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

Kara H Beaton,Michael C Schubert,W Cary Huffman

doi:10.3389/fnsys.2015.00081

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.

Highlights

Spaceflight elicits adaptive changes across all physiological systems (White, 1998)
Our model is based on the one proposed by von Baumgarten and Thümler, which we describe in brief to define the terms and equations that will be extended to our model in Sections Incorporating a Nonlinear Gravity Component into Central Compensation Facilitates the Parabolic Flight Results and Specifying the Central Compensation Inputs
Our primary metric of interest was whether subjects expressed a change in ocular misalignments in novel gravity conditions, as this could be representative of an underlying otolith asymmetry masked by central compensation in 1 g

Summary

Introduction

Spaceflight elicits adaptive changes across all physiological systems (White, 1998). In particular, the altered gravity levels (g-levels) modulate otolith signaling, disrupting multiple sensorimotor subsystems simultaneously until the central nervous system (CNS) becomes properly calibrated to the current g-level (Young et al, 1986; Reschke et al, 1994). Studying sensorimotor responses in altered gravity and developing computational models for how the CNS facilitates adaptation to such environments will allow us to quantify physiologic commonalities and individual differences. Countermeasures must be developed to enable astronauts to live in space for prolonged periods of time during transit and prepare them to re-enter a gravitational field upon arrival at their destination. This means that crewmembers will require appropriate technologies to perform accurate self-assessments and rehabilitations inflight. The two objectives of this work were to (1) evaluate a novel, portable device for quantifying ocular positioning misalignments during altered-gravity exposure, and (2) explore through the development of a computational model how oculomotor pathways might be organized such that these misalignments arise

Objectives

Methods

Results

Conclusion