Learning dynamic balancing in the roll plane with and without gravitational cues

Vibrotactile feedback as a countermeasure for spatial disorientation

Spaceflight can make astronauts susceptible to spatial disorientation which is one of the leading causes of fatal aircraft accidents. In our experiment, blindfolded participants used a joystick to balance themselves while inside a multi-axis rotation device (MARS) in either the vertical or horizontal roll plane. On Day 1, in the vertical roll plane (Earth analog condition) participants could use gravitational cues and therefore had a good sense of their orientation. On Day 2, in the horizontal roll plane (spaceflight analog condition) participants could not use gravitational cues and rapidly became disoriented and showed minimal learning and poor performance. One potential countermeasure for spatial disorientation is vibrotactile feedback that conveys body orientation provided by small vibrating devices applied to the skin. Orientation-dependent vibrotactile feedback provided to one group enhanced performance in the spaceflight condition but the participants reported a conflict between the accurate vibrotactile cues and their erroneous perception of their orientation. Specialized vibrotactile training on Day 1 provided to another group resulted in significantly better learning and performance in the spaceflight analog task with vibrotactile cueing. In this training, participants in the Earth analog condition on Day 1 were required to disengage from the task of aligning with the gravitational vertical encoded by natural vestibular/somatosensory afference and had to align with randomized non-vertical directions of balance signaled by vibrotactile feedback. At the end of Day 2, we deactivated the vibrotactile feedback after both vibration-cued groups had practiced with it in the spaceflight analog condition. They performed as well as the group who did not have any vibrotactile feedback. We conclude that after appropriate training, vibrotactile orientation feedback augments dynamic spatial orientation and does not lead to any negative dependence.

Quantifying virtual self-motion sensations induced by galvanic vestibular stimulation

BACKGROUND

The vestibular system provides a comprehensive estimate of self-motion in 3D space. Widely used to artificially stimulate the vestibular system, binaural-bipolar square-wave Galvanic Vestibular Stimulation (GVS) elicits a virtual sensation of roll rotation. Postural responses to GVS have been clearly delineated, however quantifying the perceived virtual rotation vector has not been fully realised.

OBJECTIVE

We aimed to quantify the perceived virtual roll rotation vector elicited by GVS using a psychophysical approach on a 3D turntable.

METHODS

Participants were placed supine on the 3D turntable and rotated around the naso-occipital axis while supine and received square-wave binaural-bipolar GVS or sham stimulation. GVS amplitudes and intensities were systematically manipulated. The turntable motion profile consisted of a velocity step of 20°/s2 until the trial velocity between 0-20°/s was reached, followed by a 1°/s ramp until the end of the trial. In a psychophysical adaptive staircase procedure, we systematically varied the roll velocity to identify the exact velocity that cancelled the perceived roll sensation induced by GVS.

RESULTS

Participants perceived a virtual roll rotation towards the cathode of approximately 2°/s velocity for 1 mA GVS and 6°/s velocity for 2.5 mA GVS. The observed values were stable across repetitions.

CONCLUSIONS

Our results quantify for the first time the perceived virtual roll rotations induced by binaural-bipolar square-wave GVS. Importantly, estimates were based on perceptual judgements, in the absence of motor or postural responses and in a head orientation where the GVS-induced roll sensation did not interact with the perceived direction of gravity. This is an important step towards applications of GVS in different settings, including sensory substitution or Virtual Reality.

Crash Prediction Using Deep Learning in a Disorienting Spaceflight Analog Balancing Task

Were astronauts forced to land on the surface of Mars using manual control of their vehicle, they would not have familiar gravitational cues because Mars’ gravity is only 0.38 g. They could become susceptible to spatial disorientation, potentially causing mission ending crashes. In our earlier studies, we secured blindfolded participants into a Multi-Axis Rotation System (MARS) device that was programmed to behave like an inverted pendulum. Participants used a joystick to stabilize around the balance point. We created a spaceflight analog condition by having participants dynamically balance in the horizontal roll plane, where they did not tilt relative to the gravitational vertical and therefore could not use gravitational cues to determine their position. We found 90% of participants in our spaceflight analog condition reported spatial disorientation and all of them showed it in their data. There was a high rate of crashing into boundaries that were set at ± 60° from the balance point. Our goal was to see whether we could use deep learning to predict the occurrence of crashes before they happened. We used stacked gated recurrent units (GRU) to predict crash events 800 ms in advance with an AUC (area under the curve) value of 99%. When we prioritized reducing false negatives we found it resulted in more false positives. We found that false negatives occurred when participants made destabilizing joystick deflections that rapidly moved the MARS away from the balance point. These unpredictable destabilizing joystick deflections, which occurred in the duration of time after the input data, are likely a result of spatial disorientation. If our model could work in real time, we calculated that immediate human action would result in the prevention of 80.7% of crashes, however, if we accounted for human reaction times (∼400 ms), only 30.3% of crashes could be prevented, suggesting that one solution could be an AI taking temporary control of the spacecraft during these moments.

