Motion detection in human vision: a unifying approach on energy and features A. T. Smith* and T. Ledgeway Department of Psychology, Royal Holloway, University of London, EghamTW20 0EX, UKMost studies of human motion perception have been based on the implicit assumption that the brain has only one motion-detection system; or at least that only one is operational in any given instance. We show, in the context of direction perception in spatially altered two-frame random-dot kinematograms, that two quite different mechanisms operate simultaneously in the detection of such patterns. One mechanism causes reversal of the perceived direction (reversed-phi motion) when the image contrast is reversed between frames, and is highly dependent on the spatial-frequency content of the image. These characteristics are both signatures of detection based on motion energy. The other mechanism does not produce reversed-phi motion and is unaffected by spatial altering. This appears to involve the tracking of unsigned complex spatial features. The perceived direction of an altered dot pattern typically reflects a mixture of the two types of behavior in any given instance. Although both types of mechanism have previously been invoked to explain the perception of motion of different types of image, the simultaneous involvement of two mechanisms in the detection of the same simple rigid motion of a pattern suggests that motion perception in general results from a combination of mechanisms working simultaneously on different principles in the same circumstances.