PERCEPTION OF SPATIAL ORIENTATION IN SPASMODIC TORTICOLLIS .1. THE POSTURAL VERTICAL

Estimates of points of entering and exiting from upright posture were obtained from 25 seated, restrained patients with idiopathic spasmodic torticollis (ST) and matched normal subjects exposed to cycles of 1.5 degrees/s tilts in a flight simulator. Estimates were obtained for di splacements in roll and pitch about upright and for yaw tilts about a rostrocaudal, ''barbecue,'' axis with the subjects supinated. For both pitch and roll, normal subjects estimated entering upright when they were still similar to 1 degrees from machine upright and perceived the mselves to be upright through a mean are of 6 degrees. In barbecue til t, entering upright was estimated at 0.2 degrees for an are of 6 degre es. Patients estimated entering upright at 2.8 degrees in roll and 3 d egrees in pitch but estimated exiting upright at the same tilt as norm al subjects; that is, they were less specific in detecting verticality . Patients were normal in barbecue tilt. No relationship between tilt estimates and head deviation was found. There were no differences betw een normal subjects when tested with their head in normal posture and with an assumed tilt of 20 degrees. Normal subjects probably based the ir estimates on combined vestibular-somatosensory signals, whereas tor ticollis patients appeared to derive more from a vestibular signal. Ho wever, patients referred the vestibular signals to the trunk long axis when asked to indicate the whole-body vertical. The findings suggest disruption of the normal combined vestibuloproprioceptive mechanism fo r detecting body uprightness in ST.