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.

The experiments in this thesis used behavioural measures and event-related potentials (ERPs) to investigate the influence of race on face processing in the brain. Previous behavioural research has highlighted an own-race effect in face processing, whereby individuals are more accurate at recognizing own-race compared to other-race faces. The current Thesis examined the own-race effect at perceptual and neural levels. Social influences on the own-race effect were also investigated, such as other-race experience, anxiety and implicit social bias, as these may account for differential own- versus other-race face processing. The main aim of the experiments contained in this thesis was to delve deeper into the examination of own and other-race face perception through a series of original experiments. Participants performed a variety of perceptual discrimination and identification tasks, and completed measures of explicit other-race experience and implicit racial bias to record their perceptions of other-race individuals. Chapters 2-4 saw the development of a new paradigm that tested the own-race effect in perception, in contrast to traditional recognition memory investigations. In Chapter 2 the own-race effect was investigated developmentally and found across three age-groups, and was larger in the two older age-groups. Chapters 3 and 4 found that the own-race effect differed across racial groups, and that social variables such as other-race experience influenced the strength of the own-race effect. In the latter experimental chapters, ERPs revealed that the behavioural own-race effect was evident at a neural level. Chapter 7 demonstrated that face-related stages of processing in the brain were sensitive to race of face. In Chapters 8 and 9, the sensitivity of face processing to own and other-race emotional expression processing was also examined. The additional social factor of emotional expression was explored in order to further the investigation of socially relevant information processing from the face. Findings from the last two experimental chapters demonstrated differential emotional face processing for own- versus other-race faces. Confirming the findings of the behavioural experiments, own- versus other-race emotion processing varied across racial groups and was subject to social influences such as other-race experience, intergroup anxiety and implicit racial bias. Overall, behavioural and neural investigations of the own-race effect demonstrated the influence of social variables such as other-race experience, intergroup anxiety and implicit racial bias on the way in which individuals processed own- versus other-race faces in the human brain.

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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.

Background. A visual prosthesis is a conceptual device designed to activate residual functional neurons in the visual pathway of blind individuals to produce artificial vision. Such device, when applied to stimulate the vitreous surface of the retina, has proven feasible in producing patterned light perception in blind individuals suffering from dystrophic diseases of the retina, such as aged-related macular degeneration (AMD). However the practicality of such approach has been challenged by the difficulty of surgical access and the risks of damaging the neuroretina. Positioning a visual implant over the scleral surface of the eye could present a safer alternative but this stimulation modality has not been tested in diseased retinas. Additionally, recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its visual input and the potential deterioration in visual capabilities as a result of such experience modification may undermine the overall bionic rescue strategy. Methods. Two animal models mimicking the principal pathologies found in AMD, namely photoreceptor degeneration and reduced retinal ganglion cell mass, were used to evaluate the efficacy of trans-scleral stimulation of the retina by recording electrical evoked potentials in the visual cortex. The visual performance following the loss of pattern vision induced by bilateral eyelid suturing in adult mice was examined by analysing visual evoked potentials. Findings. Spatially differentiated cortical activations were obtained notwithstanding the underlying retinopathy in the experiment animals. The charge density thresholds were found to be similar to controls and below the bioelectric safety limit. After prolonged visual deprivation (weeks) in the mouse, the visual cortical responses evoked by either electrical or photic stimuli were both significantly reduced. An assessment of different visual capabilities using patterned stimuli demonstrated that whilst visual acuity and motion sensitivity were preserved, significant depression in luminance and contrast sensitivities was detected. Conclusion. Trans-scleral stimulation of the retina is a feasible approach for the development of a visual prosthesis. Following visual loss the adult brain exhibits significant experience-dependent modifications. These new insights may force a revision on the current bionic rescue strategy.

Background. A visual prosthesis is a conceptual device designed to activate residual functional neurons in the visual pathway of blind individuals to produce artificial vision. Such device, when applied to stimulate the vitreous surface of the retina, has proven feasible in producing patterned light perception in blind individuals suffering from dystrophic diseases of the retina, such as aged-related macular degeneration (AMD). However the practicality of such approach has been challenged by the difficulty of surgical access and the risks of damaging the neuroretina. Positioning a visual implant over the scleral surface of the eye could present a safer alternative but this stimulation modality has not been tested in diseased retinas. Additionally, recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its visual input and the potential deterioration in visual capabilities as a result of such experience modification may undermine the overall bionic rescue strategy. Methods. Two animal models mimicking the principal pathologies found in AMD, namely photoreceptor degeneration and reduced retinal ganglion cell mass, were used to evaluate the efficacy of trans-scleral stimulation of the retina by recording electrical evoked potentials in the visual cortex. The visual performance following the loss of pattern vision induced by bilateral eyelid suturing in adult mice was examined by analysing visual evoked potentials. Findings. Spatially differentiated cortical activations were obtained notwithstanding the underlying retinopathy in the experiment animals. The charge density thresholds were found to be similar to controls and below the bioelectric safety limit. After prolonged visual deprivation (weeks) in the mouse, the visual cortical responses evoked by either electrical or photic stimuli were both significantly reduced. An assessment of different visual capabilities using patterned stimuli demonstrated that whilst visual acuity and motion sensitivity were preserved, significant depression in luminance and contrast sensitivities was detected. Conclusion. Trans-scleral stimulation of the retina is a feasible approach for the development of a visual prosthesis. Following visual loss the adult brain exhibits significant experience-dependent modifications. These new insights may force a revision on the current bionic rescue strategy.

Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2007. / Title from first page of PDF file (viewed January 9, 2008). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 258-271).

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 59-64).

Thesis (M.S.)--University of Texas at El Paso, 2007. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (Ph.D.)--Tufts University, 2004. / Adviser: Phillip J. Holcomb. Submitted to the Dept. of Psychology. Includes bibliographical references (leaves 211-216). Access restricted to members of the Tufts University community. Also available via the World Wide Web;