The main aim of this thesis is to evaluate the effect of meridional blur, using refractive
induced astigmatism, on visual performance at far and close distances.
Visual performance was evaluated using letter discrimination tasks at distance and
near (visual acuity, VA) and a reading task at near on subjects with pharmacologically
blocked (young) or absent accommodation (presbyopic and pseudophakic). The effect
of astigmatism was tested using positive cylindrical lenses oriented at 180 and 90
degrees, these simulating with- (WTR) and against-the-rule (ATR) astigmatism. Other
refractive status were also evaluated, namely, in-focus and spherical defocus. The
visual performance data were correlated with biometric measurements (pupil size,
anterior chamber depth (ACD), corneal and ocular aberrations, corneal multifocality,
patient age, axial length). Further, the functionality of meridional blur was evaluated for
alphabets in addition to the standard Roman alphabet using a VA task.
The results confirm that myopic astigmatism contributes to a better visual performance
at closer distances, with ATR astigmatism providing higher performance for reading
tasks compared to other forms of astigmatism. Anatomical factors such as pupil size,
corneal multifocality and ACD were significantly correlated visual performance, while
other ocular characteristics were not. Ray tracing modelling using wavefront data was a
moderate predictor of VA and reading acuity. The results of the effect of meridional blur
orientation on alphabets other than the Roman alphabet, suggest that visual
performance is dependent on the interaction between blur orientation and letter¿s
spatial characteristics.
In conclusion, pseudoaccommodation is a multifactorial phenomenon with pupil size
being the major contributor for the improvement in visual performance. Against-the-rule
shows advantages over WTR astigmatism, by providing higher reading performance,
however extending the present and previous findings for clinical application will require
further investigation on the effect of meridional blur in common and socio-culturally
adapted tasks. / Bradford School of Optometry and Vision Sciences
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/6331 |
| Date | January 2013 |
| Creators | Serra, Pedro M.F.N. |
| Contributors | Cox, Michael J., Chisholm, Catharine M. |
| Publisher | University of Bradford, Bradford School of Optometry and Vision Sciences |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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