THE CONTRIBUTION OF OTOLITHS AND SEMICIRCULAR CANALS TO THE PERCEPTION OF 2-DIMENSIONAL PASSIVE WHOLE-BODY MOTION IN HUMANS

1. Perception of two-dimensional (2-D) whole-body passive motion in th e horizontal plane was studied in twelve blindfolded healthy volunteer s: pure rotation in place (180 deg), linear motion (4.5 m) and a semic ircular trajectory (radius, 1.5 m; angular acceleration, 0.2 rad s(-2) ) were applied in random sequence by means of a remote-controlled robo t equipped with a racing-car seat. The seat orientation in the horizon tal plane was controlled by the experimenter, independent of the robot trajectory. Thus different degrees of otolith-canal interaction were obtained. The maximal linear acceleration during the semicircular traj ectory was 0.1 g; however, the linear acceleration vector was complex as it rotated relative to the subject's head. 2. In the first of two s essions, subjects were instructed to maintain an angular pointer orien ted towards a remote (15 m) previously seen target during the passive movements. In the second session they had to make a drawing of the pat h of the perceived trajectory, after the movement was finished. 3. The results showed that, on average, the movement of the pointer matched the dynamics of the rotatory component of the 2-D motion well. This su ggests that, in the range of linear accelerations used in this study, no appreciable influence of otolith input on canal-mediated perception of angular motion occurred. 4. The curvature of the drawn paths was m ostly explained by the input to the semicircular canals. Subjects' rec onstruction of motion did not account for the directional dynamics of the input to the otoliths occurring during passive motion. 5. This fin ding proves that reconstructing trajectory in space does not imply a m athematically perfect transformation of the linear and angular motion- related inputs into a Cartesian or polar 2-D representation. Physiolog ical constraints on the interaction between motion direction and chang e of heading play an important role in motion perception.