Vestibular signal processing in a subject with somatosensory deafferentation: the case of sitting posture

Abstract

Background: The vestibular system of the inner ear provides information about head translation/rotation in space and about the orientation of the head with respect to the gravitoinertial vector. It also largely contributes to the control of posture through vestibulospinal pathways. Testing an individual severely deprived of somatosensory information below the nose, we investigated if equilibrium can be maintained while seated on the sole basis of this information.

Results: Although she was unstable, the deafferented subject (DS) was able to remain seated with the eyes closed in the absence of feet, arm and back supports. However, with the head unconsciously rotated towards the left or right shoulder, the DS's instability markedly increased. Small electrical stimulations of the vestibular apparatus produced large body tilts in the DS contrary to control subjects who did not show clear postural responses to the stimulations.

Conclusion: The results of the present experiment show that in the lack of vision and somatosensory information, vestibular signal processing allows the maintenance of an active sitting posture (i.e. without back or side rests). When head orientation changes with respect to the trunk, in the absence of vision, the lack of cervical information prevents the transformation of the head-centered vestibular information into a trunk-centered frame of reference of body motion. For the normal subjects, this latter frame of reference enables proper postural adjustments through vestibular signal processing, irrespectively of the orientation of the head with respect to the trunk.

Figure 1

Length of CoP displacements. The graphs show the mean CoP displacement over time in the different experimental conditions for the control subjects (left panel) and the deafferented subject (right panel). As the control subjects were remarkably stable while seated in all experimental conditions, the different traces are superimposed. The deafferented subject maintained a relatively stable seated posture with the eyes open. Closing the eyes markedly deteriorated her stability, especially when she had the head unconsciously turned towards a shoulder. Electric stimulation of the vestibular apparatus when the deafferented subject had the eyes closed allowed her to roughly recover the body stability she had with the eyes open. See Additional file 1: Table 1 for the statistical analyses.

Figure 2

Medio-lateral CoP displacements in GVS trials. The graph show the mean medio-lateral CoP displacement of the deafferented subject recorded in trials with 0.75 mA or 1.5 mA galvanic vestibular stimulations. The GVS produced large lateral shifts of the CoP towards the anode side. The data from the healthy control subjects are not represented as the stimulations were not large enough to induce clear postural responses when seated. See additional file 2: Table 2 for the statistical analyses.