What does autonomic arousal tell us about locomotor learning?

Age and self-selected walking speed impact the generalization of locomotor memories across contexts

Previous work has shown that compared with young adults, older adults generalize their walking patterns more across environments that impose different motor demands (i.e., split-belt treadmill vs. overground). However, in this previous study, all participants walked at a speed that was more comfortable for older adults than young participants, which leads to the question of whether young adults would generalize more their walking patterns than older adults when exposed to faster speeds that are more comfortable for them. To address this question, we examined the interaction between healthy aging and walking speed on the generalization of a pattern learned on a split-belt treadmill (i.e., legs moving at different speeds) to overground. We hypothesized that walking speed during split-belt walking regulates the generalization of walking patterns in an age-specific manner. To this end, groups of young (<30 yr old) and older (65+ yr old) adults adapted their gait on a split-belt treadmill at either slower or faster walking speeds. We assessed the generalization of movements between the groups by quantifying their aftereffects during overground walking, where larger overground aftereffects represent more generalization, and zero aftereffects represent no generalization. We found an interaction between age and walking speed in the generalization of walking patterns. More specifically, older adults generalized more when adapted at slower speeds, whereas younger adults did so when adapted at faster speeds. These results suggest that comfortable walking speeds lead to more generalization of newly acquired motor patterns beyond the training contexts.NEW & NOTEWORTHY The generalization of motor learning in humans depends on both internal factors, such as age, and external factors imposed by experimenters, like treadmill speeds. However, the interaction between these factors remains unclear. Our study revealed that older adults showed increased generalization when adapting to slower speeds, whereas younger adults exhibited this behavior at faster speeds. This implies that walking at unusual speeds acts as a contextual cue, restricting the generalization of motor patterns.

The effects of auditory consequences on visuomotor adaptation and motor memory

Sensorimotor adaptation is a form of motor learning that is essential for maintaining motor performance across the lifespan and is integral to recovery of function after neurological injury. Recent research indicates that experiencing a balance-threatening physical consequence when making a movement error during adaptation can enhance subsequent motor memory. This is perhaps not surprising, as learning to avoid injury is critical for our survival and well-being. Reward and punishment can also differentially modify aspects of motor learning. However, it remains unclear whether other forms of non-physical consequences can impact motor learning. Here we tested the hypothesis that a loud acoustic stimulus linked to a movement error during adaptation could lead to greater generalization and consolidation. Two groups of participants (n = 12 each) adapted to a new, prism-induced visuomotor mapping while performing a precision walking task. One group experienced an unexpected loud acoustic stimulus (85 dB tone) when making foot-placement errors during adaptation. This auditory consequence group adapted faster and showed greater generalization with an interlimb transfer task, but not greater generalization to an obstacle avoidance task. Both groups showed faster relearning (i.e., savings) during the second testing session one week later despite the presence of an interference block of trials following initial adaptation, indicating successful consolidation. However, we did not find significant differences between groups with relearning during session 2. Overall, our results suggest that auditory consequences may serve as a useful method to improve motor learning, though further research is required.

The influence of postural threat-induced anxiety on locomotor learning and updating

The ability to adapt our locomotion in a feedforward (i.e., "predictive") manner is crucial for safe and efficient walking behavior. Equally important is the ability to quickly deadapt and update behavior that is no longer appropriate for the given context. It has been suggested that anxiety induced via postural threat may play a fundamental role in disrupting such deadaptation. We tested this hypothesis, using the "broken escalator" phenomenon: Fifty-six healthy young adults walked onto a stationary walkway ("BEFORE" condition, 5 trials), then onto a moving walkway akin to an airport travelator ("MOVING" condition, 10 trials), and then again onto the stationary walkway ("AFTER" condition, 5 trials). Participants completed all trials while wearing a virtual reality headset, which was used to induce postural threat-related anxiety (raised clifflike drop at the end of the walkway) during different phases of the paradigm. We found that performing the locomotor adaptation phase in a state of increased threat disrupted subsequent deadaptation during AFTER trials: These participants displayed anticipatory muscular activity as if expecting the platform to move and exhibited inappropriate anticipatory forward trunk movement that persisted during multiple AFTER trials. In contrast, postural threat induced during AFTER trials did not affect behavioral or neurophysiological outcomes. These findings highlight that actions learned in the presence of postural threat-induced anxiety are strengthened, leading to difficulties in deadapting these behaviors when no longer appropriate. Given the associations between anxiety and persistent maladaptive gait behaviors (e.g., "overly cautious" gait, functional gait disorders), the findings have implications for the understanding of such conditions.NEW & NOTEWORTHY Safe and efficient locomotion frequently requires movements to be adapted in a feedforward (i.e., "predictive") manner. These adaptations are not always correct, and thus inappropriate behavior must be quickly updated. Here we showed that increased threat disrupts this process. We found that locomotor actions learned in the presence of postural threat-induced anxiety are strengthened, subsequently impairing one's ability to update (or "deadapt") these actions when they are no longer appropriate for the current context.