The role of spatial acuity in a dynamic balancing task without gravitational cues

In earlier studies, blindfolded participants used a joystick to orient themselves to the direction of balance in the horizontal roll plane while in a device programmed to behave like an inverted pendulum. In this spaceflight analog situation, position relevant gravitational cues are absent. Most participants show minimal learning, positional drifting, and failure of path integration. However, individual differences are substantial, some participants show learning and others become progressively worse. In Experiment 1, our goal was to determine whether spatial acuity could explain these individual differences in active balancing. We exposed blindfolded participants to passive movement profiles, with different frequency components, in the vertical and horizontal roll planes. They pressed a joystick trigger to indicate every time they passed the start point. We found greater spatial acuity for higher frequencies but no relation between passive spatial accuracy and active balance control in the horizontal roll plane, suggesting that spatial acuity in the horizontal roll plane does not predict performance in a disorienting spaceflight condition. In Experiment 2, we found significant correlations between passive spatial acuity in the vertical roll plane, where participants have task relevant gravitational cues, and early active balancing in the horizontal roll plane. These correlations appeared after participants underwent brief provocative vestibular stimulation by making a pitch head movement during vertical yaw rotation. Our findings suggest that vestibular stimulation may be a valuable part of assessments of individual differences in performance during initial exposure to disorienting spaceflight conditions where there are no reliable gravity dependent positional cues.

Where is my hand in space? The internal model of gravity influences proprioception

Knowing where our limbs are in space is crucial for a successful interaction with the external world. Joint position sense (JPS) relies on both cues from muscle spindles and joint mechanoreceptors, as well as the effort required to move. However, JPS may also rely on the perceived external force on the limb, such as the gravitational field. It is well known that the internal model of gravity plays a large role in perception and behaviour. Thus, we have explored whether direct vestibular-gravitational cues could influence JPS. Participants passively estimated the position of their hand while they were upright and therefore aligned with terrestrial gravity, or pitch-tilted 45° backwards from gravity. Overall participants overestimated the position of their hand in both upright and tilted postures; however, the proprioceptive bias was significantly reduced when participants were tilted. Our findings therefore suggest that the internal model of gravity may influence and update JPS in order to allow the organism to interact with the environment.

The Importance of Being in Touch

This paper describes a series of studies resulting from the finding that when free floating in weightless conditions with eyes closed, all sense of one's spatial orientation with respect to the aircraft can be lost. But, a touch of the hand to the enclosure restores the sense of spatial anchoring within the environment. This observation led to the exploration of how light touch of the hand can stabilize postural control on Earth even in individuals lacking vestibular function, and can override the effect of otherwise destabilizing tonic vibration reflexes in leg muscles. Such haptic stabilization appears to represent a long loop cortical reflex with contact cues at the hand phase leading EMG activity in leg muscles, which change the center of pressure at the feet to counteract body sway. Experiments on dynamic control of balance in a device programmed to exhibit inverted pendulum behavior about different axes and planes of rotation revealed that the direction of gravity not the direction of balance influences the perceived upright. Active control does not improve the accuracy of indicating the upright vs. passive exposure. In the absence of position dependent gravity shear forces on the otolith organs and body surface, drifting and loss of control soon result and subjects are unaware of their ongoing spatial position. There is a failure of dynamic path integration of the semicircular canal signals, such as occurs in weightless conditions.

Effects of white Gaussian noise on dynamic balance in healthy young adults

It has been known that short-time auditory stimulation can contribute to the improvement of the balancing ability of the human body. The present study aims to explore the effects of white Gaussian noise (WGN) of different intensities and frequencies on dynamic balance performance in healthy young adults. A total of 20 healthy young participants were asked to stand at a dynamic balance force platform, which swung along the x-axis with an amplitude of ± 4° and frequency of 1 Hz. Their center of pressure (COP) trajectories were recorded when they were stimulated by WGN of different intensities (block 1) and different frequencies (block 2). A traditional method and detrended fluctuation analysis (DFA) were used for data preprocessing. The authors found that only with 75–85 dB WGN, the COP parameters improved. WGN frequency did not affect the dynamic balance performance of all the participants. The DFA results indicated stimulation with 75 dB WGN enhanced the short-term index and reduced the crossover point. Stimulation with 500 Hz and 2500 Hz WGN significantly enhanced the short-term index. These results suggest that 75 dB WGN and 500 Hz and 2500 Hz WGN improved the participants’ dynamic balance performance. The results of this study indicate that a certain intensity of WGN is indispensable to achieve a remarkable improvement in dynamic balance. The DFA results suggest that WGN only affected the short-term persistence, indicating the potential of WGN being considered as an adjuvant therapy in low-speed rehabilitation training.

