Adaptation in a rotating artificial gravity environment

The Effects of 30 Minutes of Artificial Gravity on Cognitive and Sensorimotor Performance in a Spaceflight Analog Environment

The altered vestibular signaling and somatosensory unloading of microgravity result in sensory reweighting and adaptation to conflicting sensory inputs. Aftereffects of these adaptive changes are evident postflight as impairments in behaviors such as balance and gait. Microgravity also induces fluid shifts toward the head and an upward shift of the brain within the skull; these changes are well-replicated in strict head-down tilt bed rest (HDBR), a spaceflight analog environment. Artificial gravity (AG) is a potential countermeasure to mitigate these effects of microgravity. A previous study demonstrated that intermittent (six, 5-mins bouts per day) daily AG sessions were more efficacious at counteracting orthostatic intolerance in a 5 day HDBR study than continuous daily AG. Here we examined whether intermittent daily AG was also more effective than continuous dosing for mitigating brain and behavioral changes in response to 60 days of HDBR. Participants (n = 24) were split evenly between three groups. The first received 30 mins of continuous AG daily (cAG). The second received 30 mins of intermittent AG daily (6 bouts of 5 mins; iAG). The third received no AG (Ctrl). We collected a broad range of sensorimotor, cognitive, and brain structural and functional assessments before, during, and after the 60 days of HDBR. We observed no significant differences between the three groups in terms of HDBR-associated changes in cognition, balance, and functional mobility. Interestingly, the intermittent AG group reported less severe motion sickness symptoms than the continuous group during centrifugation; iAG motion sickness levels were not elevated above those of controls who did not undergo AG. They also had a shorter duration of post-AG illusory motion than cAG. Moreover, the two AG groups performed the paced auditory serial addition test weekly while undergoing AG; their performance was more accurate than that of controls, who performed the test while in HDBR. Although AG did not counteract HDBR-induced gait and balance declines, iAG did not cause motion sickness and was associated with better self-motion perception during AG ramp-down. Additionally, both AG groups had superior cognitive performance while undergoing AG relative to controls; this may reflect attention or motivation differences between the groups.

Convergent models of handedness and brain lateralization

The pervasive nature of handedness across human history and cultures is a salient consequence of brain lateralization. This paper presents evidence that provides a structure for understanding the motor control processes that give rise to handedness. According to the Dynamic Dominance Model, the left hemisphere (in right handers) is proficient for processes that predict the effects of body and environmental dynamics, while the right hemisphere is proficient at impedance control processes that can minimize potential errors when faced with unexpected mechanical conditions, and can achieve accurate steady-state positions. This model can be viewed as a motor component for the paradigm of brain lateralization that has been proposed by Rogers et al. (MacNeilage et al., 2009) that is based upon evidence from a wide range of behaviors across many vertebrate species. Rogers proposed a left-hemisphere specialization for well-established patterns of behavior performed in familiar environmental conditions, and a right hemisphere specialization for responding to unforeseen environmental events. The dynamic dominance hypothesis provides a framework for understanding the biology of motor lateralization that is consistent with Roger's paradigm of brain lateralization.

Absence of equifinality of hand position in a double-step unloading task

Equifinality, during arm reaching movements, relates to the capacity of the neuromuscular system to attain the same final position in the presence or absence of transient perturbations. There have been several controversies regarding equifinality in the literature. A brief elastic perturbation, applied during a fast arm movement or just before its initiation, typically does not affect final arm position. On the other hand, several experiments have shown that velocity-dependent perturbations, such as Coriolis force or negative damping, while transient in nature, have a significant effect on final arm position when compared to unperturbed movements. In this study, an unloading paradigm was used to study the role of reflexes with respect to equifinality. The effects on final arm position of suddenly decreasing a static load maintained by fourteen subjects were analyzed. Subjects maintained an initial load produced by a double-joint manipulandum moving in the horizontal plane. The load was suddenly decreased, either in one or in two successive steps with different time intervals, resulting in a rapid reflex-mediated change in arm position. Unloading led to short-latency changes in the activity of shoulder and elbow muscles and significant variations in tonic activity. It was found that the final hand position was shorter for double- versus single-step unloading if the time between two successive changes in load was greater than 100 ms. With a shorter time interval, the final hand positions were the same. This difference in final hand positions was inversely proportional to the hand velocity at the time of the second change in load. Further, agonist/antagonist co-activation increased in double-step unloading. Thus, the change in both the load and the movement velocity may influence the magnitude of the unloading reflex. This may be indicative of a dependence of stretch reflexes on velocity. Perturbation may cause a reflex-mediated increase in joint stiffness, which could explain why equifinality is not preserved after some perturbations, such as velocity-dependant external forces.

