The present program of investigation is concerned with the optical support for the perception and control of heading during constant-altitude approach to a stationary target. Of particular interest is the functional contribution of differential optical motion. The results of four experiments in which participants were given control over the direction of computer-simulated self-motion are reported. Results of Experiment 1 served to establish the potential performance range in terms of heading

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... or. In addition, they provided a first indication of the functionality of motion parallax for the experimental task and demonstrated that target drift can be used for accurate approach. Results of Experiments 2 and 4 showed that successful target approach is not possible in the absence of both target drift and differential optical motion. Furthermore, they served to eliminate two alternative potentially functional types of information (rate of target expansion and apparent rotation of target), as well as provide a first indication of optimizing performance in the top end of the global optical flow velocity range commonly available during human bipedal self-motion. The latter result was replicated in Experiment 3. Experiments 3 and 4 were specifically designed to evaluate the functionality of simple motion parallax (SMP) and differential motion parallax (DMP). A separation ratio (σ) indexing the separation of two objects in depth was able to account for (a) performance improvements with decreasing distance to the target, and (b) most of the performance differences among all simulated environments. With the effect of σ accounted for, the addition of DMP information to events that already carried SMP information did not affect performance. The rate of change in horizontal optical separation between at least two discontinuities is identified as the most likely candidate for the optical foundation of the perception and control of heading during target approach. In conclusion, suggestions for the development of formal descriptio [...]