Adaptation to the prismatic effects of refractive lenses.

Adaptation to the simple visual displacement of prisms was compared to that for refractive lenses, which have a varied prismatic effect. Subjects were made myopic using contact lenses, then corrected using spectacle lenses. The effect on the perceived direction of a randomly located target was assessed from pointing behavior. Prism adaptation showed a negative directional aftereffect but lacked intermanual transfer. Lens adaptation lacked a negative aftereffect but exhibited intermanual transfer. The results suggest that lens adaptation involves a recalibration of extraretinal eye movement information and multiple sets of lens adaptation can be retained for short periods.

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.