Abstract
D.Phil. (Optometry)
The measurement of Contrast Visual Acuities (contrast VA) is recognised in
various studies as an important psychophysical measure of visual function, and
contrast VA is often used to understand visual limitations or functional disability.
Contrast VA is also useful for monitoring the effects of disease modifying therapies.
High and medium contrast levels are generally used in studies to evaluate contact lens
performance, the outcomes of surgical procedures and for assessing activities of daily
living. Measurement of stimuli with low contrast levels are also sometimes applied in
diagnosing, monitoring and evaluating disease processes and their management,
especially where high contrast visual acuity remains intact. This is believed to be the
first study that comprehensively investigates the reliability (or repeatability) of four
contrast levels using the computerized Thomson Test Chart 2000 XPert. (A similar
study with four contrast levels and both univariate and multivariate analysis as applied
in this thesis has not been performed elsewhere). Although the main emphasis of this
study was to explore various issues relating to short-term repeatability of contrast VA,
both within and across individuals, both univariate and multivariate statistical
analyses were also used in this thesis to investigate age and gender related changes in
measurements of contrast VA. Together the results from this thesis provide test and
re-test contrast VA reliability measures and some basic or preliminary statistical
normative contrast VA values, which should aid clinicians to confidently detect
abnormal measurements which, in turn, promotes good clinical practice.
For this thesis and within a clinical environment to investigate inter-subject
variation in contrast VA, two measurements of contrast VA at four specified contrast
levels (100%, 10%, 5% and 2.5%) at a 6 metre distance were obtained for the right
eyes only of 155 healthy participants. Thus, at each contrast level 155 test and re-test
contrast VA measurements were determined using the computerized Thomson Test
Chart 2000 XPert. All measurements were determined through the optimal refractive
compensation for each right eye of the 155 participants concerned. In a subset of ten
subjects or participants, samples of thirty consecutive measurements of contrast VA at
each of the four contrast levels were also obtained to explore short-term intra-subject
variation in contrast VA. A simple questionnaire was administered to all subjects to
obtain biographical, general and ocular health histories. Visual assessment included
II
subjective clinical refraction, stereopsis, colour vision, direct ophthalmoscopy and
biomicroscopy to understand the eyes of the participants and exclude possible factors
that could cause ocular or neurological changes in the retina or in vision thereby
influencing contrast VA in a detrimental fashion...