What is straight ahead to a patient with torticollis?

Abstract

Vestibular and neck proprioceptive signals are known to be used in judging the locations of objects in space and relative to the body. Given that these signals are asymmetric in patients with spasmodic torticollis, one would expect such patients to have abnormal spatial perception. We tested this idea by measuring patients' perception of visual straight ahead (VSA) under various conditions: with the body in its primary position, i.e. with the head and trunk as closely aligned as possible, and after well defined passive rotations of the head and/or trunk. In the primary body position, patients' VSA direction showed considerable variations which were similar, however, to those of normal subjects; it was independent of torticollis direction, of the head torque it produced, and of the weak spontaneous nystagmus recorded in seven of the 10 patients. After whole-body rotations, i.e. where head and trunk underwent the same motion, the VSA was shifted in both patients and normal subjects, and in both groups the shift was symmetrical after rotations to the right or left. After motions where the trunk rotated under the stationary head (neck proprioceptive stimulation) or the head on the stationary trunk (combined vestibular and neck stimulus), the VSAs of normal subjects coincided rather well with their head midsagittal planes, whereas the VSAs of patients were shifted considerably towards the trunk, again in a symmetrical way. We suggest two mechanisms to explain the findings in patients: (i) a central compensation which restores symmetry of the afferent inflow in the patients (unlike the motor efference); (ii) shifting of the reference for the VSA from the head towards the trunk, because the trunk is a more reliable egocentric reference than the head in the patients. Our findings do not support the assumption that asymmetries in afferent inflow are responsible for the asymmetry of motor output in spasmodic torticollis.