Motion perception is the process of inferring the "true" velocity and direction of motion in a visual scene given some visual input. While humans appear to perform this task effortlesly in most cases, it has proven to be a hard problem from an information processing perspective.
The problem of motion perception is hard because the visual input is generally insufficient to uniquely determine the 'true' velocity in a visual scene. For example, when we consider monocular vision, the visual input will be a 2D projection of the 3D scene. The motion cues present in the 2D projection are by default insufficient to reconstruct the motion in a 3D scene. In other words, many 3D scenes will be compatible with any 2D projection. The problem of motion estimation generalizes to binocular vision when we consider occlusion or motion perception at relatively large distances, where binocular disparity is a poor cue to depth.
These problems are an issue in motion perception in humans as well. We are not as good at motion perception as we think. This becomes apparent when we look at visual illusions involving motion.
A well known example is the barberpole illusion. When a diagonally-striped pole is rotated around its longer axis, so that the stripes are moving in the direction of the pole's shorter axis, it nonetheless appears the stripes are moving in the direction of its longer axis.
In addition to the fundamental problems of motion perception given occlusion or poor binocular disparity cues, a number of issues arise in human motion perception given the way visual information is processed in the brain. Individual neurons in the visual system are each only senstive to visual input in a small part of our visual field. For each neuron it is as if it was looking at the visual input through a small apperture. At the resolution of this apperture visual cues can often be approximated by straight lines. The motion direction of a straight line is fundamentally ambiguous, because the motion component along this line cannot be inferred based on the visual input.
In cases where motion cannot be inferred in a particular direction, the visual system is thought to rely on prior assumptions. In this case the assumption could be that since motion along the line cannot be inferred, it could be set to zero. By contrast the motion perpendicular to the line can be reliably inferred. As a consequence this perpendicular motion is the only motion perceived when looking at the barberpole through an aperture.
Motion estimation has connections to both psychology (i.e. visual perception) and computer science.
