The ability to stabilize vision while walking requires that three systems – the eye-head, head-trunk and lower limbs – function and coordinate together. Dr. Jacob J. Bloomberg is studying how these systems interact by altering one system and observing how this change affects a subject’s gaze stabilization during walking. The research will lead to an understanding of how the eye-head, head-trunk and lower limbs are coordinated to stabilize gaze during walking. This research will also aid in the development of post-flight testing procedures to evaluate an astronaut’s walking ability and to determine the effectiveness of proposed countermeasures.
Overview
Understanding Full-Body Gaze Control During Locomotion
Principal Investigator:
Jacob J. Bloomberg, Ph.D.
Organization:
NASA-Johnson Space Center
Technical Summary
Control of locomotion requires precise interaction between several sensorimotor subsystems. Exposure to the microgravity environment during space flight leads to postflight adaptive alterations in these multiple subsystems leading to postural and gait disturbances along with decrement in visual acuity during body motion. Countermeasures designed to mitigate these postflight gait alterations need to be assessed with a new generation of functional tests that evaluate the interaction of various elements central to locomotor control.
Traditionally, gaze stabilization has been studied almost exclusively as a problem of eye-head-trunk coordination. However, coordination between the eye, head and trunk are not the only mechanism aiding gaze stabilization, particularly during locomotion. Therefore, the first goal of this study was to develop new techniques to evaluate the multiple, interdependent, full-body sensorimotor subsystems that aid gaze stabilization during locomotion. The second goal was to develop new tests to evaluate changes in dynamic visual acuity during body movement.
To address the first goal, two experiments were performed. In the first study, we investigated how alterations in gaze tasking changes full-body locomotor control strategies. Subjects performed two discreet gaze-stabilization tasks while walking on a motorized treadmill: 1). focusing on a central point target, and; 2). reading numeral characters, presented at two meters in front at eye level.
The second study investigated the potential of adaptive remodeling of the full-body gaze control systems following exposure to visual-vestibular conflict known to adaptively modify vestibulo-ocular reflex (VOR) function. Subjects walked on the treadmill before and after they were exposed to 0.5X minifying lenses worn for 30 minutes during self-generated sinusoidal vertical head rotations performed while seated. In both studies, we measured temporal parameters of gait, full-body sagittal-plane segmental kinematics of the head, trunk, thigh, shank and foot, accelerations along the vertical axis at the head and the shank, and the vertical forces acting on the support surface.
Results showed that while reading numeral characters as compared to the central point target, A). compensatory head-pitch movements increased, B). the peak acceleration measured at the head was significantly reduced, and C). the knee-joint total movement was greater during the period from the heel strike event to the peak knee-flexion event in stance phase of the gait cycle. Results also indicate that following exposure to visual-vestibular conflict known to adaptively modify the VOR subjects predominantly: 1), decreased the amplitude of head movement with respect to space, and; 2). increased the amount of knee and ankle flexion during the initial stance phase of the gait cycle. These responses serve to aid gaze stabilization during locomotion.
Taken together, results from studies 1 and 2 provide evidence that the full body contributes to gaze stabilization during locomotion and that different functional elements are responsive to changes in visual task constraints and are subject to adaptive alterations following exposure to visual-vestibular conflict. These studies successfully validated new integrated methodologies designed to assess locomotor function for countermeasure evaluation and validation.
To address the second goal of this study, we developed a new test to measure dynamic visual acuity during treadmill walking. Upon their return to Earth, astronauts experience the effects of vestibular adaptation to microgravity. The postflight changes in vestibular information processing affect postural and locomotor stability and lead to reduced visual acuity during body movement due to alterations in gaze control. These changes in acuity have significant operational implications - the inability to see clearly during body motion can impair the ability to operate spacecraft, conduct EVAs and perform an emergency egress soon after landing following a long-duration space flight. However, it is likely that time spent in microgravity affects canal and otolith function differently. As a result, the isolated rotational stimuli used in traditional tests of canal function may fail to identify vestibular deficits after space flight.
In a gaze-control task, the relative contributions of the canal and otolith organs are modulated with viewing distance. The ability to stabilize gaze during a perturbation on visual targets placed at different distances from the head may therefore provide independent insight into the function of these systems. Our newly developed dynamic visual acuity test allows us to measure changes both in static and dynamic visual acuity for both near (one-half meters) and far (four meters) visual target positions. This test was evaluated in both normal subjects and in patients with bilateral vestibular impairment. Results show a significant ability to reliably differentiate normal from clinical behavior.
We have used results obtained from this research to develop an inflight measure of dynamic visual acuity. This test will measure static visual acuity while subjects stand on the ISS treadmill and dynamic visual acuity during treadmill walking. We are currently developing an integrated testing system using a computer-driven microdisplay screen. This inflight test will measure changes in static and dynamic visual acuity during the initial adaptation phase to space flight and during the full duration of the flight. The newly developed test will be performed on the ISS following delivery of the hardware via an upcoming Shuttle flight.
Both our full-body treadmill locomotion and near/far dynamic visual acuity tests have been accepted by the Clinical Status Evaluation (CSE) review committee and are now going through the final approval process to include these evaluation techniques as part of the standard complement of tests that all astronauts undergo to evaluate their recovery.
Earth Applications
Our experiments are yielding results that may be compared and contrasted with the impairment experienced by the elderly or clinical populations. The investigation of neural adaptation to microgravity will lead to better understanding of neural alterations associated with aging and other neurological disorders. The development of unique research protocols to investigate neural alterations in the control of gaze can aid clinicians in diagnosis of neurological and neurovestibular pathology, and in monitoring post-surgical recovery.
The development of a unique research protocol to determine how normal subjects adapt to altered sensory information can be used by clinicians to develop enhanced rehabilitation techniques for patients with balance disorders. Development of this new technology can lead to the establishment of worldwide clinical, vestibular testing norms that can be used in medical facilities. In addition, this research can lead to the formulation of models of neural activity based on known pathways and substrates. These models can be used to make predictions about response properties of a variety of motor subsystems following exposure to microgravity or as a predictive tool for clinical populations. Specifically, our newly developed integrated tests of locomotor function serve to better assess how multiple sensorimotor systems change response characteristics both in aging and during various disease states.
Our NEAR/FAR dynamic visual acuity test serves as a very sensitive marker of vestibular dysfunction. Decrement in dynamic visual acuity is a common complaint of vestibular-deficient patients, and through this project, we have developed a unique method to characterize these changes in addition to using the data as a tool to guide rehabilitation interventions.
This project's funding ended in 2004