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S. K. Randhawa, S. H. Schwartz; Simultaneous Intraocular Brightness Discrimination is Vertically Symmetric. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2003.
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Previous studies have pointed to the utility of simultaneous interocular brightness discrimination as a test of retinal function (e.g., Danesh-Meyer et al, 2007). In the current study, we examined simultaneous intraocular brightness discrimination, comparing the superior and inferior nasal visual fields in a sample of healthy subjects. Target location, subject age, and between-session reliability were examined.
A pair of vertically aligned squares (2º each), centered on the horizontal meridian and separated by 10 or 15 º, was flashed for 250 msec. The subject’s task was to indicate if the top or bottom square was brighter, or if they were equally bright. Interleaved staircases were employed to determine the ascending and descending boundaries of the brightness matching range. Initial intensities of the squares were 51 and 11 cd/m2, with the brighter stimulus decreased in 0.20 log steps, which was reduced to 0.05 after the second reversal. Ten reversals were obtained for each staircase, and the ratio of top to bottom luminance for the final 7 were averaged to arrive at the boundaries of the matching range. To assess validity, data were fit with Weibull functions. The midpoint between an ascending and descending boundary was taken as the brightness match point (BMP) between superior and inferior retina. A BMP of zero would indicate perfect vertical symmetry. Fourteen subjects participated in the study (range: 22 - 68 yrs of age; mean 38 yrs). BMP was determined both 5 and 15 º nasally (4 measurements at each location) during a session, and two sessions, separated by at least a week, were conducted for each subject.
The average subject BMP, across all sessions/conditions (16 measurements per subject) ranged from 0.77 (inferior square more intense at equal brightness) to 1.57, with a mean value of 1.07 +/-0.26. Neither retinal location (F = 0.127; p = 0.724; within-subjects ANOVA) nor subject age (r = 0.226; p = 0.438) significantly affected BMP. Between-session variability ranged from 0.004 to 0.682 (absolute values) with a mean of 0.157, and was not affected by retinal location (F = 0.848; p = 0.366). A Bland-Altman plot shows that between-session reliability is not affected by extreme values.
Intraocular brightness discrimination is vertically symmetrical for the retinal loci tested. It is not affected significantly by age, and between-session reliability is strong. These data suggest simultaneous intraocular brightness discrimination as a potential test of visual function in the diseased eye.
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