SPATIAL ORIENTATION IN HUMANS - PERCEPTION OF ANGULAR WHOLE-BODY DISPLACEMENTS IN 2-DIMENSIONAL TRAJECTORIES

Vestibular perception of whole-body passive rotation in the horizontal plane was studied by applying two-dimensional (2D) motion to eight bl indfolded healthy volunteers: purl rotations in place, corner-like tra jectories and arcs of a circular trajectory were randomly applied by m eans of a remotely controlled robot. Angles embedded in the 2D traject ories were 45 degrees, 90 degrees, 135 degrees and 180 degrees. Stimul ation of semicircular canals was the same for all trajectories but was accompanied by concurrent otolith stimulation during circular motion. Subjects participated in two successive experimental sessions. In the first session they were instructed to use a pointer to reproduce the total angular displacement after the motion (REPRODUCTION); in the sec ond session they had to keep pointing towards a remote (15 m) memorise d target during the motion (TRACKING). In REPRODUCTION subjects tended to overestimate their rotation angle by 28 +/- 11% (mean +/- SD). The re was no systematic effect of the trajectory. Overestimation also occ urred when subjects were required to rotate in darkness by 180 degrees (by controlling a joystick). In TRACKING there was virtually no overe stimation (6 +/- 17%) and the movement of the pointer matched the dyna mics of angular motion. We conclude that (a) the brain can separate an d memorise the angular component of complex 2D motion; however, a larg e inter-individual variability in estimating its amplitude exists; (b) in the range of linear accelerations used in the study, no appreciabl e effect of otolith-canal perceptual interaction was shown; (c) angula r displacements can be dynamically transformed into matched pointing m ovements; (d) overestimation seems to be typical of delayed judgements of angular displacement and of self-controlled rotations in place. Th is could be due to the characteristics of the physiological calibratio n of the vestibular input.