The influence of otoliths and somatic graviceptors on angular velocity estimation.

In three psychophysical experiments subjects (Ss), blindfolded and earphoned with white noise, estimated their angular speed A) after brief acceleration to constant centric or eccentric rotation, and B) after deceleration to a full stop. Ss either indicated whenever they were rotated through 180 degrees, or manipulated the objective velocity such that the subjective one stayed constant. With Ss in an earth-vertical attitude, subjective speed declined exponentially with a time constant that depended on eccentricity in paradigm A, but not in B. The time constant depended linearly on the amount of the centrifugal force, but not on its direction. Thus, centrifugal force has an enhancing effect on perceived angular speed. The relevant sense organs were identified with Ss in a radial, earth-horizontal attitude. The enhancing effect was minimal when the axis of rotation was caudal of the otoliths, indicating an effect of graviceptors in the trunk. This effect, just as the effect of recently discovered truncal graviceptors on the perception of posture, turned out to depend on leg position: The minimum of the enhancing effect shifted from about 30 cm to about 60 cm caudal of the otoliths, when leg position changed from flexed to extended. It is concluded that the centrifugal force, measured by otoliths and truncal graviceptors, serves to provide information on angular velocity at eccentric rotation, at least as long as the output of the velocity storage is not yet zero.