Sensitivity to full-field visual movement compatible with head rotation: variations among axes of rotation

Perceiving a stable world during active rotational and translational head movements

When a person moves through the world, the associated visual displacement of the environment in the opposite direction is not usually seen as external movement but rather as a changing view of a stable world. We measured the amount of visual motion that can be tolerated as compatible with the perception of moving within a stable world during active, sinusoidal, translational and rotational head movement. Head movements were monitored by means of a low-latency, mechanical head tracker and the information was used to update a helmet-mounted visual display. A variable gain was introduced between the head tracker and the display. Ten subjects adjusted this gain until the visual display appeared stable during sinusoidal yaw, pitch and roll head rotations and naso-occipital, inter-aural and dorso-ventral translations at 0.5 Hz. Each head movement was tested with movement either orthogonal to or parallel with gravity. A wide spread of gains was accepted as stable (0.8 to 1.4 for rotation and 1.1 to 1.8 for translation). The gain most likely to be perceived as stable was greater than that required by the geometry (1.2 for rotation; 1.4 for translation). For rotational motion, the mean gains were the same for all axes. For translation there was no effect of whether the movement was inter-aural (mean gain 1.6) or dorso-ventral (mean gain 1.5) and no effect of the relative orientation of the translation direction relative to gravity. However translation in the naso-occipital direction was associated with more closely veridical settings (mean gain 1.1) and narrower standard deviations than in other directions. These findings are discussed in terms of visual and non-visual contributions to the perception of an earth-stable environment during active head movement.

The visual consequences of deviations in the orientation of the axis of rotation of the human vestibulo-ocular reflex

The 3D orientation and amplitude of the movement of each eye evoked by predictable, sinusoidal and non-predictable, sum-of-sines rotation about roll, yaw, pitch and intermediate axes were measured in seven subjects. The rotation axis of the eyes was not always perfectly aligned with the stimulus axis but showed systematic deviations that depended on the orientation of the rotation axis of the head. Misalignment of the oculomotor response with the stimulus axis is equivalent to adding an orthogonal, non-compensatory vector that potentially could introduce retinal slip, rather than compensate for it. The variations in orientation could not be readily explained as an artifact arising from the differential processing of the roll component. Instead, differences in the movements of the left and right eyes had trends appropriate for compensating for the geometrical demands of the translation of the eyes that must necessarily accompany natural head rotation.

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.

Sensitivity to full-field visual movement compatible with head rotation: variations with eye-in-head position

Variations in velocity detection thresholds for full-field visual rotation about various axes are compatible with a simple channel-based system for coding the axis and velocity of the rotation (Harris & Lott, 1995). The present paper looks at the frame of reference for this system. The head-centered, craniotopic reference system and the retinal-based, retinotopic reference systems were separated by using eccentric eye positions. We measured the threshold for detecting full-field visual rotation about a selection of axes in the sagittal plane with the eyes held either 22 1/2 degs up, straight ahead or 22 1/2 degs down in the head. The characteristic features of the variation in detection thresholds did not stay stable in craniotopic coordinates but moved with the eyes and were constant in retinotopic coordinates. This suggests that the coding of head rotation by the visual system is in retinotopic coordinates.