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J. McAnany, K. R. Alexander; Using External Noise to Characterize Visual Performance for Landolt C and Tumbling E Optotypes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5517.
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© ARVO (1962-2015); The Authors (2016-present)
Previous evidence indicates that the threshold-elevating effect of externally added luminance noise on the detection of Gabor patches depends strongly on the temporal characteristics of the noise, with sustained and transient temporal responses obtained in dynamic and static noise, respectively. The purpose of the present study was to determine whether a corresponding pattern of findings is obtained for two optotypes used clinically: the Landolt C and the tumbling E.
Two control observers made four-alternative forced-choice orientation judgments for Landolt C and tumbling E optotypes of 1.2 log MAR equivalence presented either in 2D additive white noise or against a uniform background of 50 cd/m2. The noise had a constant RMS contrast and was either static or dynamic. In addition, the temporal relationship between target and noise was either synchronous (simultaneous onset and offset of both) or asynchronous (noise preceded the target by 100 ms). Contrast thresholds were obtained using the QUEST algorithm using presentation durations ranging from 20 to 280 ms.
In the absence of noise, the temporal integration duration was shorter for the tumbling E than for the Landolt C. The effect of noise on contrast threshold was complex and depended jointly on the temporal characteristics of the noise and the target duration. For target durations greater than approximately 100 ms, contrast threshold was higher for the Landolt C than the tumbling E, independent of the noise paradigm. For durations shorter than 100 ms, however, contrast thresholds for the two optotypes strongly depended on the noise paradigm, such that threshold for the Landolt C was equivalent to that of the tumbling E in dynamic noise, but was as much as 1 log unit higher in static noise.
Striking differences between the contrast thresholds for Landolt C and tumbling E optotypes were obtained in the presence of noise, and the magnitude of the threshold difference varied as a function of the noise temporal characteristics and the target duration. Similar to results with Gabor patches, sustained and transient responses were obtained in dynamic and static noise, respectively, suggesting that the distinct visual mechanisms underlying performance for these two optotypes can be targeted by varying the noise properties. These findings show that the extent to which these optotypes produce similar measures of visual function depends on several factors, which has important practical implications in the design and use of procedures that measure performance for optotypes presented in noise.
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