Thresholds for perception of lateral motion in normal subjects and patients with bilateral loss of vestibular function.

Thresholds for detection of direction of whole-body lateral linear acceleration were determined for normal (N) and labyrinthine defective (LD) subjects. Thresholds for 67% correct detection of direction of acceleration steps for 5 LDs (mean 5.65 cm/s2, peak gradient = 25 cm/s3) were not significantly different from 8 Ns (mean 4.84 cm/s2, peak gradient = 22 cm/s3). High inter-subject variability was found both among the 7 Ns and 3 LDs for detection of parabolic accelerations with some individuals being unable to detect their motion direction. Mean Ns thresholds were 15.2 cm/s2 for a ramp with gradient of acceleration = 2.8 cm/s3, 26.4 cm/s2 for a ramp with gradient = 7.9 cm/s3 and 20.2 cm/s2 for a parabola with second derivative = 1.52 cm/s4. Thresholds for LDs were respectively 19.1 cm/s2, 32 cm/s2 and 26.7 cm/s2. The lower thresholds for acceleration steps demonstrate the important effect of acceleration gradient on motion detection. For all stimuli, thresholds for some LDs could be in the range of Ns showing that somatosensory signals can play a significant role in detecting lateral acceleration.