Dissociating Vestibular and Somatosensory Contributions to Spatial Orientation

Inferring object orientation in the surroundings heavily depends on our internal sense of direction of gravity. Previous research showed that this sense is based on the integration of multiple information sources, including visual, vestibular (otolithic) and somatosensory signals. The individual noise characteristics and contributions of these sensors can be studied using spatial orientation tasks, such as the subjective visual vertical (SVV) task. A recent study reported that patients with complete bilateral vestibular loss perform similar as healthy controls on these tasks, from which it was conjectured that the noise levels of both otoliths and body somatosensors are roll-tilt dependent. Here, we tested this hypothesis in ten healthy human subjects by roll-tilting the head relative to the body to dissociate tilt-angle dependencies of otolith and somatosensory noise. Using a psychometric approach, we measured bias and variability in perceived orientation of a briefly flashed line relative to the gravitational vertical (SVV task). Measurements were taken at multiple body-in-space orientations (-90 to 90deg, steps of 30deg) and head-on-body roll-tilts (30deg left-ear-down, aligned, 30deg right-ear-down). Results showed that verticality perception is processed in a head-in-space reference frame, with a bias that increased with larger head-in-space orientations. Variability patterns indicated a larger contribution of the otolith organs around upright and a more substantial contribution of the body somatosensors at larger body-in-space roll-tilts. Simulations show that these findings are consistent with a statistical model that involves tilt-dependent noise levels of otolith and somatosensory signals, confirming dynamic shifts in the weights of sensory inputs with tilt angle.

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