Effect on movement selection of an evolving sensory representation: a multiple controller model of skill acquisition

Change in behavior and neural activity in skill acquisition suggests that control is transferred from cortical planning areas (e.g., the prefrontal cortex, PFC) to the basal ganglia (BG). Planning has large computational and representational requirements but requires little experience with a task. The BG are thought to employ a simpler control scheme and reinforcement learning; these mechanisms rely on extensive experience. Many theoretical accounts of behavior in the face of uncertainty invoke planning mechanisms that explicitly take uncertainty into account. We suggest that the simpler mechanisms of the BG can also contribute to the development of behavior under such conditions. We focus on learning under conditions in which sensory information takes time to resolve, e.g., when a poorly perceived goal stimulus takes non-negligible time to identify. It may be advantageous to begin acting quickly under uncertainty--possibly via decisions that are suboptimal for the actual goal--rather than to wait for sensory information to fully resolve. We present a model of skill acquisition in which control is transferred, with experience, from a planning controller (denoted A), corresponding to the PFC, to a simpler controller (B), corresponding to the BG. We apply our model to a task in which a learning agent must execute a series of actions to achieve a goal (selected randomly at each trial from a small set). Over the course of a trial, the agent's goal representation evolves from representing all possible goals to only the selected goal. A is restricted to select movements only when goal representation is fully resolved. Model behavior is similar to that observed in humans accomplishing similar tasks. Thus, B can by itself account for the development of behavior under an evolving sensory representation, suggesting that the BG can contribute to learning and control under conditions of uncertainty.

Hypergravity resistance exercise: the use of artificial gravity as potential countermeasure to microgravity

The aims of this study were to 1) determine if hypergravity (HG) squats can produce foot forces similar to those measured during 10-repetition maximum (10RM) squats using weights under normal 1-G z condition, and 2) compare the kinematics (duration and goniometry) and EMG activities of selected joints and muscles between 10RM and HG squats of similar total foot forces. Eight men and six women [27 yr (SD 4), 66 kg (SD 10)] completed ten 10RM [83 kg (SD 23)] and 10 HG squats (2.25–3.75 G z). HG squats were performed on a human-powered short-arm centrifuge. Foot forces were measured using insole force sensors. Hip, knee, and ankle angles were measured using electrogoniometers. EMG activities of the erector spinae, biceps femoris, rectus femoris, and gastrocnemius were also recorded during both squats. All subjects were able to achieve similar or higher average total foot forces during HG squats compared with those obtained during 10RM squats. There were no differences in total duration per set, average duration per repetition, and goniometry and EMG activities of the selected joints and muscles, respectively, between 10RM and HG squats. These results demonstrate that HG squats can produce very high foot forces that are comparable to those produced during 10RM squats at 1 G z. In addition, the technique and muscle activation are similar between the two types of squats. This observation supports the view that HG resistance training may represent an important countermeasure to microgravity.

