Common reference system for estimation of the postural and subjective visual vertical

When tilted subjects are asked to set a luminous line to the perceived earth-vertical in a dark surrounding, they systematically underestimate the true direction of earth-vertical at large tilt angles, a phenomenon first described by Aubert (A-phenomenon). At small tilt angles, subjects usually overestimate the direction of earth-vertical. Overestimation has been first reported by Müller, who termed the notion of E-phenomenon. Since these first reports, this rather remarkable error behavior has been studied extensively. The prevailing notion in most earlier studies was that the erroneous estimation of verticality results from otolith signals, which are thought to represent the major input for spatial orientation, and their interaction with somatosensory signals. To bring the subjects into tilted positions, most investigators used slow tilt velocities or waited for some time to prevent interaction with semicircular canal activity. Here, we tested the hypothesis that vestibular cues about self-orientation relative to gravity are most reliable when both the semicircular canals and the otolith organs are optimally activated. To compare the error behavior in estimations of the visual vertical and perceived body position, we used self-controlled passive tilts at constant velocity or acceleration.