Electrophysiologic responses are used to estimate hearing thresholds and fit hearing aids in young infants, but these estimates are not exact. An objective test of speech encoding could be used to validate infant fittings by showing that speech has been registered in the central auditory system. Such a test could also show the effects of auditory processing problems on the neural representation of speech. This thesis describes techniques for recording electrophysiologic responses to natural speech stimuli from the brainstem and auditory cortex. The first technique uses a Fourier analyzer to measure steady-state brainstem responses to periodicities and envelope changes in vowels, and the second uses a windowed cross-correlation procedure to measure cortical responses to the envelopes of sentences. Two studies were conducted with the Fourier analyzer. The first measured responses to natural vowels with steady and changing fundamentals, and changing formants. Significant responses to the fundamental were detected for all of the vowels, in all of the subjects, in 19 – 73 s (on average). The second study recorded responses to a vowel fundamental and harmonics. Vowels were presented in opposite polarities to distinguish envelope responses from responses to the spectrum. Significant envelope responses were detected in all subjects at the fundamental. Significant spectral responses were detected in most subjects at harmonics near formant peaks. The third study used cross-correlation to measure cortical responses to sentences. Significant envelope responses were detected to all sentences, at delays of roughly 180 ms. Responses were localized to the posterior auditory cortices. A model based on a series of overlapping transient responses to envelope changes could also account for the results, suggesting that the cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The strengths and weaknesses of both techniques are discussed in relation to their potential clinical applications.

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This thesis investigates the involvement of primary auditory cortex (A1) in sound localisation. Experiments were carried out both to assess the effect of A1 inactivation on sound localisation, and to measure the sensitivity of single A1 neurons to sound source location. Ferrets were trained to localise bursts of broadband noise, of varying intensity and duration, from an array of loudspeakers that spanned 360 degrees in azimuth. Bilateral A1 inactivation caused an impairment on this task, but only for short-duration stimuli. Unilateral A1 inactivation also resulted in an impairment for short-duration stimuli, but this was limited to the side of space contralateral to the inactivation, and was only seen in animals which had been highly trained prior to surgery. A feature of the impairment in all animals was the increased number of "front-back confusions", where the animal's response was on the correct side of the midline but the wrong side of the interaural axis. Recordings from ferret A1 showed that the firing rate of individual neurons varied little as sound source location was changed. Further, the neurons' location sensitivity was affected by changes in stimulus intensity and duration. However, mathematical techniques were used to measure the information these neurons provided about sound source location, and it was found that this information was not sensitive to intensity or duration changes. The analysis also showed that the amount of information provided by response latency was greater than that carried by firing rate. Similar mathematical treatment tentatively suggested that the information from different neurons was only slightly redundant, so it may be possible to account for whole-animal localisation performance by assuming that the output of large numbers of neurons is considered. It is concluded that A1 is involved in processing the location of sound sources, but it seems unlikely that sound localisation is A1's primary or only role within the auditory system.

Electrophysiologic responses are used to estimate hearing thresholds and fit hearing aids in young infants, but these estimates are not exact. An objective test of speech encoding could be used to validate infant fittings by showing that speech has been registered in the central auditory system. Such a test could also show the effects of auditory processing problems on the neural representation of speech. This thesis describes techniques for recording electrophysiologic responses to natural speech stimuli from the brainstem and auditory cortex. The first technique uses a Fourier analyzer to measure steady-state brainstem responses to periodicities and envelope changes in vowels, and the second uses a windowed cross-correlation procedure to measure cortical responses to the envelopes of sentences. Two studies were conducted with the Fourier analyzer. The first measured responses to natural vowels with steady and changing fundamentals, and changing formants. Significant responses to the fundamental were detected for all of the vowels, in all of the subjects, in 19 – 73 s (on average). The second study recorded responses to a vowel fundamental and harmonics. Vowels were presented in opposite polarities to distinguish envelope responses from responses to the spectrum. Significant envelope responses were detected in all subjects at the fundamental. Significant spectral responses were detected in most subjects at harmonics near formant peaks. The third study used cross-correlation to measure cortical responses to sentences. Significant envelope responses were detected to all sentences, at delays of roughly 180 ms. Responses were localized to the posterior auditory cortices. A model based on a series of overlapping transient responses to envelope changes could also account for the results, suggesting that the cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The strengths and weaknesses of both techniques are discussed in relation to their potential clinical applications.

