Optic Flow

Translation

When traveling on a straight path with no eye, head, or body rotation, the focus of expansion (FOE) in the resulting radial flow (x in Figure 1A) indicates one's instantaneous direction of self-motion (heading) and can thus be used for the control of self motion. [Page 574]To illustrate, to steer toward a target, we keep the FOE on the target; to stay in a lane during driving, we keep the FOE at the center of road; and to steer to avoid an obstacle, we make sure the FOE is not on the obstacle. Research by William Warren and others has shown that humans can indeed use the FOE in optic flow to estimate their heading within 1° of visual angle during simulated translation. Note that good heading performance for pure translation may not involve the perception of self-motion, because the task could be performed by locating the FOE in the 2-D velocity field on the screen without any 3-D interpretation of the velocity field.

Translation and Rotation

When one is traveling on a curved path or is traveling on a straight path but rotating one's eyes to track an object off to one side, the retinal flow pattern is not radial any more. The flow field now contains both translational and rotational components, and the lamellar flow generated by the path or eye rotation (Figure 1B) shifts the FOE in the retinal flow field away from the heading direction (Figure 1C). To recover heading in this case, many mathematical models have been proposed that use information such as global flow rate and motion parallax in the flow field to compensate for the rotation, a computation that has been implemented with neurophysiological models of primate extrastriate visual cortex.

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