Eye movements and the Pulfrich phenomenon.

The path of a target oscillating in the fronto-parallel plane and differentially filtered to the two eyes appears elliptical in depth when the eyes fixate a stationary point. When the eyes rrnck the target the path flattens out. Binocular records of sye movements indicate rhat the eyes follow the tru: physical path making only conjugate movements with no change in convergence. It is generally accepted that the apparent elliptical path of a pendulum swinging in the front+parallel plane, when viewed with a neutral density filter over one eye, is due to the increase in visual latency of the attenuated eye (Pulfrich. 192; Lit. 1949; Wilson and Anstis, 1969; Rogers and Anstis, 1972). The increased latency, either as a result of the decreased luminance level or the state of adaptation of the eye (Rogers and Anstis. 1972). alters the apparent position of any moving object in the field of view. creating a binocular disparity which is interpreted as a change in the apparent depth relative to the plane of convergence of the eyes. This explanation is satisfactory when the eyes fixate a stationary point and the target image sweeps across the retina [Fig. l(a)], but it has also been reported that the iltusion can be seen if the eyes follow the target (Gregory. 1966: Kirkwood. Ellis and Nichof, 1970). ’ Present address: Psychological Laboratory, University of St. Andrews. Scotland. Two questions arise from these findings. Firstly. do the eyes follow the real (flat) path of the oscillating target [Fig. l(b)] or do they follow the apparent elliptical path which wouid involve a continuous change in the convergence of the eyes as well as conjugate tracking movements [Fig. l(c)]. The latter description is imptied in Gregory (1966) and it is wei1 known that the eyes can follow a target which physically moves in an elliptical path in depth (Rashbass and Westheimer, 1961). If. however, only conjugate tracking movements are involved, then the second question arises as to why the illusion is still observed, since the target wili stay on the foveas of both eyes and any difference in latency is of no consequence, assuming good tracking. Altematively, if the eyes follow the apparent path as in Fig. l(c) then one might expect the change in depth signalled by the convergence system to be compensated for by the disparate position of the target on the two retinae again yielding the percept of the target moving along a flat path.