Semicircular canal and saccular influence on the subjective visual horizontal during gondola centrifugation

Abstract

Measurements of the subjective visual horizontal (SVH) were performed in 11 healthy test persons during an increase of the resultant gravitoinertial force vector in a large swing-out gondola centrifuge. Three levels of hypergravity (1.5g, 2.0g, 2.5g) were used, each with a duration of 4 minutes and with 1–2 minute pauses at 1.0g in between. The direction of the resultant gravitoinertial force vector was always parallel with the head and body length axis. Hence, there was no roll stimulus to the otolith organs. The swing-out of the gondola during acceleration, however, is sensed by the vertical semicircular canals as a change in roll head position, thus creating an otolith-semicircular canal conflict. After acceleration of the centrifuge there was a tilt of the SVH relative to the resultant gravitoinertial horizontal. This tilt gradually decayed during the 4-minute period of recordings. For a subgroup of seven test subjects who had completely normal ENG-recordings in 1g environment, the initial offset of SVH and the time constants for exponential decay were determined for each g level; initial offsets: $9.9^{\circ }$ (1.5g), $7.7^{\circ }$ (2.0g), $6.1^{\circ }$ (2.5g); time constants: 89s (1.5g), 74s (2.0g), 37s (2.5g). The offset of SVH is interpreted as being the result of mainly the stimulus to the vertical semicircular canals during acceleration of the centrifuge. The slow decay, however, does not correspond to the dynamics of the semicircular canal system, and is suggested to reflect some kind of central position storage mechanism. A smaller offset and more rapid decay for the higher g loads may be explained by an increasing dominance of graviceptive input, presumably from the saccules. In conclusion, these results might suggest the role of the vertical semicircular canals as well as the sacculus in the formation of SVH. They may also have relevance with regard to the spatial disorientation problem in aviators.