the majority of the population language processing is lateralized to the left hemisphere. Evidence suggests that lateralization is also present in the brainstem. In the current study, the syllable /da/ was presented monaurally to the right and left ears and electrophysiological responses from the brainstem were recorded in adults with symmetrical interaural click-evoked responses. Responses to the right-ear presentation occurred earlier than those to left-ear presentation in two peaks of the frequency following response (FFR) and approached significance for the third peak of the FFR and the offset peak. Interestingly, there were no differences in interpeak latencies indicating the response to right-ear presentation simply occurred earlier over this region. Analyses also showed more robust frequency encoding when stimuli were presented to the right ear than the left ear. The effect was found for the harmonics of the fundamental that correspond to the first formant of the stimulus, but was not seen in the fundamental frequency range. The results suggest that left lateralization of processing acoustic elements important for discriminating speech extends to the auditory brainstem and that these effects are speech specific., (Copyright 2008 S. Karger AG, Basel.)

Musicians have a variety of perceptual and cortical specializations compared to non-musicians. Recent studies have shown that potentials evoked from primarily brainstem structures are enhanced in musicians, compared to non-musicians. Specifically, musicians have more robust representations of pitch periodicity and faster neural timing to sound onset when listening to sounds or both listening to and viewing a speaker. However, it is not known whether musician-related enhancements at the subcortical level are correlated with specializations in the cortex. Does musical training shape the auditory system in a coordinated manner or in disparate ways at cortical and subcortical levels? To answer this question, we recorded simultaneous brainstem and cortical evoked responses in musician and non-musician subjects. Brainstem response periodicity was related to early cortical response timing across all subjects, and this relationship was stronger in musicians. Peaks of the brainstem response evoked by sound onset and timbre cues were also related to cortical timing. Neurophysiological measures at both levels correlated with musical skill scores across all subjects. In addition, brainstem and cortical measures correlated with the age musicians began their training and the years of musical practice. Taken together, these data imply that neural representations of pitch, timing and timbre cues and cortical response timing are shaped in a coordinated manner, and indicate corticofugal modulation of subcortical afferent circuitry.

Development of the human auditory brainstem is thought to be primarily complete by the age of approximately 2 years, such that subsequent sensory plasticity is confined primarily to the cortex. However, recent findings have revealed experience-dependent developmental plasticity in the mammalian auditory brainstem in an animal model. It is not known whether the human system demonstrates similar changes and whether experience with sounds composed of acoustic elements relevant to speech may alter brainstem response characteristics. We recorded brainstem responses evoked by both click and speech syllables in children between the ages of 3 and 12 years. Here, we report a neural response discrepancy in brainstem encoding of these two sounds, observed in 3- to 4-year-old children but not in school-age children. Whereas all children exhibited identical neural activity to a click, 3- to 4-year-old children displayed delayed and less synchronous onset and sustained neural response activity when elicited by speech compared with 5- to 12-year-olds. These results suggest that the human auditory system exhibits developmental plasticity, in both frequency and time domains, for sounds that are composed of acoustic elements relevant to speech. The findings are interpreted within the contexts of stimulus-related differences and experience-dependent plasticity.

A substantial proportion of children with language-based learning problems [learning disabilities (LD)] display abnormal encoding of speech at rostral levels of the auditory brainstem (i.e. midbrain) as measured by the auditory brainstem response (ABR). Of interest here is whether these timing deficits originate at the rostral brainstem or whether they reflect deficient sensory encoding at lower levels of the auditory pathway. We describe the early brainstem response to speech (waves I and III) in typically developing 8- to 12-year-old children and children with LD. We then focus on the early brainstem responses in children with LD found to show abnormal components of the rostral speech-evoked ABR (waves V and A). We found that wave I was not reliably evoked using our speech stimulus and recording parameters in either typically developing children or those with LD. Wave III was reliably evoked in the large majority of subjects in both groups and its timing did not differ between them. These data are consistent with the view that the auditory deficits in the majority of LD children with abnormal speech-evoked ABR originate from corticofugal modulation of subcortical activity., (Copyright 2008 S. Karger AG, Basel.)

