Previous studies have found that spatial processing in children and adults with Down syndrome is different in comparison to the normal population. Some previous studies have also found that there is a high prevalence of colour vision deficiencies in people with Down syndrome. The aim of the present study was to use an objective test, the transient visual evoked potential (VEP), to assess achromatic and chromatic visual processing in adults with Down syndrome. Achromatic VEPs were recorded in response to black-white stimuli presented in patternreversal mode. Chromatic VEPs were recorded in response to two types of colour pattern, presented in pattern onset-offset mode. The two colour types were intended to preferentially stimulate the two principal chromatic pathways of the visual system, the ???redgreen??? and ???blue-yellow??? colour-opponent pathways. These stimuli are here termed the ???LM??? and ???S-(L+M) stimuli, respectively, reflecting the cone types that input to the pathways they are intended to stimulate. Each subject also completed two subjective colour vision tests, the Colour Vision Test Made Easy (CVTME) and the City University Colour Vision Test (CUT). Morphology of the achromatic and chromatic VEPs was found to differ between the group with Down syndrome and an age-matched control group. The latency of the P100 component of the achromatic VEP was found to be significantly later in the group with Down syndrome compared to the control group (the N75 latency was earlier in the group with Down syndrome, but not significantly so). The group-averaged peak-to-peak amplitude of the achromatic VEP was significantly lower in the group with Down syndrome compared to the control group. The major positive component of the VEP in response to the L-M stimulus was of significantly longer latency compared to that of the control group. The major negative component and the peak-to-peak amplitude of this response were not significantly different between the groups. For the response to S-(L+M) stimuli, the latency of the major negativity was significantly earlier in the group with Down syndrome and the major positivity was later, but not significantly so. Amplitude of this response was significantly higher in adults with Down syndrome compared to the control group. Most subjects in both groups passed both the CVTME and CUT. Our findings indicate that chromatic VEPs are abnormal in Down syndrome, and this may reflect abnormal processing of chromatic stimuli in this population. Alternatively, these abnormalities may arise due to abnormal cortical morphology, which may occur with normal or abnormal processing of chromatic signals. These findings further indicate that abnormality of chromatic VEPs may be expected in Down syndrome, and is not necessarily indicative of pathology or other abnormal function that is unrelated to the syndrome.

Introduction: Psychophysical and electrophysiological techniques have shown that chromatic contrast sensitivity improves between infancy and adolescence. In adults, electrophysiological and psychophysical methods usually agree. However, in infants electrophysiological techniques may underestimate ability to see chromatic contrast (Suttle et al., 2002). It is not known if the discrepancy between electrophysiological and psychophysical methods continues during childhood nor whether the chromatic VEP can be used as an indicator of colour perception in children. Purpose: To investigate the transient L-M chromatic visual evoked potential and its ability to indicate perception (psychophysical thresholds) of chromatic stimuli in children and adults. In particular, to determine whether a discrepancy between VEP and psychophysical L-M thresholds exists during childhood and if so, to gain some understanding about the nature of the discrepancy. Methods: Transient chromatic VEPs were recorded in children (aged 4.5-13 years) and adults (aged 20-40 years). VEP thresholds were compared with psychophysical thresholds (within-subjects comparison). Because the VEPs of the children were less intra-individually repeatable in morphology than those of the adults, post-hoc objective analysis of the VEPs, linear (Fourier) and nonlinear dynamical (Grassberger and Procaccia's (1983) correlation dimension) analyses, was conducted. Results: VEP and psychophysical estimates of chromatic contrast thresholds agreed using a variety of methods in the adults. In the children, however, the objective methods of assessment (extrapolation from Fourier-derived amplitudes and the correlation dimension) were more accurate than the methods that employed subjective evaluations of VEP morphology. Conclusion: The L-M transient chromatic VEPs of both children (aged 4.5-13 years) and adults appear to contain chromatic information, even in the absence of repeatable VEP morphology and should therefore be able to indicate chromatic perception (psychophysical thresholds). However, the chromatic information may be present as a nonlinear dynamical signal, which may require objective methods (Fourier analysis, the correlation dimension) to reveal the chromatic signal. The greater intra-individual variability of VEP morphology in children compared to adults may reflect poorer precision when switching between cortical states in children's brains. Alternatively, interactions between the immature visual system of the children and their general EEG may occur. Children's VEPs should therefore be interpreted differently to adult VEPs.