Roll tilt self-motion direction discrimination training: First evidence for perceptual learning

Perceptual learning, the ability to improve the sensitivity of sensory perception through training, has been shown to exist in all sensory systems but the vestibular system. A previous study found no improvement of passive self-motion thresholds in the dark after intense direction discrimination training of either yaw rotations (stimulating semicircular canals) or y-translation (stimulating otoliths). The goal of the present study was to investigate whether perceptual learning of self-motion in the dark would occur when there is a simultaneous otolith and semicircular canal input, as is the case with roll tilt motion stimuli. Blindfolded subjects (n = 10) trained on a direction discrimination task with 0.2-Hz roll tilt motion stimuli (9 h of training, 1,800 trials). Before and after training, motion thresholds were measured in the dark for the trained motion and for three transfer conditions. We found that roll tilt sensitivity in the 0.2-Hz roll tilt condition was increased (i.e., thresholds decreased) after training but not for controls who were not exposed to training. This is the first demonstration of perceptual learning of passive self-motion direction discrimination in the dark. The results have potential therapeutic relevance as 0.2-Hz roll thresholds have been associated with poor performance on a clinical balance test that has been linked to more than a fivefold increase in falls.

Velocity storage: its multiple roles

Our research described in this article was motivated by the puzzling finding of the Skylab M131 experiments: head movements made while rotating that are nauseogenic and disorienting on Earth are innocuous in a weightless, 0-g environment. We describe a series of parabolic flight experiments that directly addressed this puzzle and discovered the gravity-dependent responses to semicircular canal stimulation, consistent with the principles of velocity storage. We describe a line of research that started in a different direction, investigating dynamic balancing, but ended up pointing to the gravity dependence of angular velocity-to-position integration of semicircular canal signals. Together, these lines of research and the theoretical framework of velocity storage provide an answer to at least part of the M131 puzzle. We also describe recently discovered neural circuits by which active, dynamic vestibular, multisensory, and motor signals are interpreted as either appropriate for action and orientation or as conflicts evoking motion sickness and disorientation.

Gravity modulates behaviour control strategy

Human behaviour is a trade-off between exploitation of familiar resources and exploration of new ones. In a challenging environment-such as outer space-making the correct decision is vital. On Earth, gravity is always there, and is an important reference for behaviour. Thus, altered gravitational signals may affect behaviour control strategies. Here, we investigated whether changing the body's orientation to the gravitational vector would modulate the balance between routine and novel behaviour. Participants completed a random number generation task while upright or supine. We found decreased randomness when participants were supine. In particular, the degree of equiprobability of pairs of consecutive responses was reduced in the supine orientation. Online gravitational signals may shape the balance between exploitation and exploration, in favour of more stereotyped and routine responses.

Learning dynamic control of body yaw orientation

To investigate the role of gravitational cues in the learning of a dynamic balancing task, we placed blindfolded subjects in a device programmed with inverted pendulum dynamics about the yaw axis. Subjects used a joystick to try and maintain a stable orientation at the direction of balance during 20 100 s-long trials. They pressed a trigger button on the joystick to indicate whenever they felt at the direction of balance. Three groups of ten subjects each participated. One group balanced with their body and the yaw axis vertical, and thus did not have gravitational cues to help them to determine their angular position. They showed minimal learning, inaccurate indications of the direction of balance, and a characteristic pattern of positional drifting away from the balance point. A second group balanced with the yaw axis pitched 45° from the gravitational vertical and had gravity relevant position cues. The third group balanced with their yaw axis horizontal where they had gravity-dependent cues about body position in yaw. Groups 2 and 3 showed better initial balancing performance and more learning across trials than Group 1. These results indicate that in the absence of vision, the integration of transient semicircular canal and somatosensory signals about angular acceleration is insufficient for determining angular position during dynamic balancing; direct position-dependent gravity cues are necessary.