Prism adaptation and viewing distance

Previous studies have shown that subjects with normal binocular vision adapt to prism-induced phorias. However, there are conflicting reports as to the amount of adaptation shown to horizontal prisms. Some investigations found a greater amount of adaptation to base out prisms compared with base in, whilst others found the reverse and one study found no adaptation to base out prism at all. This study investigated the adaptation response to horizontal prisms at distance and near in a larger number of subjects (n = 40). The results confirmed our previous findings and showed that at distance the adaptation responses are asymmetrical, there being a significantly greater amount of adaptation to base out prism compared with base in, whilst at near the responses are virtually symmetrical. Possible reasons for these findings are discussed.