Independent detectors for expansion and rotation, and for orthogonal components of deformation

It is well known that optic flow-the smooth transformation of the retinal i mage experienced by a moving observer-contains valuable information about t he three-dimensional layout of the environment. From psychophysical and neu rophysiological experiments, specialised mechanisms responsive to component s of optic flow (sometimes called complex motion) such as expansion and rot ation have been inferred. However, it remains unclear (a) whether the visua l system has mechanisms for processing the component of deformation and (b) whether there are multiple mechanisms that function independently from eac h other. Here, we investigate these issues using random-dot patterns and a forced-choice subthreshold summation technique. In experiment 1, we manipul ated the size of a test region that was permitted to contain signal and fou nd substantial spatial summation for signal components of translation, expa nsion, rotation, and deformation embedded in noise. In experiment 2, little or no summation was found for the superposition of orthogonal pairs of com plex motion patterns (eg expansion and rotation), consistent with probabili ty summation between pairs of independent detectors. Our results suggest th at optic-flow components are detected by mechanisms that are specialised fo r particular patterns of complex motion.