Accuracy of Orientation in Space under Increased Acceleration in the Absence of Visual Reference Frame

Abstract

The perception of the apparent vertical without visual cues depends on the position of the longitudinal axes of body and head to the direction of the resultant acceleration above 1 G. With dissociation of the direction of the longitudinal axes of body and head, and exposing them separately in varying angles to the direction of the resultant acceleration, information is obtained on the contribution of systems involved in the perception of the vertical. Conclusions are derived from results of experiments on 15 volunteers subjected to a total of 397 different combinations of body and head positions with resultant accelerations between 1.0 and 3.0 G. The position of the longitudinal axis of body or head influences the direction of the apparent vertical. The effect of the position of body and head is additive. The accurarcy of estimation of the direction of the resultant acceleration is optimal up to 1. 5 G, when body and head are held in the direction of the resultant acceleration. Above 1.5 G the direction of the resultant acceleration is underestimated when the vector of forces moved transiently through a frontal plane to its final position. The direction of the resultant acceleration is increasingly overestimated when the vector of forces moved transiently through a sagittal plane to its final position. The precision of estimation of the apparent vertical is higher during lateral acceleration than during backward acceleration. The precision decreases slightly with tilt of body and head away from the direction of gravity.