The influence of stimulus size on perimetric detection of small scotomata

The influence of perimetric stimulus size on the detection of small scotomata was examined, and the blind spot was used as a paradigm of a small scotoma. A customized point pattern for perimetry was created on a Humphrey field analyzer. The point pattern had a density of 1.4 degrees and was arranged in a grid in the area of the blind spot. Four normal persons were tested with full-threshold strategy in the points of this grid using Goldmann stimulus sizes ranging from I to V. The delimitation of the blind spot was found to depend strongly on stimulus size. For small stimulus sizes, the blind spot could be delimited as an absolute scotoma with almost sharp borders. However, with larger spot sizes a relative scotoma zone gradually developed, extending centrally from the borders of the blind spot. The central absolute scotoma component of the blind spot totally disappeared for the largest stimulus sizes. We could establish that some of these larger stimuli would be projected totally within the optic nerve head, if perfect imaging through the refractive media of the eye, was assumed. We interpret our findings to be a result of light scattering in the refractive media of the eye, causing a blur to appear around the image of the light stimulus at the retinal plane. We conclude that perimetric resolution power is highly dependent on stimulus size. The smallest stimulus size possible should be used when dense stimulus patterns are used for perimetric detection of small scotomata.