Visual Field Analysis Using Digital Signal Processing of Visual Evoked Potentials

Bradnam, Michael Stephen (1994) Visual Field Analysis Using Digital Signal Processing of Visual Evoked Potentials. PhD thesis, University of Glasgow.

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Abstract

Perimetry is the measurement of the visual field and has important clinical applications in the diagnosis and management of many conditions causing visual impairment. Current clinical methods of perimetry are subjective and are difficult or impossible to perform in some groups of patients such as the young, the elderly and those with severe learning difficulties. This thesis describes the development of an alternative objective form of perimetry, called electroperimetry, which records the cortical potentials evoked by visual stimuli in different parts of the visual field. No commercial electrophysiology system was suitable for the research, so a completely new system based on a personal computer was developed. The system described has two novel features: it uses a Digital Signal Processing (DSP) card to enable real-time signal processing and it provides a wide range of software generated stimuli. Hitherto, a major limitation of electroperimetry had been the time taken to recover visual evoked cortical potential (VECP) signals from electroencephalographic activity and muscle noise. In order to minimise the recording time, the signal-to-noise ratio (SNR) had to be increased. This was achieved by using steady-state stimuli, by analysing the VECP in the frequency domain and by using four recording electrodes. A new DSP method based on adaptive noise cancelling was developed which has a number of advantages over conventional Fourier analysis for real-time signal measurement. Visual stimuli were created using computer graphics and presented to the subject on a computer monitor screen. Work concentrated on optimising pattern-reversal checkerboard stimuli for testing visual field quadrants, and the optimum check-size for testing different size fields was determined. As a result, a dartboard pattern-reversal stimulus was developed in which the most effective check-size was presented at each location in the visual field. The increased SNR enabled the testing of smaller areas within each visual field quadrant. It has been proposed that motion stimuli may be useful for testing the extramacular area of the visual field, so motion-onset stimuli were developed and compared with pattern-reversal stimuli. It was concluded that pattern-reversal stimuli were superior for electroperimetry because the motion-onset VECP is reduced by adaptation to moving stimuli. Accurate positioning of the stimulus on the retina is essential for electroperimetry. A new method of fixation monitoring was introduced which also helped patients to concentrate and reduced fatigue. In addition, a novel system employing a miniature LCD display and an indirect ophthalmoscope has been developed to enable VECPs to be recorded whilst the stimulus pattern is simultaneously positioned on the retina under direct view of the operator. VECPs from transient dark pattern-onset stimuli were investigated for this system. The steady-state stimulus and signal analysis techniques developed were clinically evaluated using a large number of subjects including 17 patients with complete and absolute hemifield and quadrantic field defects. Quadrant field stimuli were used and were presented on a computer monitor screen. A unique aspect of this study was the use of objective signal detection techniques. The results confirmed patterns of subjective visual field loss. The system currently has a sensitivity and specificity of at least 85% and 80% respectively for detecting non-seeing areas of the visual field. The results have demonstrated that when subjective perimetry is not possible the developed system is of potential clinical value to the ophthalmologist. Current developments are aimed at further improving the sensitivity and specificity of the system whilst keeping the recording time to a minimum.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J O Rowan
Keywords: Ophthalmology, Biomedical engineering
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-75008
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 14:38
Last Modified: 27 Sep 2019 14:38
URI: http://theses.gla.ac.uk/id/eprint/75008

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