Binocular misalignments elicited by altered gravity provide evidence for nonlinear central compensation

Abstract

Increased ocular positioning misalignments upon exposure to altered gravity levels (g-levels) have been strongly correlated with space motion sickness (SMS) severity, possibly due to underlying otolith asymmetries uncompensated in novel gravitational environments. We investigated vertical and torsional ocular positioning misalignments elicited by the 0 and 1.8 g g-levels of parabolic flight and used these data to develop a computational model to describe how such misalignments might arise. Ocular misalignments were inferred through two perceptual nulling tasks: Vertical Alignment Nulling (VAN) and Torsional Alignment Nulling (TAN). All test subjects exhibited significant differences in ocular misalignments in the novel g-levels, which we postulate to be the result of healthy individuals with 1 g-tuned central compensatory mechanisms unadapted to the parabolic flight environment. Furthermore, the magnitude and direction of ocular misalignments in hypo-g and hyper-g, in comparison to 1 g, were nonlinear and nonmonotonic. Previous linear models of central compensation do not predict this. Here we show that a single model of the form a + bgε, where a, b, and ε are the model parameters and g is the current g-level, accounts for both the vertical and torsional ocular misalignment data observed inflight. Furthering our understanding of oculomotor control is critical for the development of interventions that promote adaptation in spaceflight (e.g., countermeasures for novel g-level exposure) and terrestrial (e.g., rehabilitation protocols for vestibular pathology) environments.

A rapid quantification of binocular misalignment without recording eye movements: Vertical and torsional alignment nulling

“…In particular, we suggest that VAN and TAN are a more applicable test of adult patients with acquired vestibular causes of vertical and torsional misalignment, rather than strabismus, in part because VAN and TAN only consider the subjective angles of vertical and torsional deviation, which may suffer from a false negative reading due to the adaptation capacity in strabismus patients. Our VAN and TAN software and hardware are stable and perform reliably in challenging settings (Beaton et al 2015).…”

“…The VAN and TAN hardware consists of a small (8.1 x 5.3 x 0.3in, 12.3oz) active-matrix organic light-emitting diode (AMOLED) tablet computer (e.g. Toshiba AT270, Android OS) and a customized, light-occluding, portable shroud to ensure complete darkness (Beaton et al 2015). When a dark room is available, use of the shroud is unnecessary.…”

Prototype tests of vertical and torsional alignment nulling for screening vestibular function

“…Therefore, in the quest for a better marker of space motion sickness Beaton et al developed the Vertical Alignment Nulling (VAN) and Torsional Alignment Nulling (TAN) tests [1]. In each test the final positions of a movable horizontal or rotated line was compared to a fixed horizontal line.…”

Spatial Updating Depends on Gravity

“…These results are also supported by parabolic flights, showing a tendency for upward slow phase velocity to be attenuated and downward optokinetic responses to be augmented (Clément et al, 1992a,b). Since otolith signaling drives not only vertical but also torsional eye movements, it may not be surprising that parabolic flight has also been shown to induce torsional misalignments during 0 g and 1.8 g (Markham et al, 2000;Beaton et al, 2015). To better understand the effects of changing g-levels on task performance, future studies should consider tracking eye movements during neurobehavioral testing.…”