Visual and vestibular factors influencing vestibular "navigation".

In order to elucidate the role of the "starting point" in path integration, normal subjects underwent a self-rotational task in a motor-driven turntable rotating around an earth-vertical axis. They were passively rotated ("stimulus") and had to return to the starting point, controlling the direction and velocity of the turntable by means of a joystick ("response"). The test included conditions with an earth-fixed target (EFT) as starting point, shown before the stimulus, and conditions without EFT presentation. The subject's response always took place in total darkness. Subjects succeeded in returning to the starting point in all conditions but were more precise (i.e. had smaller variability of responses) with the EFT than in the other conditions. The larger data scatter (inaccuracy) in these latter tasks was directly related to the return peak velocity, whereas with EFT there was no relationship between amplitude and velocity of the return motion. These results suggest that the presentation of the starting point (the EFT) allows a real time integration to take place, thereby improving accuracy during self-controlled motion in the dark. Five subjects were also tested with the same rotational paradigm in total darkness throughout, but with the head in a different position during stimulus and response motions. Thus, motion detection was performed by different semicircular canals during stimulus and response. The conditions used were head upright during stimulus, hyper-extended backward during response motion, and head backward during stimulus and upright during response motion. It was found that the accuracy during these tasks did not differ from that during stimulus/response motion without change in the upright or backward head position. These data indicate that estimates of trajectory are, within limits, independent of canal plane.