Direction specific error patterns during continuous tracking of the subjective visual vertical

The aim of this study was to characterize the error pattern of continuously tracking the perceived earth-vertical during roll rotations from upright to right or left ear-down and from right or left ear-down to upright. We compared the tracking responses of two paradigms, which either continuously activated the otoliths organs alone (constant velocity tilt) or both the otolith organs and the semicircular canals (constant acceleration tilt). The tracking responses of the subjective visual vertical showed characteristic differences depending on starting position and tilt direction relative to gravity. The error patterns in the constant-velocity and constant-acceleration tilt paradigm were reversed. Estimations during tracking, when otolith information was continuously changing, were more precise compared to estimations following fast tilts to fixed roll tilt positions. We conclude that the central processing underlying these perceptual tracking responses requires, besides the otolith input, information from the vertical semicircular canals.