Vestibular contribution to the planning of reach trajectories

Reaching for an object while simultaneously rotating induces Coriolis and centrifugal inertial forces on the arm that require compensatory actions to maintain accuracy. We investigated whether the nervous system uses vestibular signals of head rotation to predict inertial forces. Human subjects reached in darkness to a remembered target 33 cm distant. Subjects were stationary, but experienced a strong vestibular rotation signal. We achieved this by rotating subjects at 360 degrees /s in yaw for 2 min and then stopping, and subjects reached during the 'post-rotary' period when the deceleration is interpreted by the vestibular system as a rotation in the opposite direction. Arm trajectories were straight in control trials without a rotary stimulus. With vestibular stimulation, trajectory curvature increased an average of 3 cm in the direction of the vestibular stimulation (e.g., to the right for a rightward yaw stimulus). Vestibular-induced curvature returned rapidly to normal, with an average time constant of 6 s. Movements also became longer as the vestibular stimulus diminished, and returned towards normal length with an average time constant of 5.6 s. In a second experiment we compared reaching with preferred and non-preferred hands, and found that they were similarly affected by vestibular stimulation. The reach curvatures were in the expected direction if the nervous system anticipated and attempted to counteract the presence of Coriolis forces based on the vestibular signals. Similarly, the shorter reaches may have occurred because the nervous system was attempting to compensate for an expected centrifugal force. Since vestibular stimulation also alters the perceived location of targets, vestibular signals probably influence all stages of the sensorimotor pathway transforming the desired goal of a reach into specific motor-unit innervation.

The dominant and nondominant arms are specialized for stabilizing different features of task performance

We have previously proposed a model of motor lateralization, in which the two arms are differentially specialized for complementary control processes. During aimed movements, the dominant arm shows advantages for coordinating intersegmental dynamics as required for specifying trajectory speed and direction, while the nondominant arm shows advantages in controlling limb impedance, as required for accurate final position control. We now directly test this model of lateralization by comparing performance of the two arms under two different tasks: one in which reaching movement is made from one fixed starting position to three different target positions; and the other in which reaching is made from three different starting positions to one fixed target position. For the dominant arm, performance was most accurate when reaching from one fixed starting position to multiple targets. In contrast, nondominant arm performance was most accurate when reaching toward a single target from multiple start locations. These findings contradict the idea that motor lateralization reflects a global advantage of one "dominant" hemisphere/limb system. Instead, each hemisphere/limb system appears specialized for stabilizing different aspects of task performance.

Robot-enhanced motor learning: accelerating internal model formation during locomotion by transient dynamic amplification

When adapting to novel dynamic environments the nervous system learns to anticipate the imposed forces by forming an internal model of the environmental dynamics in a process driven by movement error reduction. Here, we tested the hypothesis that motor learning could be accelerated by transiently amplifying the environmental dynamics. A novel dynamic environment was created during treadmill stepping by applying a perpendicular viscous force field to the leg through a robotic device. The environmental dynamics were amplified by an amount determined by a computational learning model fit on a per-subject basis. On average, subjects significantly reduced the time required to predict the applied force field by approximately 26% when the field was transiently amplified. However, this reduction was not as great as that predicted by the model, likely due to nonstationarities in the learning parameters. We conclude that motor learning of a novel dynamic environment can be accelerated by exploiting the error-based learning mechanism of internal model formation, but that nonlinearities in adaptive response may limit the feasible acceleration. These results support an approach to movement training devices that amplify rather than reduce movement errors, and provide a computational framework for both implementing the approach and understanding its limitations.

Robotics, motor learning, and neurologic recovery

Robotic devices are helping shed light on human motor control in health and injury. By using robots to apply novel force fields to the arm, investigators are gaining insight into how the nervous system models its external dynamic environment. The nervous system builds internal models gradually by experience and uses them in combination with impedance and feedback control strategies. Internal models are robust to environmental and neural noise, generalized across space, implemented in multiple brain regions, and developed in childhood. Robots are also being used to assist in repetitive movement practice following neurologic injury, providing insight into movement recovery. Robots can haptically assess sensorimotor performance, administer training, quantify amount of training, and improve motor recovery. In addition to providing insight into motor control, robotic paradigms may eventually enhance motor learning and rehabilitation beyond the levels possible with conventional training techniques.