Musical training is known to modify cortical organization. Here, we show that such modifications extend to subcortical sensory structures and generalize to processing of speech. Musicians had earlier and larger brainstem responses than nonmusician controls to both speech and music stimuli presented in auditory and audiovisual conditions, evident as early as 10 ms after acoustic onset. Phase-locking to stimulus periodicity, which likely underlies perception of pitch, was enhanced in musicians and strongly correlated with length of musical practice. In addition, viewing videos of speech (lip-reading) and music (instrument being played) enhanced temporal and frequency encoding in the auditory brainstem, particularly in musicians. These findings demonstrate practice-related changes in the early sensory encoding of auditory and audiovisual information.

We have developed a data-driven conceptual framework that links two areas of science: the source-filter model of acoustics and cortical sensory processing streams. The source-filter model describes the mechanics behind speech production: the identity of the speaker is carried largely in the vocal cord source and the message is shaped by the ever-changing filters of the vocal tract. Sensory processing streams, popularly called 'what' and 'where' pathways, are well established in the visual system as a neural scheme for separately carrying different facets of visual objects, namely their identity and their position/motion, to the cortex. A similar functional organization has been postulated in the auditory system. Both speaker identity and the spoken message, which are simultaneously conveyed in the acoustic structure of speech, can be disentangled into discrete brainstem response components. We argue that these two response classes are early manifestations of auditory 'what' and 'where' streams in the cortex. This brainstem link forges a new understanding of the relationship between the acoustics of speech and cortical processing streams, unites two hitherto separate areas in science, and provides a model for future investigations of auditory function.

Auditory brainstem responses were recorded in normal children (NL) and children clinically diagnosed with a learning problem (LP). These responses were recorded to both a click stimulus and the formant transition portion of a speech syllable /da/. While no latency differences between the NL and LP populations were seen in responses to the click stimuli, the syllable /da/ did elicit latency differences between these two groups. Deficits in cortical processing of signals in noise were seen for those LP subjects with delayed brainstem responses to the /da/, but not for LPs with normal brainstem measures. Preliminary findings indicate that training may be beneficial to LP subjects with brainstem processing delays.

Objective: To determine whether the N1-P2 complex reflects training-induced changes in neural activity associated with improved voice-onset-time (VOT) perception., Design: Auditory cortical evoked potentials N1 and P2 were obtained from 10 normal-hearing young adults in response to two synthetic speech variants of the syllable /ba/. Using a repeated measures design, subjects were tested before and after training both behaviorally and neurophysiologically to determine whether there were training-related changes. In between pre- and post-testing sessions, subjects were trained to distinguish the -20 and -10 msec VOT /ba/ syllables as being different from each other. Two stimulus presentation rates were used during electrophysiologic testing (390 msec and 910 msec interstimulus interval)., Results: Before training, subjects perceived both the -20 msec and -10 msec VOT stimuli as /ba/. Through training, subjects learned to identify the -20 msec VOT stimulus as "mba" and -10 msec VOT stimulus as "ba." As subjects learned to correctly identify the difference between the -20 msec and -10 msec VOT syllabi, an increase in N1-P2 peak-to-peak amplitude was observed. The effects of training were most obvious at the slower stimulus presentation rate., Conclusions: As perception improved, N1-P2 amplitude increased. These changes in waveform morphology are thought to reflect increases in neural synchrony as well as strengthened neural connections associated with improved speech perception. These findings suggest that the N1-P2 complex may have clinical applications as an objective physiologic correlate of speech-sound representation associated with speech-sound training.

