The properties of perimetric thresholds in normal and glaucomatous eyes.

In perimetry, threshold is defined as the stimulus intensity at which the frequency of perception is 50%. Around the threshold, there is a transition zone in which the frequency of perception turns from 0% to 100%. We examined the frequency-of-seeing-function in 21 persons with and without pathological damage at various locations in the visual field. We used the classic method of constant stimuli: 12 different stimulus intensities were presented 25-50 times each. The results were: (1) The frequency-of-seeing curve showed the typical S-shaped transition zone as is known from other psychophysical thresholds. The slope of the curve and thereby the size of the transition zone varied by a factor of 17. The 'threshold coefficient,' which is the difference between the intensities that provoke a 50% and a 84% frequency-of-seeing, ranged from 1.13 to 19.83 dB. (2) The threshold coefficient was strongly correlated with the threshold level itself (r2 = 0.596). (3) Other influencing factors such as pathological damage or peripheral location appeared to be covariant, because they influenced the threshold level. A substantial influence of these factors on the threshold coefficient could not be found. Different fluctuations of the threshold are therefore mainly an effect of the threshold level. This may serve as a anticipatory knowledge that allows the optimization of staircase measurement procedures and enables the improvement of the statistical analysis of visual fields.