Optokinetic stimulation in microgravity: torsional eye movements and subjective visual vertical.

Introduction: Microgravity provides unique sensory inputs to the vestibular and oculomotor systems. We sought to determine the effects of long-term spaceflight on sensing of spatial orientation.

Methods: Two cosmonauts participated in experiments on human vestibulo-visual interactions during a long-term mission (178 d) in the MIR station in 1995. During circular optokinetic stimulation (OKS) the tonic torsional eye position (torsional beating field, TBF) and the subjective visual vertical (SVV) were recorded on several days of the space mission as well as pre- and post-flight. A reference data set was obtained from healthy subjects on Earth, in whom the TBF was measured in upright and in prone positions.

Results: Neither cosmonaut showed changes in the SVV or the TBF values during the first days in microgravity. On flight day 149, cosmonaut A showed an increase of both values, which continued to rise by 4- and 10-fold until the end of the flight (TBF: 8.1 degrees; SVV: 216.8 degrees). This cosmonaut reported that the increase was accompanied by a loss of spatial orientation. In contrast, cosmonaut B's values remained at pre-flight levels (TBF: 1.6 degrees; SVV: 4.4 degrees). Post-flight values of the TBF did not significantly differ from pre-flight values for either cosmonaut. Subjects showed an increase of the TBF by more than a factor of 2 in prone position (range -7.7 degrees to +10.2 degrees) compared with upright position (range -3.7 degrees to +3.4 degrees).

Conclusions: Pre-flight, post-flight and during the first part of the flight, both cosmonauts exhibited values similar to those of normal subjects in an upright position. The increased TBF values of cosmonaut A from flight day 110 on were within the range of the normal subjects in prone (face-down) position, when the gravity vector cannot be used to stabilize the TBF against the rotating stimulus (the axis of rotation is parallel to the gravity vector). The increasing deviations of cosmonaut A's SVV values in-flight suggest the presence of an internal body reference system, which weakened throughout the flight and thus lost its stabilizing effect.