Oculomotor adjustments in darkness and the specific distance tendency
Two experiments were performed to investigate the relationship between the oculomotor adjustments assumed in total darkness and perceived distance under reduced visual conditions. Experiment I compared the dark focus of accommodation with the perceived distance of a monocular light point presented in a dark environment. Experiment II compared the convergence angle assumed in darkness (dark convergence) with the perceived distance of the light point. Both accommodation and convergence were found to assume intermediate values in darkness. Perceived distance of the monocular light point was significantly correlated with dark convergence and unrelated to the dark focus of accommodation. It was suggested that ocular vergence is a major determinant of perceived distance under reduced visual conditions, and thus provides a possible mechanism for the specific distance tendency.