STEREOPSIS AND POSITIONAL ACUITY UNDER DARK-ADAPTATION

Though experience tells us we can perceive depth in dim light, it is n ot so obvious that one of the chief mechanisms for depth perception, s tereopsis, is possible under scotopic conditions. The only studies on human stereopsis in the dark adapted state seem to be those of Nagel [ (1902) Zeitschrift fur Psychologie, 27, 264-266] and Mueller and Lloyd [(1948) Proceedings of the National Academy of Science, U.S.A., 34, 2 23-227], both of which used real objects or line stereograms. We teste d stereopsis using both random-dot and Line stereograms and, in agreem ent with these studies, found that stereopsis is indeed possible in da rk adaptation. We also measured stereo acuity and positional acuity (b oth of which are examples of hyperacuity) and compared these with grat ing acuity at several levels of Light and dark adaptation. At all illu mination levels tested, acuities for stereopsis and relative line posi tion were both higher than for grating acuity. As light levels decreas ed, positional and grating acuity declined in parallel fashion, wherea s stereoacuity declined more steeply.