Perceived heading during simulated torsional eye movements

Observer translation through the environment can be accompanied by rotation of the eye about any axis. For rotation about the vertical axis (horizontal rotation) during translation in the horizontal plane, it is known that the absence of depth in the scene and an extra retinal signal leads to a systematic error in the observer's perceived direction of heading. This heading error is related in magnitude and direction to the shift of the centre of retinal flow (CF) that occurs because of the rotation. Rotation about any axis that deviates from the heading direction results in a CF shift. So far, however, the effect of rotation about the line of sight (torsion) on perceived heading has not been investigated. We simulated observer translation towards a wall or cloud, while simultaneously simulating eye rotation about the vertical axis, the torsional axis or combinations thereof. We find only small systematic effects of torsion on the set of 2D perceived headings, regardless of the simulated horizontal rotation. In proportion to the CF shift, the systematic errors are significantly smaller for pure torsion than for pure horizontal rotation. In contrast to errors caused by horizontal rotation, the torsional errors are hardly reduced by addition of depth to the scene. We suggest the difference in behaviour reflects the difference in symmetry of the field of view relative to the axis of rotation: the higher symmetry in the case of torsion may allow for a more accurate estimation of the rotational flow. Moreover, we report a new phenomenon. Simulated horizontal rotation during simulated wall approach increases the heading-dependency of errors, causing a larger compression of perceived heading in the horizontal direction than in the vertical direction.