Human vestibular memory studied via measurement of the subjective horizontal during gondola centrifugation

Measurements of the subjective visual horizontal (SVH) were made in a large swing-out gondola centrifuge. Rotation of the centrifuge was anti-clockwise, as seen from above. Test subjects were seated upright in the gondola, facing forwards. In front of the subject, at a straight-ahead eye-level position, there was a narrow luminous line, which could be rotated, by remote control, about the visual axis. At gravitoinertial force levels of 1.1-1.3G the subjects were asked to indicate, by repeatedly setting the line in darkness, what they perceived as horizontal (the SVH). During gondola centrifugation, the head and body length axis is always parallel with the resultant gravitoinertial force vector (vectorial sum of earth gravity force and the centrifugal force) i.e., the horizontal plane of the head or body does not change with respect to the gravitoinertial horizontal. Hence, the otolith organs, as well as the somatosensory system, continually signal upright position. However, the swing-out of the gondola during acceleration of the centrifuge (25 degrees at 1.1G) is a roll (frontal plane) change-in-position stimulus to the vertical semicircular canals, thus creating an otolith-semicircular canal conflict. After acceleration of the centrifuge, the SVH was initially tilted up to 20 degrees to the right relative to the gravitoinertial horizontal. Since there was no roll-tilt stimulus to gravity receptors, this SVH tilt must be related to stimulation of the semicircular canals. However, it decayed much more slowly than any known effects of angular-velocity stimulation of the semicircular canals. The decay was bi-phasic with two time constants, the smaller in the region of 1-2 min, the other being too large to be reliably estimated on the basis of data collected during only 10 min. This persistence of the SVH tilt suggests a memory for angular changes in roll head position detected by the semicircular canals-a position-storage mechanism. Further, the SVH seems to be dependent on two different mechanisms related to semicircular canal stimulation.