Four functional magnetic resonance imaging (fMRI) studies of pitch processing in auditory cortex were designed to reduce the impact of a number of methodological issues that have hitherto limited previous research findings. Due to adaptation effects, it is necessary to repeatedly present short stimulus bursts rather than long-duration stimuli. Thus, conventionally, in neuroimaging studies of pitch perception, a number of short bursts of the pitch stimulus, separated by silent intervals, are compared to a Gaussian noise presented in the same way. The results of the first experiment indicate that replacing the silent intervals with an energetically matched noise context increases the pitch-specific response by removing the 'energy-onset response' that saturates the overall response if silent intervals are used. In the second experiment, a particular pitch-evoking stimulus, iterated ripple noise (IRN), which is commonly used in neuroimaging studies of pitch perception, was examined. Hall and Plack (Cerebral Cortex 2009;19:576-585) showed that IRN contains slowly varying spectro-temporal features unrelated to pitch, and suggested that these features could account for at least some of the cortical activation produced by IRN. The results support this hypothesis, but also suggest that there is an additional pitch-dependent effect in the same region of auditory cortex.The third experiment assessed the effect of using a different control stimulus to the usual Gaussian noise. The new matched controls were a pulse train with randomly jittered inter-pulse intervals and a random-phase unresolved harmonic complex tone. These low-pitch-salience controls were compared to a regular interval pulse train, which is identical to a cosine-phase unresolved harmonic complex tone. The third experiment did not provide evidence for sensitivity to pitch-salience in pitch-responsive regions of auditory cortex. The fourth and final experiment was a factorial design seeking to answer two main questions: 1) Is the pitch-sensitive region of auditory cortex responsive to the salience of other sound features (e.g. modulation)? 2) Are the responses to pitch and to modulation within this region co-located? Two different pitch-evoking stimuli with different levels of pitch salience were used, presented in a noise context. Results indicate that the pitch-sensitive region contains representations for both pitch and modulation. Furthermore, there was no evidence for an interaction between pitch and modulation, suggesting that the two responses are independent. Overall, the results suggest that careful stimulus design, and appropriate experimental control, is necessary to obtain reliable information on the cortical response to pitch. In addition, the results have shed further light on the likely neural substrates of pitch processing in the cortex.

The mammalian auditory system generates representations of the physical world in terms of auditory objects. To decide which object class a particular sound belongs to, the auditory system must recognise the patterns of acoustic components that form the acoustic “fingerprint” of the sound’s auditory class. Where in the central auditory system such patterns are detected and what form the neural processing takes that underlies their detection are unanswered questions in sensory neurophysiology. In the research conducted for this thesis I used artificial vowel sounds to explore the neural and perceptual characteristics of auditory object recognition in rats. I recorded cortical responses from the primary auditory cortex (A1) in anaesthetised rats and determined how well the spiking responses, evoked by artificial vowels, resolve the spectral components that define vowel classes in human perception. The recognition of an auditory class rests on the ability to detect the combination of spectral components that all member sounds of the class share. I generated and evaluated models of the integration by A1 responses of the acoustic components that define human vowels classes. The hippocampus is a candidate area for neural responses that are specific to particular object classes. In this thesis I also report the results of a collaboration during which we investigated how the hippocampus responds to vowels in awake behaving animals. Finally, I explored the processing of vowels behaviourally, testing the perceptual ability of rats to discriminate and classify vowels and in particular whether rats use combinations of spectral components to recognise members of vowel classes. For the behavioural training I built a novel integrated housing and training cage that allows rats to train themselves in auditory recognition tasks. Combining the results and methods presented in this thesis will help reveal how the mammalian auditory system recognises auditory objects.

