Abstract
Precision and accuracy of the subjective visual vertical (SVV) modulate in the roll-plane. At large roll angles, systematic SVV errors are biased toward the subject's body-longitudinal axis and SVV precision is decreased. To explain this, SVV models typically implement a bias signal, or a prior, in a head-fixed reference frame and assume the sensory input to be optimally tuned along the head-longitudinal axis. We tested the pattern of SVV adjustments both in terms of accuracy and precision in experiments where the head and the trunk reference frame were not aligned. Twelve subjects were placed on a turntable with the head rolled ~28 degrees counter-clockwise relative to the trunk by lateral tilt of the neck to dissociate the orientation of head- and trunk-fixed sensors relative to gravity. Subjects were brought to various positions (roll of head- or trunk-longitudinal axis relative to gravity: 0 degrees , +/-75 degrees ) and aligned an arrow with perceived vertical. Both accuracy and precision of the SVV were significantly (p < 0.05) better when the head-longitudinal axis was aligned with gravity. Comparing absolute SVV errors for clockwise and counter-clockwise roll-tilts, statistical analysis yielded no significant differences (p > 0.05) when referenced relative to head-upright, but differed significantly (p < 0.001) when referenced relative to trunk-upright. These findings indicate that the bias signal, which drives the SVV towards the subject's body longitudinal axis, operates in a head-fixed reference frame. Further analysis of SVV precision supports the hypothesis that head-based graviceptive signals provide the predominant input for internal estimates of visual vertical.
Tarnutzer, A A