Roll-tilt perception during gondola centrifugation: influence of steady-state acceleration (G) level.
BACKGROUND
Spatial disorientation is an important problem in aviation. The significance of the C level for illusions elicited from the semicircular canals is not clear. The aim of the present investigation was to elucidate how a gravitoinertial force, acting in parallel with the subject's long (z) axis, may influence the magnitude and persistence of canal-induced tilts of the subjective visual horizontal (SVH) present after acceleration in a gondola centrifuge.


METHODS
The SVH was measured by means of an adjustable luminous line in darkness. Two series of experiments were performed. In series 1, the SVH was measured in 13 subjects at 1.1 G, 1.7 G, and 2.5 G. In series 2, it was measured in 8 subjects at 2.5 G and 4.5 G.


RESULTS
After acceleration of the centrifuge the SVH was tilted relative to the gravitoinertial horizontal. The direction of tilt was compensatory to the gondola inclination. In series 1 the initial SVH tilt was: 16.2 +/- 7.0 degrees (1.1 G), 24.2 +/- 10.2 degrees (1.7 G), and 27.1 +/- 13.9 degrees (2.5 G). In series 2 it was: 27.6 +/- 14.6 degrees (2.5 G), and 31.2 +/- 18.8 degrees (4.5 G). The gain for this response, defined as the ratio between the initial tilt and the inclination of the gondola, was: 0.65 +/- 0.28 (1.1 G), 0.45 +/- 0.19(1.7 G), 0.41 +/- 0.21 (2.5 G) (series 1); and 0.42 +/- 0.22 (2.5 G), and 0.40 +/- 0.24 (4.5 G) (series 2). Thus, an increase from 1.1 G to 1.7 G caused a reduction in the gain, but at G levels beyond 1.7 G there was no further decrease. The time constant for exponential decay tended to increase with the G level. It was: 61 +/- 31 s (1.1 G), 84 +/- 36 s (1.7 G), 89 +/- 42 s (2.5 G) (series 1); and 67 +/- 49 s (2.5 G), and 101 +/- 73 s (4.5 G) (series 2).


CONCLUSIONS
It appears that otolithic stimulation via an increased gravitoinertial force vector, acting in parallel with the head and body long axis, does not substantially influence the magnitude of the canal-mediated sensation of roll-tilt after acceleration in a swing-out gondola centrifuge. Nor does it reduce the duration of this sensation.
Tribukait
Arne
A
Swedish Defence Research Agency, Karolinska Institutet, Stockholm, Sweden. arne.tribukait@foi.se
Eiken
Ola
O
D000054:Acceleration; D000328:Adult; D002498:Centrifugation; D005260:Female; D018466:Gravity Sensing / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010037:Otolithic Membrane / Q000502:physiology; D012399:Rotation; D012665:Semicircular Canals / Q000502:physiology*; D013028:Space Perception / Q000502:physiology*