Some Mechanisms Underlying Sensory and Postural Stability in Man. In: Held R, Leibowitz HW, Teuber H, editors. Perception. Berlin: Springer Berlin Heidelberg; 1978. pp. 805-45. [DOI: 10.1007/978-3-642-46354-9_26]

The relative role of visual and non-visual cues in determining the perceived direction of "up": experiments in parabolic flight

In order to measure the perceived direction of "up", subjects judged the three-dimensional shape of disks shaded to be compatible with illumination from particular directions. By finding which shaded disk appeared most convex, we were able to infer the perceived direction of illumination. This provides an indirect measure of the subject's perception of the direction of "up". The different cues contributing to this percept were separated by varying the orientation of the subject and the orientation of the visual background relative to gravity. We also measured the effect of decreasing or increasing gravity by making these shape judgements throughout all the phases of parabolic flight (0 g, 2 g and 1 g during level flight). The perceived up direction was modeled by a simple vector sum of "up" defined by vision, the body and gravity. In this model, the weighting of the visual cue became negligible under microgravity and hypergravity conditions.

Vestibular influence on human auditory space perception

We investigated the effect of vestibular stimulation on the lateralization of dichotic sound by cold-water irrigation of the external auditory canal in human subjects. Subjects adjusted the interaural level difference of the auditory stimulus to the subjective median plane of the head. In those subjects in whom dizziness and nystagmus indicated sufficient vestibular stimulation, these adjustments were significantly shifted toward the cooled ear compared with the control condition (irrigation with water at body temperature); i.e., vestibular stimulation induced a shift of the sound image toward the nonstimulated side. The mean magnitude of the shift was 7.3 dB immediately after vestibular stimulation and decreased to 2.5 dB after 5 min. As shown by an additional control experiment, this effect cannot be attributed to a unilateral hearing loss induced by cooling of the auditory periphery. The results indicate the involvement of vestibular afferent information in the perception of sound location during movements of the head and/or the whole body. We thus hypothesize that vestibular information is used by central-nervous mechanisms generating a world-centered representation of auditory space.

Eye position and visual attention influence perceived auditory direction

The singular and joint effects of eccentric visual fixation and peripheral visual attention upon the perceived auditory median plane (PAMP) of the head were examined in four subjects. They positioned a hidden, horizontally-moving sound source to the perceived median plane of the head, while fixating a light 20 degrees or 45 degrees to the right or left of the actual median plane, and attending to another light in peripheral vision. Analysis by multiple linear regression yielded a model for each subject which indicated the nature and weighting of those factors contributing to variance reduction in PAMP judgments. Three subjects showed a strong linear attention effect, resulting in shifts of the PAMP away from the locus of attention. Similarly, a strong linear fixation effect was discovered in two subjects, a weak effect in another, and a nonlinear, inverse fixation effect in the last. Additional factors were noted for some subjects. Possible explanations of the observed effects were discussed. The study indicates that both visual attention and visual fixation are critical factors in experiments on auditory space perception although inter-individual differences do exist in the nature and strength of the effects.