Vestibular and vestibulo-proprioceptive perception of motion in the horizontal plane in blindfolded man--I. Estimations of linear displacement.

Perception of linear displacement in the horizontal plane was studied in blindfolded human subjects. Subjects were transported and walked with guidance along 2-6 m straight lines and then had to retrace these paths walking backwards. Subjects solved a double orientational task: firstly, they defined the direction of the backward path and, secondly, estimated its length. Following passive transportation overestimation of shorter distances was observed, which tended towards underestimation with path lengthening. The absolute average error in estimating a 2-m path was 0.4 +/- 0.18 m (M +/- m), a 6-m path -0.2 +/- 0.19 m. Guided subjects constantly overestimated the path length with an averaged error of 0.51 +/- 0.064 m. In defining the direction of the backward path subjects made errors scattered over a range of 0-30 degrees. These errors did not vary in relation to path length. A reverse proportionality was revealed between errors in direction and estimation of path lengths following passive transportation. It is suggested that accuracy in perceiving a linear displacement is dependent upon subjective perception of the preceding change in the trajectory of movement.