O implante coclear tem sido indicado para a reabilitação de crianças com Espectro da Neuropatia Auditiva, com ampla variação no desempenho na percepção de fala. Caso o desenvolvimento das estruturas auditivas centrais não ocorra normalmente, pode-se presumir que as habilidades perceptuais que são a base para a percepção e produção da fala também não se desenvolverão normalmente. Nesse contexto, é possível questionar se a maneira como o sistema auditivo dessas crianças responde à estimulação elétrica após o período de privação sensorial poderia ser uma das justificativas para a variabilidade no resultado obtido com o implante coclear. O objetivo deste estudo foi caracterizar o componente P1 dos potenciais evocados auditivos de longa latência em indivíduos com Espectro da Neuropatia Auditiva usuários de implante coclear e correlacioná-los com o desempenho na percepção de fala e secundariamente a outras variáveis relacionadas ao implante coclear. Participaram do estudo 14 crianças com Espectro da Neuropatia Auditiva, usuárias de implante coclear, de ambos os sexos, na faixa etária de 4 a 11 anos. Foi realizada a pesquisa dos potenciais evocados auditivos de longa latência com estimulação acústica, utilizando estímulo de fala /da/ apresentado em campo livre, e a avaliação da percepção de fala por meio do protocolo GASP (BEVILACQUA; TECH, 1996). Como resultado, foi constatado que foi possível registrar o componente P1 em 85,7% desta população, sendo que o valor da latência do componente P1 apresentou correlação significante com o tempo de privação sensorial demonstrando que quanto maior o tempo de privação sensorial, maior a latência do componente P1. Não foi observada correlação significante entre o componente P1 e o tempo de uso do implante coclear. Foi observado que quando separados os indivíduos em grupos de acordo com o desempenho na percepção de fala, aqueles que apresentaram um melhor desempenho possuíam a latência do componente P1 estatisticamente menor, comparados ao de pior desempenho. Com os resultados obtidos neste estudo foi possível caracterizar o componente P1 dos potenciais evocados auditivos de longa latência em crianças com Espectro da Neuropatia Auditiva usuárias de implante coclear e demonstrar a correlação com o desempenho de percepção de fala e tempo de privação sensorial. Em crianças com Espectro da Neuropatia Auditiva, o componente P1 pode servir como um preditor do desempenho da criança usuária de implante coclear para a percepção de fala. / The cochlear implant has been recommended for the rehabilitation of children with Auditory Neuropathy Spectrum, with wide variation in performance in speech perception. If the development of central auditory structures does not occur normally, one can assume that the perceptual abilities that are the basis for the perception and speech production will not develop normally either. In this context one might question if the way the auditory system of these children respond to electrical stimulation after a period of sensory deprivation could be one of the reasons for this variability in the results obtained with the cochlear implant. The aim of this study was to characterize the P1 component of long-latency auditory evoked potentials in cochlear implanted individuals with Auditory Neuropathy Spectrum and correlate it with performance in speech perception and secondarily to other variables related to cochlear implant. The study included 14 cochlear implanted children with Auditory Neuropathy Spectrum, of both sexes, aged from four to eleven years. We studied the long-latency auditory evoked potential with acoustic stimulation, using speech stimuli presented in open field, and assessment of speech perception by means of GASP test (BEVILACQUA; TECH, 1996). As a result, we found that it was possible to register the P1 component in 85.7% of this population, and the P1 component latency value showed a significant correlation with the duration of sensory deprivation showing that the longer the duration of sensory deprivation, the greater the latency of the P1 component. There was no significant correlation between the P1 component and the time of cochlear implant use. It was observed that when the individuals were separated into groups according to performance on speech perception, those who performed better had the P1 component latency statistically lower when compared to the worst performers. With the results obtained from this study it was possible to characterize the P1 component of long-latency auditory evoked potentials in cochlear implanted children with Auditory Neuropathy Spectrum and demonstrate a correlation with the performance of speech perception. In children with Auditory Neuropathy Spectrum, the P1 component may serve as a predictor of the performance of a cochlear implanted child for speech perception.

Frequency-modulated (FM) sweeps play a key role in species specific communication. Evidence from previous studies have shown that central auditory processing has been shown to vary based on the language spoken, which leads to the idea of experience-driven pitch encoding. Other studies have also shown that there is a decrease in this pitch encoding with aging. Using both iterated rippled noise (IRN) and frequency modulated amplitude modulation (FM/AM) methods to create complex pitch sweeps mimicking speech, allows for the processing of pitch to be determined. Neuromodulation using pharmacogenetics allows for the targeted inhibition of a specific neural pathway. Based on previous studies, the primary auditory cortex to inferior colliculus (A1/IC) pathway is hypothesized to be important in pitch encoding. However, there is a lack of evidence on specifically how the pitch information is encoded in the auditory system and how aging impacts the processing. To solve these issues, age-related changes in pitch encoding and maintaining pitch encoding through neuromodulation were characterized in the using behavioral and electrophysiology methods. Behavioral discrimination abilities, measured by modulation of the acoustic startle response, between pitch sweep direction and pitch sweep creation methods highlighted a reduced discrimination in aging and A1/IC inhibited rats. Electrophysiology changes was assessed using envelope-following responses (EFRs) and suggested a decreased initial frequency locking in aging and decrease in frequency locking overall with A1/IC pathway inhibition. Comparison of behavioral and electrophysiology to IRN and FM/AM stimuli show that the decrease in age-related processing as well as A1/IC pathway processing is larger in the behavioral pitch sweep discrimination than in the reduction in EFRs.

