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published_or_final_version / abstract / toc / Speech and Hearing Sciences / Master / Master of Science in Audiology

Thesis (PhD) - Swinburne University of Technology, Brain Sciences Institute, 2007. / A thesis for Doctor of Neuropsychopharmacology, Brain Sciences Institute, Swinburne University of Technology - 2007. Typescript. Includes bibliographical references (p. 138-164).

Thesis (M.S.)--University of Texas at Dallas, 2008. / Includes vita. Includes bibliographical references (leaves 77-78)

Potentials generated by the eye cause unwanted artifact in Visual Evoked Response (VER) recordings. These artifacts often contaminate the data in a systematic way that can lead to spurious experimental results. Although it is widely agreed that ocular artifact must be accounted for, the methods used to deal with this problem are varied. The present study compared four methods used to control ocular artifact; blink rejection, eyes closed, subtraction and regression. Twenty normal, female subjects were tested twice within the same session. Subjects watched light flashes of 4 intensities; 2, 30, 80, and 240 ft lamberts. The lights were presented at 1 hertz, reached maximum brightness in 0.5 msec and lasted for 0.5 sec. During testing the VER, and electroocculographic (EOG) response generated by a blink, were recorded. In the blink rejection method, any VER epoch that contained blink artifact was excluded from the average. The eyes closed method consisted of having subjects watch the stimuli through closed eyelids. The subtraction method corrects blink artifact by digitally subtracting the averaged EOG from the EEG. The proportion of EOG subtracted was determined by the EEG/EOG ratio estimated while subjects blinked spontaneously in a darkened environment. The regression method determines what proportion of EOG is to be subtracted on the basis of the correlation between EOG and EEG within VER epochs. Two correction, factors are calculated, one to correct for vertical movements and one to correct for horizontal movements. The blink rejection method was found to be useful with subjects who had 40% or more blink-free epochs, but was an unreliable method for the majority of subjects. The eyes closed method was also found to produce poor VER data. The eyelids appear to attenuate the light reaching the retina and there may be eyeball movement despite having the eyes closed. Both the subtraction and regression methods substantially reduced the ocular artifact. Horizontal eye movements do not appear to be a significant problem over the short intervals of VER recording because the regression method was not superior to the subtraction method in removing artifact. Although the subtraction and regression methods effectively reduce ocular artifact, both are less effective at posterior electrode placements. The reason for this may be that ocular potential is not propagated across the scalp in a linear fashion, as often assumed. Using spontaneously generated blinks in a darkened environment, it was found that the ocular potential waveform changes shape as it moves towards the back of the head. / Arts, Faculty of / Psychology, Department of / Graduate

The auditory brainstem response (ABR) is an evoked potential that has achieved widespread acceptance as a technique for evaluating the status and function of the auditory nervous system. For many diagnostic applications, the latency of an obtained ABR peak is compared to clinical norms. One who uses this approach makes some basic assumptions regarding between-subject and within-subject variability of latency. Although a great deal is known about between-subject variability of ABR latency, virtually nothing is known about such variability within a single subject. The purpose of this investigation was to describe the nature of within-subject variability of ABR latency. Nine male subjects participated in the study. Each met the following criteria: 10-12 years of age; normal speech and language development; normal academic progress; normal hearing; and, normal middle ear pressure. A repeated measures design was employed. Four sessions were scheduled for each subject and five ABRs were obtained at each session for each of three stimulus conditions: monaural left, monaural right, and binaural. Stimuli were 100 μs condensation clicks presented at 80 dB nHL. For each ABR peak, the within-subject distribution of latencies was analyzed with regard to symmetry, kurtosis, range, and standard deviation using the SPSSx "Descriptives" procedure. For every subject, variability of latency was observed. Most often, the latencies were normally distributed and the magnitude of variability was small. The variability of latency, as indexed by the standard deviation, was less within any single subject than is commonly reported for groups of subjects. It was concluded that: (a) standard parametric techniques would be appropriate for subsequent analysis of such data; and, (b) by establishing a baseline, the sensitivity of the ABR might be increased for certain within-subject monitoring applications.

Object recognition memory (ORM) refers to both recognition of an object and the memory of having seen it before. In humans, ORM has been investigated using functional neuroimaging and electrophysiological techniques with tests of episodic memory retrieval involving recollection of previously studied items. Processes involved in the maintenance of a mental state adopted for the performance of a retrieval task (retrieval mode) appear to involve right frontal neural regions. More transient processes occurring at the time of item recollection (retrieval success) have shown scalp activity over parietal and right frontal regions. This activity is thought to originate in the medial temporal lobes and the underlying right frontal cortex respectively. The aforementioned findings have been derived mainly from studies using verbal stimuli. It is uncertain whether the same neural regions are involved in object recollection. It is also uncertain whether sustained modal and transient item-related activity involve the same or different right frontal regions. In this study, steady-state probe topography (SSPT) was used to investigate both sustained and transient processes involved in the retrieval of abstract pictorial objects from memory. The ability to vary the evaluation period of the steady-state visually evoked potential (SSVEP) allows investigation of cognitive processes occurring over different time scales. Neural regions involved in sustained modal processes were identified by examining the SSVEP values averaged over the duration of a memory retrieval task. Sustained SSVEP effects were observed over right fronto-temporal regions. Neural regions involved in transient retrieval success processes were identified by comparing the transient SSVEP responses to tasks with different memory loads. Comparison of a higher with a lower memory load condition showed SSVEP effects over parieto-temporal and right inferior frontal regions. Larger differences between memory loads gave effects that were larger and more right lateralized. Retrieval mode and retrieval success processes showed SSVEP effects over different right frontal regions. It was also found that, in contrast to the left lateralized parietal ERP response to recollected verbal stimuli, the SSVEP effects produced with abstract pictorial shapes showed a more bilateral pattern. This was considered to reflect the relatively non-verbalizable pictorial nature of the stimuli.

Novices and experts show distinct differences in the performance of many tasks. Experts may perform a task quickly and accurately with seemingly little attention or effort, whilst novices will perform the same task more slowly and with great effort. The transition from novice to expert performance occurs only after extended practice and has been conceptualized as a transition from controlled to automatic processing, and has been modeled as a reduction in attention or cognitive resources. Alternatively, based on findings relating to learning in the domain of number arithmetic, it has also been modeled as a transition from an algorithmic, or computationally-based process, to the use of memory retrieval. However, relatively few studies have investigated the changes in brain activity associated with such a transition. In this study, the Steady-State Probe Topography technique was used to investigate differences in the topography of the Steady-State Visual Evoked Potential (SSVEP) between an unpracticed and a well-practiced analogue of number arithmetic, Alphabet arithmetic. Subjects showed decreases in response time with practice that followed a power law and were suggestive of automatization. During initial, unpracticed performance of the task, processing of the Alphabet Arithmetic equations was characterised by increased SSVEP amplitude and decreased latency in frontal regions, whilst after extended practice, performance was characterised by reduced SSVEP amplitude and increased latency. It is suggested that the frontal activity during the initial, unpracticed stage of the task implicates a role for working memory, whilst the amplitude decrease and latency increase observed in the well-practiced task may reflect a reduction in excitation, consistent with ideas of an improvement in brain efficiency, and possibly an increase in inhibitory processes.

Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2006. / Title from first page of PDF file (viewed December 13, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references.