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J. Wang, T.R. Candy; Behavioral Sensitivity to Defocus in Human Infants . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5607.
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Purpose: Young human infants should have a larger sensory depth of focus than adults as a result of their immature contrast sensitivity function (eg. Green, Powers & Banks, 1980). Thus, infants should be less sensitive to optical defocus than adults are. The goal of this study was to estimate infants’ behavioral threshold for defocus in natural images. Methods: Four–month–old full term infants were recruited as subjects. Defocused natural images were computed using OTFs that combine the effects of the subjects’ mean pupil size (mean=4.2mm,SD=0.5mm), the viewing distance, and the desired levels of defocus. Two–alternative forced–choice preferential looking (FPL) was used to determine whether the infants’ could discriminate a focused image from a defocused one. Six levels of defocused images were randomized into a blocked design and an observer judged the infants’ responses by observing the first Purkinje images generated by a light source in the centre of the display. Results: The psychometric functions were fitted to find the 75% correct threshold for defocus. The thresholds for defocus were less than 1.5D. Conclusions: While predictions of blur sensitivity based on infants’ acuity predict performance significantly better than a threshold of 1.5D, it is not possible to direct infants to perform to their true threshold. The results of this study do, however, demonstrate a clear upper bound to their sensitivity that is of significance in clinical decision–making.
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