The effect of stimulus size on the measured threshold values in automated perimetry

Patients with severely depressed visual fields as determined by automated static perimetry are often tested with larger test stimuli in order to obtain meaningful data. "Normal" threshold values are known and included in some perimeters' data analysis programs for stimulus III, but not for larger stimuli. Using a user-defined program on the Octopus 201 perimeter to determine mean retinal sensitivity, a study was conducted in 17 normal subjects to determine the effect of stimulus size on measured threshold. With stimulus III, the mean retinal sensitivity within the central 30 degrees was 28.4 +/- 2.3 dB; it was 31.9 +/- 2.3 dB for stimulus IV, and measured 36.00 +/- 2.5 dB with stimulus V. The differences between the values are statistically significant (P less than 0.001). It is not known whether these differences can be generalized from normals to patients with glaucoma or other diseases.