Spatial updating during rotation: the role of vestibular information and mental activity.

The remarkable accuracy with which healthy subjects can monitor their orientation while walking in darkness has been attributed to a process whereby awareness of orientation is automatically updated by information derived from active locomotion. The aim of this study was, first, to determine the contribution of vestibular information to the perception of orientation without vision, by comparing the accuracy of judgments of orientation following passive (seated) rotation about an earth-vertical axis with those following active (locomotor) rotation. The second aim was to assess whether monitoring orientation is indeed automatic, or whether it requires some degree of mental effort. This was evaluated by assessing whether accuracy in monitoring multiple passive or active rotations was affected by asking subjects to perform a mental task (that is, counting backwards) during rotation. The results indicated that although reliance on the primarily vestibular information available during passive rotation enabled subjects to accurately monitor single turns of up to 180 degrees, subjects were able to judge orientation after multiple turns more accurately after active rotation than after passive rotation, owing to the additional sensorimotor feedback gained from active locomotion. Accuracy in judging orientation was substantially impaired by backwards counting during both passive and active locomotion. This finding confirms that monitoring orientation during multiple turns in darkness necessitates central processing and adds to the growing body of evidence for the influence of mental activity on the perception and control of orientation.