Perception of motion in depth from luminous rotating spirals: directional asymmetries during and after rotation.

Motion aftereffect (MAE) following spiral rotation is often asymmetrical: centrifugal MAE exceeds centripetal MAE. Pronounced MAE asymmetry has been reported for conditions--especially with a minimal background pattern--promoting perception of motion in depth. Such conditions are predicted to elicit motion asymmetry during adaptation. In the present study observers viewed luminous spirals monocularly in the dark; they timed, and scaled for convincingness, motion in depth during and after rotation. Motion in depth during rotation was often almost continuous, but recession was more convincing than was approach. Approaching MAE lasted longer and was more convincing than was receding MAE: the duration difference was more pronounced than has been found in other MAE studies, corroborating the link between MAE asymmetry and motion in depth. A possible line of explanation resides in comparing spiral motion in depth with real motion in depth of objects: in particular, the rapid visual change and collision with the observer that characterises real approach of an object is lacking in spiral approach. Interspecies differences for 'looming' and MAE are discussed.