Vestibular and vestibulo-proprioceptive perception of motion in the horizontal plane in blindfolded man--II. Estimations of rotations about the earth-vertical axis.

Perception of angular displacement in the horizontal plane was studied in blindfolded human subjects. Subjects sitting on a rotating chair were turned and turned themselves about the earth-vertical axis. Magnitudes of left- and rightward directed rotations were in steps of 30 degrees and overlapped an entire circle. The averaged relative angular error in estimation of passive turns was 0.19 +/- 0.014 (M +/- m), that of active turns 0.16 +/- 0.011. Passively rotated subjects tended to overestimate turns with an increase in the rotation magnitude. Estimations of passive turns were linearly related to the turn magnitude (Y= -0.357+1.065X; R2=0.864). When turning themselves, subjects tended to overestimate rotations of lower magnitudes and underestimate those of higher magnitudes. Linearity was observed between estimations of active turns and their magnitude (Y=26.456+0.862X; R2=0.857). Turn estimation is regarded as a geometrical task, which associates subjectively defined angular and linear parameters of circular motion. It is proposed that during rotation blindfolded subjects perceive the horizontal plane as a heterometric space, the extent of which depends on the estimation of a turn.