Differential impact of hypergravity on maturating innervation in vestibular epithelia during rat development

Over the past decades, the new opportunity of space flights has revealed the importance of gravity as a mechanical constraint for terrestrial organisms as well as its influence on the somatosensory system. The lack of gravitational reference in orbital flight induces changes in equilibrium, with major modifications involving neuromorphological and physiological adaptations. However, few data have illustrated the putative effect of gravity on sensory vestibular epithelial development. We asked if gravity, the primary stimulus of utricles could act as an epigenetic factor. As sensorial deprivation linked to weightlessness is technically difficult, we used a ground-based centrifuge to increase the gravitational vector, in order to hyperstimulate the vestibule. In this study, 3 days after mating, pregnant females were submitted to hypergravity, 2 g (HG). Their embryos were raised, born and postnatally developed under HG. The establishment of connections between primary vestibular afferent neurons and hair cells in the utricle of these young rats was followed from birth to postnatal day 6 (PN6) and compared to embryos developed in normogravity (NG): Immunocytochemistry for neurofilaments and microvesicles revealed the differential effects of gravity on the late neuritogenic and synaptogenic processes in utricles. Taking type I hair cell innervation as a criterion of maturation, we found that primary afferent fibres reached the vestibular epithelium and enveloped hair cells in the same way, both under NG and HG. Thus, this phenomenon of leading growth cones to their epithelial target appears to be dependent on intrinsic genetic properties and not on an external stimulus. In contrast, the maturation of connection processes between type 1 hair cells and the afferent calyx, concerning specifically the microvesicles at their apex, was delayed under HG. Therefore, gravity appears to be an epigenetic factor influencing the late maturation of utricles. These differential effects of altered gravity on the development of the vestibular epithelium are discussed.

Bias and sensitivity in the haptic perception of geometry

Our ability to recognize and manipulate objects relies on our haptic sense of the objects' geometry. But little is known about the acuity of haptic perception compared to other senses like sight and hearing. Here, we determined how accurately humans could sense various geometric features of objects across the workspace. Subjects gripped the handle of a robot arm which was programmed to keep the hand inside a planar region with straight or curved boundaries. With eyes closed, subjects moved the manipulandum along this virtual wall and judged its curvature or direction. We mapped their sensitivity in different parts of the workspace. We also tested subjects' ability to discriminate between boundaries with different degrees of curvature, to sense the rate of change of curvature, and to detect the elongation or flattening of ellipses. We found that subjects' estimates of the curvature of their hand path were close to veridical, and did not change across the workspace though they did vary somewhat with hand path direction. Subjects were less accurate at judging the direction of the hand path in an egocentric frame of reference, and were slightly poorer at discriminating between arcs of different curvature than at detecting absolute curvature. They also consistently mistook flattened ellipses and paths of decreasing curvature (inward spirals) for circles-and mistook arcs of true circles for arcs of tall ellipses or outward spirals. Nevertheless, the sensitivity of haptic perception compared well with that of spatial vision in other studies. Furthermore, subjects detected curvature and directional deviations much smaller than those that actually arise for most reaching movements. These findings suggest that our haptic sense is acute enough to guide and train motor systems and to form accurate representations of shapes.

Artificial gravity as a countermeasure in long-duration space flight

Long-duration exposure to weightlessness results in bone demineralization, muscle atrophy, cardiovascular deconditioning, altered sensory-motor control, and central nervous system reorganizations. Exercise countermeasures and body loading methods so far employed have failed to prevent these changes. A human mission to Mars might last 2 or 3 years and without effective countermeasures could result in dangerous levels of bone and muscle loss. Artificial gravity generated by rotation of an entire space vehicle or of an inner chamber could be used to prevent structural changes. Some of the physical characteristics of rotating environments are outlined along with their implications for human performance. Artificial gravity is the centripetal force generated in a rotating vehicle and is proportional to the product of the square of angular velocity and the radius of rotation. Thus, for a particular g-level, there is a tradeoff between velocity of rotation and radius. Increased radius is vastly more expensive to achieve than velocity, so it is important to know the highest rotation rates to which humans can adapt. Early studies suggested that 3 rpm might be the upper limit because movement control and orientation were disrupted at higher velocities and motion sickness and chronic fatigue were persistent problems. Recent studies, however, are showing that, if the terminal velocity is achieved over a series of gradual steps and many body movements are made at each dwell velocity, then full adaptation of head, arm, and leg movements is possible. Rotation rates as high as 7.5-10 rpm are likely feasible. An important feature of the new studies is that they provide compelling evidence that equilibrium point theories of movement control are inadequate. The central principles of equilibrium point theories lead to the equifinality prediction, which is violated by movements made in rotating reference frames.