Priming overconfidence in belief systems reveals negative return on postural control mechanisms

BACKGROUND

Modulation of postural control strategies and heightened perceptual ratings of instability when exposed to postural threats, illustrates the association between anxiety and postural control.

RESEARCH QUESTION

Here we test whether modulating prior expectations can engender postural-related anxiety which, in turn, may impair postural control and dissociate the well-established relationship between sway and subjective instability.

METHODS

We modulated expectations of the difficulty posed by an upcoming postural task via priming. In the visual priming condition, participants watched a video of an actor performing the task with either a stable or unstable performance, before themselves proceeding with the postural task. In the verbal priming paradigm, participants were given erroneous verbal information regarding the amplitude of the forthcoming platform movement, or no prior information.

RESULTS

Following the visual priming, the normal relationship between trunk sway and subjective instability was preserved only in those individuals that viewed the stable but not the unstable actor. In the verbal priming experiment we observed an increase in subjective instability and anxiety during task performance in individuals who were erroneously primed that sled amplitude would increase, when in fact it did not.

SIGNIFICANCE

Our findings show that people's subjective experiences of instability and anxiety during a balancing task are powerfully modulated by priming. The contextual provision of erroneous cognitive priors dissociates the normally 'hard wired' relationship between objective measures and subjective ratings of sway. Our findings have potential clinical significance for the development of enhanced cognitive retraining in patients with balance disorders, e.g. via modifying expectations.

Younger and Late Middle-Aged Adults Exhibit Different Patterns of Cognitive-Motor Interference During Locomotor Adaptation, With No Disruption of Savings

It has been proposed that motor adaptation and subsequent savings (or faster relearning) of an adapted movement pattern are mediated by cognitive processes. Here, we evaluated the pattern of cognitive-motor interference that emerges when young and late middle-aged adults perform an executive working memory task during locomotor adaptation. We also asked if this interferes with savings of a newly learned walking pattern, as has been suggested by a study of reaching adaptation. We studied split-belt treadmill adaptation and savings in young (21 ± 2 y/o) and late middle-aged (56 ± 6 y/o) adults with or without a secondary 2-back task during adaptation. We found that young adults showed similar performance on the 2-back task during baseline and adaptation, suggesting no effect of the dual-task on cognitive performance; however, dual-tasking interfered with adaptation over the first few steps. Conversely, dual-tasking caused a decrement in cognitive performance in late middle-aged adults with no effect on adaptation. To determine if this effect was specific to adaptation, we also evaluated dual-task interference in late middle-aged adults that dual-tasked while walking in a complex environment that did not induce motor adaptation. This group exhibited less cognitive-motor interference than late middle-aged adults who dual-tasked during adaptation. Savings was unaffected by dual-tasking in both young and late middle-aged adults, which may indicate different underlying mechanisms for savings of reaching and walking. Collectively, our findings reveal an age-dependent effect of cognitive-motor interference during dual-task locomotor adaptation and no effect of dual-tasking on savings, regardless of age. Young adults maintain cognitive performance and show a mild decrement in locomotor adaptation, while late middle-aged adults adapt locomotion at the expense of cognitive performance.

Gradual adaptation to pelvis perturbation during walking reinforces motor learning of weight shift toward the paretic side in individuals post-stroke

