Reorientation of a visually evoked postural response does not require cognitive knowledge of display relative angular position
A E I Thurrell, P Bertholon, A M Bronstein
MRC Human Movement and Balance Unit, 8-11 Queen Square, London, WC1N 3BG
Previous experiments (Wolsley et al.1996) have shown that the direction of the postural response to a roll plane rotating visual display is determined by the relative angular positions of the eyes and the feet. This angle is a combination of both the eye-in-orbit and head-on-trunk positions. These experiments were conducted in a well-lit environment, with subjects instructed to stand at various orientations relative to the disk and hence full cognitive knowledge of their position. In the present experiment the cognitive element was minimised by passively rotating the subjects in an otherwise dark room and by not specifying the relative rotation of the eyes and head. Eight normal subjects stood on a motorised rotating platform that rotated at 2 deg/s (i.e. near perceptual threshold) between –90 and +90 deg relative to the visual stimulus. This consisted of a large disk (90 cm diameter) positioned 40 cm from the subject, covered in randomly distributed fluorescent spots, that rotated at constant velocity (40 deg/s) about the visual axis. Body sway was measured with a force platform and a head mounted electromagnetic sensor (fastrack). Subjects were instructed to fixate on the central point of the disk for the duration of each trial. Each trial consisted of 15 s when the disk and rotating platform were stationary; followed by 30 s of disk rotation; a variable time (30-120 s) ensued during which the platform was rotated to a new position while disk rotation was maintained; a further 30 s period followed during which only the disk rotated; and then a 15 s period during which the disk and platform were stationary. Grand averages of sway of the head and of the centre of pressure (COP) were calculated for both onset and offset of disk rotation at each orientation of the visual stimulus, these were plotted against the orientation of the plane of the disk. The best-fit linear curves showed that body sway re-oriented to lie in the plane of the disk with a gain of 0.8. The results indicate therefore, a good non-cognitive re-orientation of the visually evoked postural response. This reorientation may be mediated by either proprioceptive signals from the neck and extra-ocular muscles or by efference copy mechanisms.
C.J., Wolsley, V., Sakellari, A.M., Bronstein (1996). Reorientation of visually evoked postural responses by different eye-in-orbit and head-on-trunk angular positions. Experimental Brain Research 111: 283-288