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I.L. Bailey, S.N. Fitz, J.T. Mao, S.M. Hamamoto, K.T. Aoyagi; Defocus and Contrast Sensitivity . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5833.
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Newly developed tests of contrast sensitivity present a sequential–number search task and a blinking square detection task on a computer screen. These tests determine contrast sensitivity thresholds and plot the relationship between response time and target contrast. We examined how thresholds and response times are affected by defocus
Contrast sensitivity was measured for 6 normally–sighted young–adult subjects as they performed the search and detection tasks using large (40mm) targets viewed from 50 cm (4.5 degree). Subjects were tested monocularly wearing supplementary lenses of powers, plano, +3 D, +5D, and +7D in randomly assigned sequences before their dominant eye
For the detection of blinking squares, the average contrast sensitivity was scarcely reduced (0.04 log units) by the +3D and + 5D lenses and there was a only a small reduction (0.18 log units) with +7D. For the search task, contrast sensitivity reduced progressively by 0.07, 0.16 and 0.30 log units with the +3, +5, and +7D lenses. For both CS tests, defocus had no effect on the patterns by which response times slowed as threshold contrasts were approached. These results are significantly different from those of Jansonius and Kooijman who reported profound reductions in contrast sensitivity with defocus with an almost linear reduction of contrast sensitivity for edge detection CS reduced by 0.69 log units for only 2D of defocus.
For large targets, the blinking–square detection test is relatively insensitive to substantial defocus. The contrast thresholds for the number search test are systematically reduced by spherical defocus, but only by a modest amount. These new computer based tests were designed for clinical use and they are robust in that they are relative insensitive to optical blur. These results suggest it is not critical to accurately correct the patient’s refraction when performing these tests.
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