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Title: Pattern visual evoked potentials : comparison of onset, reversal and offset components
Author: Shawkat, Fatima Suham
ISNI:       0000 0001 3401 6551
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1997
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Pattern stimulation has proved itself to be an effective technique of studying the visual system in health and disease. However, there are differences in the property of VEPs elicited to different forms of pattern stimulation, namely, reversal, onset and offset. Responses to these three stimuli have usually been studied independently and the relationship between the response components is uncertain. This thesis is concerned with assessing properties and interrelationships between reversal, onset and offset pattern VEPs in controls and in a clinical population of amblyopes. The three stimuli were delivered sequentially in a single recording epoch so that a direct comparison could be made for virtually identical subject and recording conditions. Half-field stimulation was adopted to separate macular and paramacular contributions. The effects of checksize, scotomata and contrast were assessed, and interocular interaction was investigated. The relationship between the three VEP modes was studied by manipulating contrast and spatial phase so that components could be traced from onset/offset modes to the reversal mode. A total of 56 normal and 18 amblyopic subjects were studied. Ipsilateral reversal (N80, P100 and N145), onset (ipsilateral CII and contralateral P105) and, to a lesser extent ipsilateral offset components were enhanced by using small checksizes. They were also susceptible to central scotomata; degraded when contrast change was low; and showed the greatest extent of interocular interaction. These features indicate that they are predominantly of macular origin. Contralateral reversal and offset potentials, and ipsilateral onset CI, were enhanced by large checks and were relatively unaffected by central scotomata, suggesting predominant contributions from paramacular activity. Onset contralateral P105 waveform was sharply defined with macular stimulation but became broad and bifid with paramacular stimulation. These findings were confirmed in amblyopes, in whom macular vision is compromised. Contrast change studies indicate offset and reversal components are closely related, and suggest similar physiological origins. Onset CI and CII could be traced through to reversal P100 and N145, respectively. When small checks (12') were used, onset Co could be traced through to reversal N80 component. The ipsilateral reversal and contralateral onset P105 components have proved to be the most consistent components in distinguishing between macular and paramacular function, and, between normals and amblyopes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Amblyopes