The purpose of this study was to determine whether the gradual versus abrupt adaptation to lateral pelvis assistance force improves weight shift toward the paretic side and enhance forced use of the paretic leg during walking. Sixteen individuals who had sustained a hemispheric stroke participated in two experimental sessions, which consisted of (1) treadmill walking with the application of lateral pelvis assistance force (gradual vs. abrupt condition) and (2) overground walking. In the "gradual" condition, during treadmill walking, the assistance force was gradually increased from 0 to 100% of the predetermined force step by step. In the abrupt condition, the force was applied at 100% of the predetermined force throughout treadmill walking. Participants exhibited significant improvements in hip abductor and adductor, ankle dorsiflexor, and knee extensor muscle activities, weight shift toward the paretic side, and overground walking speed in the gradual condition (P < 0.05), but showed no significant changes in the abrupt condition (P > 0.20). Changes in weight shift toward the paretic side were statistically different between conditions (P < 0.001), although changes in muscle activities were not (P > 0.11). In the gradual condition, the error amplitude was proportional to the improvement in weight shift during the late post-adaptation (R² = 0.32, P = 0.03), but not in the abrupt condition (R² = 0.001, P = 0.93). In conclusion, the "gradual adaptation" inducing "small errors" during constraint-induced walking may improve weight shift and enhance forced use of the paretic leg in individuals post-stroke. Applying gradual pelvis assistance force during walking may be used as an intervention strategy to improve walking in individuals post-stroke.

Dissociated motor learning and de-adaptation in patients with functional gait disorders

Walking onto a stationary platform that had been previously experienced as moving generates a locomotor after-effect—the so-called ‘broken escalator’ phenomenon. The motor responses that occur during locomotor after-effects have been mapped theoretically using a hierarchal Bayesian model of brain function that takes into account current sensory information that is weighted according to prior contextually-relevant experiences; these in turn inform automatic motor responses. Here, we use the broken escalator phenomenon to explore motor learning in patients with functional gait disorders and probe whether abnormal postural mechanisms override ascending sensory information and conscious intention, leading to maladaptive and disabling gait abnormalities. Fourteen patients with functional gait disorders and 17 healthy control subjects walked onto a stationary sled (‘Before’ condition, five trials), then onto a moving sled (‘Moving’ condition, 10 trials) and then again onto the stationary sled (‘After’ condition, five trials). Subjects were warned of the change in conditions. Kinematic gait measures (trunk displacement, step timing, gait velocity), EMG responses, and subjective measures of state anxiety/instability were recorded per trial. Patients had slower gait velocities in the Before trials (P &lt; 0.05) but were able to increase this to accommodate the moving sled, with similar learning curves to control subjects (P = 0.87). Although trunk and gait velocity locomotor after-effects were present in both groups, there was a persistence of the locomotor after-effect only in patients (P &lt; 0.05). We observed an increase in gait velocity during After trials towards normal values in the patient group. Instability and state anxiety were greater in patients than controls (P &lt; 0.05) only during explicit phases (Before/After) of the task. Mean ‘final’ gait termination EMG activity (right gastrocnemius) was greater in the patient group than controls. Despite a dysfunctional locomotor system, patients show normal adaptive learning. The process of de-adaptation, however, is prolonged in patients indicating a tendency to perpetuate learned motor programmes. The trend to normalization of gait velocity following a period of implicit motor learning has implications for gait rehabilitation potential in patients with functional gait disorders and related disorders (e.g. fear of falling).

Challenging balance during sensorimotor adaptation increases generalization

From reaching to walking, real-life experience suggests that people can generalize between motor behaviors. One possible explanation for this generalization is that real-life behaviors often challenge our balance. We propose that the exacerbated body motions associated with balance-challenged whole body movements increase the value to the nervous system of using a comprehensive internal model to control the task. Because it is less customized to a specific task, a more comprehensive model is also a more generalizable model. Here we tested the hypothesis that challenging balance during adaptation would increase generalization of a newly learned internal model. We encouraged participants to learn a new internal model using prism lenses that created a new visuomotor mapping. Four groups of participants adapted to prisms while performing either a standing-based reaching or precision walking task, with or without a manipulation that challenged balance. To assess generalization after the adaptation phase, participants performed a single trial of each of the other groups' tasks without prisms. We found that both the reaching and walking balance-challenged groups showed significantly greater generalization to the equivalent, nonadapted task than the balance-unchallenged groups. Additionally, we found some evidence that all groups generalized across tasks, for example, from walking to reaching and vice versa, regardless of balance manipulation. Overall, our results demonstrate that challenging balance increases the degree to which a newly learned internal model generalizes to untrained movements.NEW & NOTEWORTHY Real-life experience indicates that people can generalize between motor behaviors. Here we show that challenging balance during the learning of a new internal model increases the degree of generalization to untrained movements for both reaching and walking tasks. These results suggest that the effects of challenging balance are not specific to the task but instead apply to motor learning more broadly.

The "broken escalator" phenomenon: Vestibular dizziness interferes with locomotor adaptation

BACKGROUND

Although vestibular lesions degrade postural control we do not know the relative contributions of the magnitude of the vestibular loss and subjective vestibular symptoms to locomotor adaptation.

