Points of a normal visual field are not statistically independent

The aim of the present study was to determine whether the sensitivity values at individual test locations in a normal visual field are statistically independent. A total of 144 eyes of 144 normal subjects were tested with the Humphrey Field Analyzer, program 30-2. For each point in the visual field the pairwise correlation of sensitivity values to all other points was calculated together with the angular distance between points. The overall 2775 correlation coefficients were plotted as a function of angular distance. With increasing distance the correlation coefficients decrease continually from 0.63 (distance 6 degrees) to approx. 0.5 (distance 30 degrees and above). The strong relationship between adjacent points that is present at up to 30 degrees is destroyed by intraindividual randomisation of the visual field data. Interindividual randomisation abolishes any correlation. Adjacent locations in a normal visual field are strongly statistically related to each other. Two patterns may be separated; part of the correlation is an intrinsic neighbourhood effect present up to 30 degrees and part is due to the observation that the sensitivity values of a specific visual field are obtained from the same subject and are thus not independent. Therefore, for the calculation of normal values, procedures have to be developed that take the relationship between neighbouring points into consideration.