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Effects of eccentricity, contrast, orientation and the number and length of grating bars on orientation discrimination

Cui, Liu 2010. Effects of eccentricity, contrast, orientation and the number and length of grating bars on orientation discrimination. PhD Thesis, Cardiff University.

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Abstract

Ei indicates the eccentricity where stimulus size must double to maintain performance equivalent to that at fovea. An over 200-fold range of Ei has been found using spatial scaling since the introduction of method. Some later research in orientation discrimination suggested that contrast reduction elevated Ei (Sally and Gurnesy 2003, 2004 and 2007). However, it was based on very limited data. Therefore, to examine how Ej changes with contrast, two types of orientation discrimination tasks involving six experiments were studied using spatial scaling in the thesis: (i) orientation discrimination at 10-100% contrasts and 0-10 degree eccentricities using Gaussian-filtered lines and 2-16 cycles-per-image (cpi) gratings, and (ii) contrast thresholds allowing discrimination of 1.5-45 degrees orientation differences (OD) at 0-10 degree eccentricities using the same stimuli for the first task. Three hypotheses were made: (i) when the effect of contrast was taken into account, the peripheral stimulus size required for performance equivalent to that of the fovea can be obtained at a range of contrasts by spatial scaling, and (ii) for low-cycle-number (<16 cpi) grating stimulus, the number of cycles played a crucial role on the visual performance across visual field, and (iii) for the threshold contrast of a fixed orientation difference discrimination, the visual process mechanism of the visual task at large orientation difference was different from that at small difference close to orientation discrimination threshold. The results of the orientation discrimination experiments showed that (i) spatial scaling succeeded in superimposing all the threshold data across contrasts or within a contrast, meaning that there is no qualitative difference between the fovea and periphery, (ii) E2 was independent of contrast, suggesting that contrast reduction had no different influence on spatial summation in between foveal and peripheral visual field, (iii) Ei decreased and saturated with increasing cpi, indicating that for low-cycle-number grating stimulus, the cycle number played a crucial role on the visual performance. The results of the contrast threshold allowing the fixed orientation discrimination experiments showed that (i) the task complexity resulted in the failure of spatial scaling for superimposing all the threshold data across orientation differences, (ii) spatial Ei increased and saturated with increasing cpi, suggesting more size scaling needed for achieving foveal levels of performance for smaller cpi stimulus, and (iii) Ej found in 1.5 deg orientation difference was much smaller than those at other differences, suggesting that the visual process mechanism at large OD was different from that at OD as small as orientation discrimination threshold.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Optometry and Vision Sciences
Subjects: R Medicine > RE Ophthalmology
ISBN: 9781303218712
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:31
URI: http://orca-mwe.cf.ac.uk/id/eprint/55017

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