Brain, Vol 121, Issue 1 91-101, Copyright © 1998 by Oxford University Press
D Anastasopoulos, G Nasios, K Psilas, T Mergner, C Maurer and CH Lucking
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.
ARTICLES
What is straight ahead to a patient with torticollis?
Department of Neurology, University of Ioannina Medical School, Greece.
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