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Shahina Pardhan, Colm McGonigle, Ian van der Linde, Peter Allen, Sheila Rae; Resetting Visual Experience: Different Contrast Threshold Outcomes When Adapting to White Noise and Printed Text. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4807. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Contrast adaptation is an effect where sensitivity to a particular spatial frequency can be altered depending on the spatial structure of an adapting target or image. Individuals are likely to have a pre-existing level of adaptation dependent on their habitual visual experience which will be reflected in the shape of the contrast sensitivity function. Natural images have a Fourier magnitude that is inversely proportional to the spatial frequency (commonly referred to as 1/f or pink noise), and are therefore predominated by power at low spatial frequencies whereas targets such as printed text contain an unequal distribution of spatial detail with power peaks corresponding to the stroke width of the letters and at the frequency of the pattern formed by the lines and spacing of the text. Pre-adaptation and post-adaptation levels were investigated after a period of reading printed text using a white noise target containing constant spectral density across a range of spatial frequencies.
Contrast thresholds were measured using vertical Gabor patches (Cambridge Research Systems) for spatial frequencies of 1, 2, 4.8, 9.6 and 19.1 cycles per degree. Thresholds were measured in ten subjects using QUEST procedure in three conditions: i) pre-adaptation; ii) post-adaptation following 30 mins viewing white noise; iii) post-adaptation following 30 mins viewing black on white Times Roman font text at 0.5m with a stroke frequency of 9.6 cycles per degree. Subjects were fully corrected for refractive error plus an addition for the viewing distance to relax accommodation.
To investigate the effects of adaptation to white noise, contrast thresholds were compared between post and pre-adaptation using paired t-tests. A significant threshold elevation of 0.2 log units was found at 1 cycle per degree after white noise adaptation (t(9)=2.96; p=0.018). The threshold at 2 cycles was also elevated and at 19.1 cycles depressed, although these did not reach significance. Following adaptation to printed text, contrast threshold depressions were seen at 1 cycle and 9.6 cycles.
Threshold elevation and depression after white noise and text reflected changes in the spatial frequency content between the post and pre-adaptation visual experience. Adaptation to printed text caused threshold depression at the frequencies created by the stroke width of the letters and the pattern made by the line spacing. This study shows that prolonged viewing of printed text has an impact on visual function.
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