Human discrimination of rotational velocities

Vestibular reflexes are critically important for stabilizing gaze and maintaining posture, but comparatively little is known about conscious perceptions of vestibular stimuli and how they may relate to balance function. We used psychophysical methods to determine the ability of normal subjects and a vestibular-deficient subject to discriminate among velocities of earth-vertical sinusoidal rotations. Discrimination thresholds in normal subjects rose from 2.26 deg/s at a peak velocity of 20 deg/s up to 5.16 deg/s at 150 deg/s. The relationship between threshold and peak angular velocity was well described by the power law function δI = 0.88I^0.37, where I is the magnitude of the stimulus and δI is the discrimination threshold. The subject with bilateral vestibular hypofunction had thresholds more than an order of magnitude worse than normals. The performance of normal subjects is much better than that predicted by Weber's Law, which states that discrimination thresholds increase proportionally with stimulus magnitude (i.e., δI/I = C, where C is the "Weber fraction"). This represents a remarkable exception to other sensory systems and may reflect the vestibular system's ability to stabilize gaze and maintain posture even at high stimulus intensities. Quantifying this relationship may help elucidate the role of higher-level processes in maintaining balance and provide information to diagnose and guide therapy of patients with central causes for imbalance.