Objetivo: Identificar a variação das medidas da latência e da amplitude e estabelecer os valores de referência desse potencial para a população infantil. Métodos: Esse estudo foi do tipo corte transversal. Foram selecionados para esse estudo sujeitos nascidos a termo, com gestação e condições perinatais sem intercorrências, desenvolvimento neuropsicomotor adequado, ausência de queixas auditivas, limiar eletrofisiológico de 30dB e latências das ondas I, III e V do PEATE dentro do esperado para a faixa etária. Participaram deste estudo 105 crianças até seis anos e onze meses, divididas em 7 grupos etários, denominados 1, 2, 3, 4, 5, 6 e 7. Os exames foram realizados no equipamento Biologic Navigator Pro. Inicialmente foi registrado o PEATE a fim de pesquisar o limiar eletrofisiológico dos sujeitos. Em seguida foi feito o registro do potencial evocado cortical auditivo eliciado mediante paradigma oddball com estímulos tonais (tone burst), diferindo quanto à frequência, 750Hz (frequente) e 1000Hz (raro), e estímulos diferindo quanto à fala: /ba/ (frequente) e /da/ (raro). Nesse estudo foram realizadas análises descritivas, comparativas e de correlação dos estímulos tonais e de fala para os grupos etários. Resultados: Foram observadas diferenças significativas quando realizada a comparação do PECA com estímulo de fala para a amplitude de P2 em referência a CzA2 e CzA1, e para a latência de P1 em referência a CzA1, quando registrado com estímulo tonal houve diferença estatisticamente significante para a amplitude de P1 em referência a CzA1. Nas medidas de correlação quando realizadas com estímulo de fala foi verificado aumento da latência de P1 em referência a CzA1, aumento da amplitude de N1 em referência a CzA2 E CzA1, aumento da latência de N1 em referência a CzA1 e redução da amplitude de P2 em referência a CzA1 e CzA2, quando realizado o PECA com estímulo tonal foi verificada redução da amplitude de P1 em referência a CzA1, redução da amplitude de P2 em referência a CZA1 e redução da amplitude de N2 em referência a CzA2. Conclusão: Os resultados obtidos podem ser utilizados como medidas de referência da latência e da amplitude dos potenciais evocados corticais auditivos em lactentes e crianças, bem como no monitoramento de seu desenvolvimento cortical auditivo / Purpose: Identify the variation of the latency and amplitude measures and establish the reference values of this potential for the child population. Methods: This study was of the cross-sectional type. Subjects were born to term, with gestation and perinatal conditions without intercurrences, adequate neuropsychomotor development, absence of auditory complaints, electrophysiological threshold of 30dB and latencies of waves I, III and V of ABR within the expected for the age group. A total of 105 children, of up to six years and eleven months old, divided into 7 age groups, named 1, 2, 3, 4, 5, 6 and 7, participated in the study. The tests were carried out using Biologic Navigator Pro. Initially, ABR was performed in order to investigate the electrophysiological threshold of the subjects. Then, Cortical Auditory Evoked Potential was elicited through oddball paradigm with tone burst differing in frequency, 750Hz (standard) and 1000Hz (nonstandard), and stimuli differing in speech: /ba / (standard) and /da/ (non-standard). In this study, descriptive, comparative and correlation analyzes of tonal and speech stimuli were performed for the age groups. Results: Significant differences were observed when comparing the CAEP with speech stimulus for the P2 amplitude in reference to CzA2 and CzA1, and for P1 latency in reference to CzA1, when recorded with tonal stimulus there was a statistically significant difference for the amplitude of P1 in reference to CzA1. In the correlation measurements when performed with speech stimulus, there was an increase in latency of P1 in reference to CzA1, increase of N1 amplitude in reference to CzA2 E CzA1, increase in N1 latency in reference to CzA1 and reduction of P2 amplitude in Reference to CzA1 and CzA2, when the CAEP with tonal stimulus was performed, P1 amplitude reduction was verified in reference to CzA1, reduction of P2 amplitude in reference to CZA1 and reduction of N2 amplitude in reference to CzA2. Conclusions: The results obtained can be used as reference measures of the latency and amplitude of cortical auditory evoked potentials in infants and children, as well as in the monitoring of their cortical auditory development

