Quantification of prism induced metamorphopsia as a model for clinical retinal (and other) distortions

Metamorphopsia (all causes) results in perceived distortions in the visual field. A simple technique utilizing hyperacuity judgements was developed in this laboratory some years ago to allow quantification of such distortions. We ask here, how good is this assessment method at detecting a distortion? To control better investigations of metamorphopsia, a model providing constant optical distortions and resultant perceptual metamorphopsia was devised using plane prisms of 8 delta and 15 delta. The instrument includes a beamsplitter and a CRT display controlled by an IBM PC computer and 'mouse'. In this study, three short vertical lines (perpendicular to the prism base-apex line) were used to assessed image distortion in the central retinal area. Positions of individual lines of this pattern were varied in order to determine perceived distortions induced by a prism. Normal observers were tested on multiple occasions. It is possible to measure accurately prism-induced directional biases (mean locations) using this method. We separated personal setting biases from the prism-related distortions. The test method devised mimics modest clinical metamorphopsia. Prism magnitude affected settings. We were able to make judgements to less than +/- 2% error.