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Sieu Khuu, Michael Kalloniatis; Spatial Summation Across the Visual Field: Implications for Visual Field Testing. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6112.
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© ARVO (1962-2015); The Authors (2016-present)
Current visual perimetric technologies assess visual function by quantifying the contrast threshold to a spot of light of a fixed size (Goldmann III) presented to discrete locations across the visual field. However, contrast sensitivity is dependent on both the size of the test stimulus and retinal location. Ricco's law states that contrast thresholds increases with stimulus size to a critical size (Ac), beyond which little (Piper’s law) or no improvement is observed. Ac is known to increase with eccentric locations but a large normative data base is not available. In the present study we provide large normative data on how the Ac changes with retinal location and whether there are differences along different angular meridians.
A total of 28 normal subjects (mean age 20) with no history of ocular disease participated in the study. Contrast thresholds were obtained (monocularly with the left eye) for spots of light (presented briefly for 100ms on a gray background set to 30 cd/m2) of different sizes (-1.5, -1.22, -0.92, -0.619, -0.318, -0.142, -0.0177, 0.28 and 0.58 log deg2) presented at 0, 5, 10, 15 and 20 degrees of eccentricity and repeated for meridians (in an anti-clockwise direction from vertical) of 0, 45, 90, 135, 180, 225, 270, and 315 degrees. An adaptive staircase procedure was used to determine the contrast threshold.
We established the extent of Ricco’s area and Ac by plotting contrast threshold and stimulus size and fitted the data using a two line fit. Ac was defined as the stimulus size at which the line slope deviated from -1 on a log-log plot. A repeated measures ANOVA showed a main effect of both retinal eccentricity (p<0.0001) and meridian direction (p<0.0001) on the Ac. Moreover an interaction effect was observed which indicated that the Ac change with eccentricity was dependent on the meridian angle. Here Acs along meridians from the inferior visual field were smaller than the superior field. The standard Goldmann III target was outside Ac for most of the central locations.
Our results provide a comprehensive assessment of the dependency of stimulus size, retinal location and direction on contrast detection thresholds. The fact that these factors influence and modulate detection thresholds has direct implication for the design of test targets in perimetric technologies and calculations of visual field indices (see Kalloniatis & Khuu, ARVO 2014).
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