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Title: Cortical contributions to the age-related decline in contrast sensitivity
Author: Reilly, James
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1988
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A signal detection paradigm was employed to investigate the possibility that changes in decision-making may contribute to the age-related decline in contrast sensitivity. Detection of sinusoidal grating patterns was measured at 3 and 15 c/deg for a range of contrasts which were psychophysically equivalent for young and old subjects. A decline in contrast sensitivity with age at the spatial frequencies studied was confirmed for contrast thresholds obtained both by an ascending method and from the 50% hit rate for detection of the grating pattern. The criterion adopted for decision-making, expressed as both B and percentage bias, did not change significantly between young and old subjects at 15 c/deg. At 3 c/deg, criterion did not change significantly at 0. 8x, 1.0x, or 1.2x contrast threshold, but at contrast giving 50% hit rate there was a significant increase with age. Paradoxically, the percentage bias increased significantly at contrast threshold but not at 50% hit rate. It is inferred from the results that the loss of contrast sensitivity is not accountable in terms of adoption of a more conservative criterion by older subjects. Hence visual loss in ageing is attributed to changes within the visual pathway rather than within higher decision-making centres. In order to further investigate this conclusion, the transient pattern visually evoked response (PVER) was recorded in young and old subjects to the onset of a vertical sinusoidal grating pattern of spatial frequency 3 and 8 c/deg and contrasts 40%, 20%, 10%, 5% and 3%. The waveform evoked contained a series of negative and positive components which were labelled N1, P1, N2 and P2. The latencies of the N1, P1 and N2 components were generally found to be significantly longer in old subjects compared with young, while no significant age-related differences were found for P2. Amplitudes and rise times were not found to be significantly different between the age groups. Latencies of the first 3 components generally generally showed a significant inverse correlation with log contrast although the 'gain' of the relationship was not significantly different between young and old subjects. Amplitude and rise time showed no significant correlation with log contrast. The age-related latency changes were confirmed when PVERs to grating patterns of 40% contrast and spatial frequencies 1, 2, 3, 5, 8, 10 and 15 c/deg were recorded in young and old subjects. The latencies of N1 and P1 were found to be significantly correlated with increasing spatial frequency. Rise times of P1 and N2 were significantly inversely correlated with spatial frequency. Amplitudes of P1 and N2 showed a significant inverse correlation with spatial frequency only in young subjects. Age-related latency changes in the flash VER are interpreted in terms of a possible decline in cortical inhibitory function in old subjects. PVERs were also recorded to grating patterns of 3 and 8 c/deg, the contrasts of which were normalized with respect to contrast threshold and hence were psychophysically comparable between subjects of different ages. At 8 c/deg there were no significant latency, amplitude or rise time differences between young and old subjects. At 3 c/deg, however, the age-related latency differences were maintained, even though the stimuli had been made perceptually equivalent. The results indicate that the location of at least some of the physiological changes underlying the age-related deficit in visual performance must lie in or prior to the primary visual cortex where the early components of the PVER are thought to be generated. In addition, it is concluded that the results in the signal detection experiment where contrast thresholds obtained from the 50% hit rate at 3 c/deg (but not 15 c/deg) were higher than expected in old subjects, and the results in the PVER experiments where normalizing stimulus contrast failed to abolish age-related latency differences at 3 c/deg (but not 8 c/deg), can best be explained by an impairment in old people in the processing of transient stimuli at low spatial frequency.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available