City Research Online

Abstract

* The overall aim of this study was to assess how the processing of different stimulus attributes in human vision is affected by ageing and disease. Both foveal and paracentral regions of the retina were investigated, with emphasis on both pregeniculate and postgeniculate impairments.

Isolation of different stimulus attributes and the assessment of visual performance were carried out using a series of Advanced Vision and Optometric Tests (AVOT) developed at City University. A total of 133 normal controls and 59 patients participated in the study. Contrast detection (CT) and contrast acuity (CA) thresholds were assessed using Landolt ring stimuli. First-order motion was examined using moving stimuli embedded in static luminance contrast noise. Red/green (RG) and yellow/blue (YB) colour sensitivity were investigated using dynamic luminance contrast noise, a technique that isolates the use of colour signals. The effects of ageing and loss of visual function caused by disease were examined at the fovea and at each of four paracentral locations. Visual performance was assessed to establish how ageing and disease affect the thresholds for detection of stimulus structure, motion and colour. For each of the AVOT tests, non-parametric limits were established based on data from normal subjects to allow differentiation of the effects of ageing and disease in the patient group. All attributes tested were influenced by ageing, degeneration and disease. The results demonstrate the benefots of parafoveal and peripheral testing. Some conditions are reflected in the periphery first, encroaching on foveal vision with progressing disease. Stimulus attributes tested parafoveally enabled discovery of disease earlier than at the foveal location.

* Ageing affects contrast detection thresholds (CT) differently for foveal and parafoveal locations. Data were more variable for the foveal location across the sample, indicating larger intersubject differences compared to the parafovea. In general, the ageing effects on visual performance can be described by a weak linear upwards trend in threshold as age increases. Additionally, some presumed subclinical cases were included in the sample. These subjects exhibited no signs of abnormality on standard examination but had larger CT thresholds, especially in the older age groups.

* The effect of age on contrast acuity thresholds (CA) was similar for foveal and parafoveal locations. The appropriate choice of target size scaling ensured similar results with no statistically significant differences between foveal and paracentral thresholds. Data were less variable at the fovea indicating larger intersubject differences at parafoveal locations across the sample. In general, the ageing effect was accounted for by a weak linear upwards trend of CA thresholds with age, although the thresholds for presumed subclinical cases in the older subject groups departed significantly from the linear trend and increased rapidly with age.

* Motion sensitivity thresholds are influenced by age beyond the 5th decade. Statistical analysis revealed that motion data can be separated into a younger (20-49.9) and an older (50-79.9) age group. Within each of these age bands motion thresholds showed no statistically significant differences. Within each group, the thresholds were similar for the five locations tested, a finding facilitated by the larger target size employed paracentrally.

* Chromatic sensitivity thresholds were influenced by age beyond the 6th decade. Statistical analysis revealed that RG and YB colour data can again be separated into a younger (20-59.9) and an older (60-79.9) age band. Within each of these age bands, age effects are not statistically signficant. Foveal thresholds were, however, statistically significantly different from parafoveal locations for both RG and YB discrimination. Ageing had a greater effect on YB thresholds than on RG thresholds. RG thresholds for subjects within the older age band were significantly larger with a similar increment for all five locations tested.

* The following findings were established from studies in 36 patients with pregeniculate lesions (23 with Glaucoma, 6 retinal conditions, 6 optic nerve conditions and 1 chiasmal lesion): In general, loss was usually diffuse and corresponded with the location of the visual field defect. The majority of pregeniculate patients exhibited impairment of all functions tested: CT, CA and motion were all substantially impaired and chromatic discrimination was also affected symmetrically in one or both channels (RG, YB). This pattern of loss was present within the area identified by visual field loss, where visual attributes were often not seen at the phosphor limits of the display. Some pregeniculate patients also exhibited substantial loss of all visual functions in areas where perimetric loss was largely absent. In most pregeniculate patients all quadrants revealed similar loss. In some patients the least affected quadrant exhibited normal CT or motion thresholds, but CA and colour vision were always affected to some degree. In pregeniculate patients, loss in both CT and CA was a marker of profound loss in both colour and motion. Chromatic sensitivity loss was always symmetric, frequently the RG channel was more affected than YB. The YB channel was affected more in patients with early glaucoma with more advanced disease, the RG channel was a®ected most, and finally further progression of the disease resulted in large thresholds limited by the phosphor limits of the visual display.

* In 23 patients with postgeniculate lesions: the majority of those with pre-striate damage exhibited loss of all tested visual functions with symmetric chromatic impairment. Some pre-striate lesions were associated only with CA and colour loss, and in these cases the chromatic loss was symmetric and affected either one or both channels.

Striate or extra-striate lesions tended to exhibit loss of CT, CA, motion and colour vision within the area identified by visual field testing. More specific loss tended to be associated with less impaired areas that were often normal on perimetric testing. Some striate or extra-striate lesions were only associated with CA and colour loss. In such cases chromatic loss was asymmetric for one or more colour categories. When striate or extra-striate lesions were also accompanied by underlying pre-striate damage, chromatic sensitivity loss was always symmetric and sometimes accompanied by CT loss. Motion was affected least in postgeniculate conditions and was always correlated with the area of densest visual field loss.

The findings from this study show that the loss of chromatic sensitivity in cerebral achromatopsia varies considerably with location in the visual field. The same subject can exhibit loss of chromatic sensitivity that is either colour channel or colour category specific and such losses often affect only restricted areas of the visual field.

* The findings from this investigation show how ageing processes affect the most important aspects of visual performance and provide the statistical limits needed to differentiate ageing effects from disease. The study also reveals how specific, localised damage to visual pathways can produce selective loss of visual function and how the latter varies with retinal topography. The observed variation in the processing of the same stimulus attribute with retinal location as well as the differences measured for different stimulus attributes at the same location illustrate the importance of the testing paradigm employed to reveal early onset of disease.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RE Ophthalmology
Departments:	Doctoral Theses School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses

Export

Downloads