This dissertation is the documentation for a Doctoral research program undertaken at the Industrial Research Institute of the Swinburne University of Technology (IRIS), in collaboration with the School of Biophysical Sciences and Electrical Engineering (BSEE), between the years of 2000 and 2004. The objective of the research work that was undertaken was to study the auditory brainstem response (ABR) characteristics of people with Parkinson�s Disease (PD) to determine whether or not these characteristics could form the basis of a tool for determining the presence of PD; the performance of medication, and disease progression. Prior to the commencement of this research, the gold-standard for PD diagnostics was considered to be autopsy, and the only tools available for assessment of living patients were fluoro-deoxyglucose (FDG) / fluoroDopa (FDopa)-based positron emission tomography (PET) or conventional (subjective) clinical testing. The use of PET was costly and impractical for monitoring disease progression. Clinical testing approaches, on the other hand, had a degree of subjectivity, leading to potential misdiagnoses with inadequately trained clinicians. It was therefore postulated that a low-cost technique based on conventional ABR testing equipment, which was readily available within the medical environment, could provide considerable benefits. In order to be useful as a tool, ABR would ultimately need to identify uniquely the presence of PD from other potential disorders, most notably multiple system atrophy (MSA) which was sometimes confused with PD. At the time this research was conducted, the prevailing method of differentiating PD from MSA was based upon magnetic resonance imaging (MRI). This research was only concerned with the first stage of such a research process � that is, determining whether the ABR of PD patients was fundamentally different to that of a control group. In undertaking the investigation, a significant component of the research involved drawing together research literature from a large number of sources in order to present a case that could present a reasonable research path for experimentation. The next stage of the research involved developing an experimentation program that could be used to compare the ABR characteristics of people with PD against a control group and, further, to compare the ABR characteristics of PD participants against their level of impairment, as measured on a conventional mobility test. In summary, the outcomes of the experimental process that was undertaken, and the statistical analysis of the outcomes, suggested that: (1) People with mild manifestations of PD, who were in an unmedicated state, had marked abnormalities in the amplitude of Wave III of their ABR waveform. (2) The amplitude of Wave III of the ABR waveform was sensitive to variations in PD medication levels. (3) The amplitude of Wave V of the ABR waveform was, to a lesser extent than Wave III, affected by the presence of PD and medication levels. (4) The latencies of Waves III and V of the ABR waveforms did not appear to be affected by the presence of PD or by variations in medication levels. This thesis presents the results of the experimental study, together with a statistical analysis of the significance of the outcomes and an evaluation of the limitations of the work that was carried out. The recommendations arising from the research outcomes suggest ways for extending the work that has been completed in this program.

This thesis reports on several new and innovative instrumentation developments to solve some of the problems of brain activity monitoring, particularly SSVEP (Steady State Visual Evoked Potentials) studies. SSVEP systems generate suitable stimuli and record the resulting brain biopotentials from scalp electrodes. The instrumentation is configured as a 'Neuropsychiatric Workstation', supporting up to 136 scalp electrodes. Operating in the SSVEP mode, the Neuropsychiatric Workstation reported here significantly improves upon the previously reported spatial resolution and accuracy of maps related to the generated stimuli. These maps allows insights to be gained into the cognitive workings of the brain. A significant component of the work reported here covers the development of the multielectrode EEG measurement modules and the associated techniques for minimising interference and cross-talk. The techniques for synchronising recordings from all electrodes with the stimulus, interfacing to a host computer and real-time storage of the very large amounts of data generated to hard disk, are all reported. The SSVEP paradigm uses a sinusoidal-modulated visual stimuli. A novel linearised LED (light emitting diode) head-up display was developed, in addition to more conventional stimuli, such as the alternating checker-board display, all with sinusoidal modulation capability over a range of frequencies. The Neuropsychiatric Workstation described in thesis has been replicated several times and is in regular use at Brain Sciences Institute (BSI) at Swinburne University of Technology, and other collaborative research institutes.

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (M. Communication Pathology)--University of Pretoria, 2008. / Summary in English and Afrikaans. Includes bibliographical references.

Thesis (Ph. D.)--University of Chicago, Dept. of Psychology, 2001. / Includes bibliographical references. Also available on the Internet.

Thesis (D. Phil. Communication Pathology)--Universiteit van Pretoria, 2002. / Summary in English and Afrikaans. Includes bibliographical references.

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