THE INTERACTIVE CONTRIBUTION OF NECK MUSCLE PROPRIOCEPTION AND VESTIBULAR STIMULATION TO SUBJECTIVE STRAIGHT AHEAD ORIENTATION IN MAN

Seventeen normal subjects were asked to direct a laser point to the po sition 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 alig ned. For both the horizontal and vertical plane, ''straight ahead'' ju dgements were closely scattered around the objective straight ahead bo dy position. Posterior neck muscle vibration as well as caloric vestib ular stimulation with ice water led to (1) an apparent motion and hori zontal displacement of a stationary visual target to the side opposite to stimulation and (2) a horizontal deviation of subjective ''straigh t ahead'' perception toward the side of stimulation. Only those subjec ts who experienced an illusion of target motion also showed a deviatio n of their subjective body orientation. No systematic effect of a disp lacement of subjective body orientation in the vertical plane was dete cted. When vestibular stimulation and neck muscle vibration were combi ned their effects were additive, i.e. the horizontal deviation of subj ective body orientation observed when either type of stimulation was a pplied in isolation, was linearly combined either by summation or by c ancellation. The present results clearly support the assumption that a fferent visual, vestibular and proprioceptive input converge to the ne ural generation of an egocentric, body-centred coordinate system that allows us to determine our body position with respect to visual space.