Biologic processes underlying speech sound perception and learning have been addressed using the mismatch negativity (MMN) evoked response. First is a consideration of how the acoustic properties of the signal affect the neural mechanisms and brain regions engaged. Because the MMN differs depending on the acoustic characteristics of the stimuli used to elicit the response, it has been used to probe mechanisms underlying the neural representation of stimuli along the auditory pathway. Second is a consideration of neurophysiologic correlates of speech sound perception and learning. Detailed is a 'behavioral-neurophysiologic, acoustic-phonetic approach', used to link perception with underlying physiologic processes in humans. The focus here is on children and what has been learned about normal maturation of speech sound perception and its disruption in certain children with learning disorders. The last topic is a consideration of central nervous system changes with perceptual learning. This includes long-term experience with one's native language and short-term auditory training in the laboratory. Limitations and future challenges are discussed., (Copyright 2000 S. Karger AG, Basel)

An approach to understanding biological processes underlying speech perception, is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. Research from our group that pertains to the neurophysiologic representation of speech in the central pathways is reviewed here. Specifically considered is the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and neurophysiologic plasticity associated with speech-sound perceptual learning.

Historically, auditory research has focused predominantly on how relatively simple acoustic signals are represented in the neuronal responses of the auditory periphery. However, in order to understand the neurophysiology underlying speech perception, the ultimate objective is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. This paper reviews three areas pertaining to the central auditory representation of speech: (1) the differences in neural representation of speech sounds at different levels of the auditory system, (2) the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and (3) the plasticity of speech-sound neural representation and speech perception.

Children with learning problems often cannot discriminate rapid acoustic changes that occur in speech. In this study of normal children and children with learning problems, impaired behavioral discrimination of a rapid speech change (/dalpha/versus/galpha/) was correlated with diminished magnitude of an electrophysiologic measure that is not dependent on attention or a voluntary response. The ability of children with learning problems to discriminate another rapid speech change (/balpha/versus/walpha/) also was reflected in the neurophysiology. These results indicate that some children's discrimination deficits originate in the auditory pathway before conscious perception and have implications for differential diagnosis and targeted therapeutic strategies for children with learning disabilities and attention disorders.

Ongoing studies are aimed at identifying the neural pathways responsible for the middle latency response (MLR). These studies involve the analysis of surface and intracranial potentials following pharmacologic inactivation (with lidocaine) of discrete regions of the guinea pig brain. Previous investigations have shown that MLR surface waves recorded over the temporal lobe originate from pathways anatomically and functionally distinct from those that generate MLR waves recorded over the midline, and that both primary and non-primary auditory thalamo-cortical pathways contribute to the guinea pig MLR. The present investigation examines the role of the mesencephalic reticular formation (mRF) in the MLR generating system. Inactivation of the mRF was associated with disruption of the midline response. These waves have been shown to reflect activity from non-primary subdivisions of the thalamo-cortical pathway. Components recorded over the temporal lobe were also affected, consisting of amplitude reduction and latency prolongation without changes in response morphology. Changes in temporal MLR components with mRF inactivation were smaller than those associated with direct inactivation of primary and non-primary subdivisions of the medial geniculate body. These findings indicate that mRF input is essential for normal generation of those components of the MLR thought to reflect both primary and non-primary auditory pathway activity.

Ongoing studies in our laboratory, concerned with identifying the neural pathways responsible for the auditory middle latency response (MLR), have involved analysis of surface and intracranial potentials following pharmacologic inactivation (with lidocaine) of small regions in the guinea pig brain. Previous studies indicate that MLR surface waves recorded over the temporal lobe originate from pathways anatomically distinct from those that generate MLR waves recorded over the midline. The medial geniculate body (MG) contributes to both MLR responses. At issue here are the relative contributions of ventral and caudomedial subdivisions, which have been linked to primary and non-primary auditory pathways, respectively. Ventral and caudomedial subdivisions contributed to the surface-recorded MLR in a distinctive manner. Lidocaine injections to both areas reduced the amplitude of the surface temporal response. Caudomedial injections had a much greater effect on the surface midline responses than did injections in the ventral portion. Thus, the ventral division, a part of the primary auditory pathway, contributes chiefly to the temporal response. The caudomedial portion, which may be linked to non-primary auditory pathways, contributes to both responses.