Objetivo: Identificar a variação das medidas da latência e da amplitude e estabelecer os valores de referência desse potencial para a população infantil. Métodos: Esse estudo foi do tipo corte transversal. Foram selecionados para esse estudo sujeitos nascidos a termo, com gestação e condições perinatais sem intercorrências, desenvolvimento neuropsicomotor adequado, ausência de queixas auditivas, limiar eletrofisiológico de 30dB e latências das ondas I, III e V do PEATE dentro do esperado para a faixa etária. Participaram deste estudo 105 crianças até seis anos e onze meses, divididas em 7 grupos etários, denominados 1, 2, 3, 4, 5, 6 e 7. Os exames foram realizados no equipamento Biologic Navigator Pro. Inicialmente foi registrado o PEATE a fim de pesquisar o limiar eletrofisiológico dos sujeitos. Em seguida foi feito o registro do potencial evocado cortical auditivo eliciado mediante paradigma oddball com estímulos tonais (tone burst), diferindo quanto à frequência, 750Hz (frequente) e 1000Hz (raro), e estímulos diferindo quanto à fala: /ba/ (frequente) e /da/ (raro). Nesse estudo foram realizadas análises descritivas, comparativas e de correlação dos estímulos tonais e de fala para os grupos etários. Resultados: Foram observadas diferenças significativas quando realizada a comparação do PECA com estímulo de fala para a amplitude de P2 em referência a CzA2 e CzA1, e para a latência de P1 em referência a CzA1, quando registrado com estímulo tonal houve diferença estatisticamente significante para a amplitude de P1 em referência a CzA1. Nas medidas de correlação quando realizadas com estímulo de fala foi verificado aumento da latência de P1 em referência a CzA1, aumento da amplitude de N1 em referência a CzA2 E CzA1, aumento da latência de N1 em referência a CzA1 e redução da amplitude de P2 em referência a CzA1 e CzA2, quando realizado o PECA com estímulo tonal foi verificada redução da amplitude de P1 em referência a CzA1, redução da amplitude de P2 em referência a CZA1 e redução da amplitude de N2 em referência a CzA2. Conclusão: Os resultados obtidos podem ser utilizados como medidas de referência da latência e da amplitude dos potenciais evocados corticais auditivos em lactentes e crianças, bem como no monitoramento de seu desenvolvimento cortical auditivo / Purpose: Identify the variation of the latency and amplitude measures and establish the reference values of this potential for the child population. Methods: This study was of the cross-sectional type. Subjects were born to term, with gestation and perinatal conditions without intercurrences, adequate neuropsychomotor development, absence of auditory complaints, electrophysiological threshold of 30dB and latencies of waves I, III and V of ABR within the expected for the age group. A total of 105 children, of up to six years and eleven months old, divided into 7 age groups, named 1, 2, 3, 4, 5, 6 and 7, participated in the study. The tests were carried out using Biologic Navigator Pro. Initially, ABR was performed in order to investigate the electrophysiological threshold of the subjects. Then, Cortical Auditory Evoked Potential was elicited through oddball paradigm with tone burst differing in frequency, 750Hz (standard) and 1000Hz (nonstandard), and stimuli differing in speech: /ba / (standard) and /da/ (non-standard). In this study, descriptive, comparative and correlation analyzes of tonal and speech stimuli were performed for the age groups. Results: Significant differences were observed when comparing the CAEP with speech stimulus for the P2 amplitude in reference to CzA2 and CzA1, and for P1 latency in reference to CzA1, when recorded with tonal stimulus there was a statistically significant difference for the amplitude of P1 in reference to CzA1. In the correlation measurements when performed with speech stimulus, there was an increase in latency of P1 in reference to CzA1, increase of N1 amplitude in reference to CzA2 E CzA1, increase in N1 latency in reference to CzA1 and reduction of P2 amplitude in Reference to CzA1 and CzA2, when the CAEP with tonal stimulus was performed, P1 amplitude reduction was verified in reference to CzA1, reduction of P2 amplitude in reference to CZA1 and reduction of N2 amplitude in reference to CzA2. Conclusions: The results obtained can be used as reference measures of the latency and amplitude of cortical auditory evoked potentials in infants and children, as well as in the monitoring of their cortical auditory development