The sensation of self-motion is a common visual illusion which allows inferences concerning visual-vestibular interaction. It may be perceived while gazing at moving clouds, streaming water, or when a train moves on the adjacent track in a railway station. This compelling sensation of body movement can even affect postural balance. On first approaching the problem, a psychologic explanation could be suggested in the sense of Helmholtz’s (1896) Urteilstauschung. One might assume that these illusions are inferences based upon the conscious or unconscious assumption of a stable environment, so that when the environment does in fact move, the observer infers that he himself is moving. This interpretation would be consistent with the individual’s past experience as the entire visual surround seldom moves uniformly under natural conditions unless the body moves relative to the earth.

Visual-Vestibular Interaction: Effects on Self-Motion Perception and Postural Control

For the egocentric orientation of observers moving with respect to a plane (e.g., pilots and automobile drivers), the movement parallax field provides the main cue. The parallax field is split into a lamellar and a solenoidal part, and it is shown that the solenoidal part is purely propriospecific. For instance, it can be shown that this component can be completely canceled by an appropriate eye movement. Thus all exterospecific information is contained in the lamellar part, and this part is completely determined by the divergence of the parallax field. Thus the measure of expansion of the visual field as a function of direction of gaze is sufficient to provide all information available for egocentric orientation. It is further shown that the widely used focus of expansion, as introduced by Gibson, is not invariant against eye movements and does not (in general) correspond to extrema of the divergence.

Exterospecific component of the motion parallax field.

Seventeen normal subjects were asked to direct a laser point to the position they felt to lie exactly straight ahead of their body. Subjects were seated in complete darkness in an approximately spherical cabin in an upright position with the orientation of the trunk and head aligned. For both the horizontal and vertical plane, "straight ahead" judgements were closely scattered around the objective straight ahead body position. Posterior neck muscle vibration as well as caloric vestibular stimulation with ice water led to (1) an apparent motion and horizontal displacement of a stationary visual target to the side opposite to stimulation and (2) a horizontal deviation of subjective "straight ahead" perception toward the side of stimulation. Only those subjects who experienced an illusion of target motion also showed a deviation of their subjective body orientation. No systematic effect of a displacement of subjective body orientation in the vertical plane was detected. When vestibular stimulation and neck muscle vibration were combined their effects were additive, i.e. the horizontal deviation of subjective body orientation observed when either type of stimulation was applied in isolation, was linearly combined either by summation or by cancellation. The present results clearly support the assumption that afferent visual, vestibular and proprioceptive input converge to the neural generation of an egocentric, body-centred coordinate system that allows us to determine our body position with respect to visual space.

The interactive contribution of neck muscle proprioception and vestibular stimulation to subjective “straight ahead” orientation in man

Optic flow drives human visuo-locomotor adaptation.

OBJECTIVES Several studies have investigated how peripheral stimulation affects the perception of body orientation in healthy subjects. The studies showed that opposing stimulation of two different input modalities can cancel out, leaving perception of body orientation unchanged. It was ascertained whether a comparable phenomenon could be found in brain damaged patients with two distinct disorders which individually lead to opposing shifts of the perceived midline. METHODS The visual subjective straight ahead was measured in patients with pure neglect, pure hemianopia, or a combination of neglect and hemianopia. RESULTS As in previous studies, patients with pure neglect displayed an ipsilesional displacement of the perceived straight ahead. Patients with pure hemianopia showed a contralesional shift. By contrast, no significant midline shift occurred in the patients with both neglect and hemianopia. CONCLUSIONS Neglect and hemianopia interact so that opposing biases in the perception of body orientation neutralise each other. Both parietal and occipital areas seem to contribute to the perception of straight ahead body orientation and seem to have counteracting effects when lesioned in the same hemisphere.

https://jnnp.bmj.com/content/jnnp/67/5/572.full.pdf

Parietal and occipital lobe contributions to perception of straight ahead orientation

(1972). Relation of Optical and Labyrinthean Orientation. Optica Acta: International Journal of Optics: Vol. 19, No. 5, pp. 467-471.

Relation of Optical and Labyrinthean Orientation

: The problem of disorientation in a moving optical environment was examined. Egocentric disorientation can be experienced by a pilot if the entire visual environment moves relative to his body without a clue of the objective position of the airplane in respect to the ground. A simple method of measuring disorientation was devised. In this method changes in motor activity were recorded for assessing optically induced disorientation. Subjects were placed into an optokinetic drum which excluded any stationary objects from view. Within the non-moving drum the subject walked straight back and forth with open eyes. In the experimental situation the drum was rotated at various constant speeds. The subjects were requested to walk straight forward and backward while their walking pattern was traced. (Author)

M. Herbert Brecher

Papers

Relation of Optical and Labyrinthean Orientation

Motor effects from visually induced disorientation in man