Although the clinical use of the middle latency response (MLR) in adults is fairly straightforward, its use is complicated by maturational changes that continue throughout the first decade of life. In order to telescope the time period of this long developmental course, we have approached the study of MLR maturation using the gerbil as an animal model. The course of MLR obtained over the temporal lobe development was characterized in the Mongolian gerbil ranging in age from 10 days to 3 months of life. The adult gerbil MLR consists of two positive peaks (A and C) at 11 and 25 ms, respectively, and a negative component (B) at 16 ms. These components emerge in a systematic fashion as a function of age. The present work supports a strong age effect of increased MLR detectability in the gerbil, similar to findings reported for humans. Wave A was infrequently detected in young animals, but when present, it occurred at adult latencies. The latency of waves B and C decreased systematically with age. The amplitude of all components increased with age, similar to findings in humans. The fact that adult-like thresholds were obtained shortly after birth indicates that when present, MLRs may be a good index of hearing threshold. Effects of stimulating across a wide range of intensities were described. The gerbil model appears appropriate for the study of development of the central auditory system function.

Auditory brainstem (ABR) and middle latency responses (MLR) were obtained from each ear in 8 crab-eating macaques, 4 white-handed gibbons, 4 siamangs and 2 orangutans. Macaques ranged in age from 5 days to 15 years with the 6 older animals in age-matched, male-female pairs. From each animal, latency-intensity functions were obtained and multiple MLR recordings were measured at 60 and 70 dB. Latency-intensity functions, interwave intervals, thresholds and percent detectability were calculated for ABR waveforms. Waves II and IV were largest in amplitude and were most consistently detected at low stimulus intensities in all species tested. Waves I and II had adult latencies in the youngest animal tested (5-day-old macaque), while waves III and IV were prolonged in comparison to the 15-month-old macaque, in whom latencies had reached adult values. There were no apparent sex differences in evoked potentials in the age-matched, male-female pairs. A broad, negative MLR at 7-13 ms was observed in all animals. Longer latency MLRs varied among animals of the same species, but were replicable in some individuals, including the youngest macaque (5 days) and orangutan (7 months). These data were compared to responses obtained in humans, other primates and other vertebrates.

In the guinea pig and gerbil, individual components within the MLR time frame differ in optimal recording location. Specifically, MLR components obtained from the midline differ from those obtained over the temporal lobe. In the present paper midline and temporal lobe components were shown to differ not only in scalp topography but also in response to the following experimental manipulations: intracortical injection of neural inactivating agents (lidocaine and kainic acid), temporal lobe ablation, electrolytic lesions, systemic anesthesia, stimulation rate and course of development. Since midline and temporal lobe components respond differently to experimental manipulations, it can be concluded that the midline and temporal lobe responses are mediated by different generator sources. The particular orientation of the generators responsible for the MLR in the guinea pig and gerbil facilitates the identification of individual components. Results from simultaneous recordings of these components during experimental manipulations support the hypothesis of multiple MLR generators in laboratory animals and provide insight into the generators and developmental aspects of the MLR in humans.

Auditory brainstem response (ABR) reflects activation of the neural generators along the ascending auditory pathway when a sound is heard. In this study, we explored the relationship between brainstem encoding of click and speech signals in normal-learning children and in those with language-based learning problems. To that end, ABR was recorded from both types of stimuli. We found that the normal pattern of correlation between click- and speech-evoked ABRs was disrupted when speech-evoked ABRs were delayed. Thus, delayed responses to speech were not indicative of clinically abnormal responses to clicks. We conclude that these two responses reflect largely separate neural processes and that only processes involved in encoding complex signals such as speech are impaired in children with learning problems. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Highlights: [•] Auditory brainstem is part of the neural circuitry mediating rapid auditory learning. [•] The brainstem’s sensitivity to patterns is observed within minutes of exposure. [•] Individual differences in statistical learning reflect individual differences in physiology. [ABSTRACT FROM AUTHOR]