When a moving dot, which is tracked by the eyes and enclosed in a moving framework, suddenly stops while the enclosing framework continues its motion, the dot is seen to describe a curved path. This illusion can be explained only by assuming that extra-retinal signals are taken into account in interpreting retinal information. The form of the illusion, and the fact that the phenomenal path cannot be explained on the basis of positional information alone, suggests that the perceived path is computed by integrating (instantaneous) velocity information over time. A vector addition model embodying a number of simplifying assumptions is found to qualitatively fit the experimental data. A number of follow-up studies are suggested.