OBJECTIVE

To study how dizzy symptoms interfere with adaptive locomotor learning.

METHODS

We examined patients with contrasting peripheral vestibular deficits, vestibular neuritis in the chronic stable phase (n = 20) and strongly symptomatic unilateral Meniere's disease (n = 15), compared to age-matched healthy controls (n = 15). We measured locomotor adaptive learning using the "broken escalator" aftereffect, simulated on a motorised moving sled.

RESULTS

Patients with Meniere's disease had an enhanced "broken escalator" postural aftereffect. More generally, the size of the locomotor aftereffect was related to how symptomatic patients were across both groups. Contrastingly, the degree of peripheral vestibular loss was not correlated with symptom load or locomotor aftereffect size. During the MOVING trials, both patient groups had larger levels of instability (trunk sway) and reduced adaptation than normal controls.

CONCLUSION

Dizziness symptoms influence locomotor adaptation and its subsequent expression through motor aftereffects. Given that the unsteadiness experienced during the "broken escalator" paradigm is internally driven, the enhanced aftereffect found represents a new type of self-generated postural challenge for vestibular/unsteady patients.

Explicit Control of Step Timing During Split-Belt Walking Reveals Interdependent Recalibration of Movements in Space and Time

Split-belt treadmills that move the legs at different speeds are thought to update internal representations of the environment, such that this novel condition generates a new locomotor pattern with distinct spatio-temporal features compared to those of regular walking. It is unclear the degree to which such recalibration of movements in the spatial and temporal domains is interdependent. In this study, we explicitly altered subjects' limb motion in either space or time during split-belt walking to determine its impact on the adaptation of the other domain. Interestingly, we observed that motor adaptation in the spatial domain was susceptible to altering the temporal domain, whereas motor adaptation in the temporal domain was resilient to modifying the spatial domain. This non-reciprocal relation suggests a hierarchical organization such that the control of timing in locomotion has an effect on the control of limb position. This is of translational interest because clinical populations often have a greater deficit in one domain compared to the other. Our results suggest that explicit changes to temporal deficits cannot occur without modifying the spatial control of the limb.

Large Propulsion Demands Increase Locomotor Adaptation at the Expense of Step Length Symmetry

There is an interest to identify factors facilitating locomotor adaptation induced by split-belt walking (i.e., legs moving at different speeds) because of its clinical potential. We hypothesized that augmenting braking forces, rather than propulsion forces, experienced at the feet would increase locomotor adaptation during and after split-belt walking. To test this, forces were modulated during split-belt walking with distinct slopes: incline (larger propulsion than braking), decline (larger braking than propulsion), and flat (similar propulsion and braking). Step length asymmetry was compared between groups because it is a clinically relevant measure robustly adapted on split-belt treadmills. Unexpectedly, the group with larger propulsion demands (i.e., the incline group) changed their gait the most during adaptation, reached their final adapted state more quickly, and had larger after-effects when the split-belt perturbation was removed. We also found that subjects who experienced larger disruptions of propulsion forces in early adaptation exhibited greater after-effects, which further highlights the catalytic role of propulsion forces on locomotor adaptation. The relevance of mechanical demands on shaping our movements was also indicated by the steady state split-belt behavior, during which each group recovered their baseline leg orientation to meet leg-specific force demands at the expense of step length symmetry. Notably, the flat group was nearly symmetric, whereas the incline and decline group overshot and undershot step length symmetry, respectively. Taken together, our results indicate that forces propelling the body facilitate gait changes during and after split-belt walking. Therefore, the particular propulsion demands to walk on a split-belt treadmill might explain the gait symmetry improvements in hemiparetic gait following split-belt training.

Trial-to-trial adaptation in control of arm reaching and standing posture

Classical theories of motor learning hypothesize that adaptation is driven by sensorimotor error; this is supported by studies of arm and eye movements that have shown that trial-to-trial adaptation increases with error. Studies of postural control have shown that anticipatory postural adjustments increase with the magnitude of a perturbation. However, differences in adaptation have been observed between the two modalities, possibly due to either the inherent instability or sensory uncertainty in standing posture. Therefore, we hypothesized that trial-to-trial adaptation in posture should be driven by error, similar to what is observed in arm reaching, but the nature of the relationship between error and adaptation may differ. Here we investigated trial-to-trial adaptation of arm reaching and postural control concurrently; subjects made reaching movements in a novel dynamic environment of varying strengths, while standing and holding the handle of a force-generating robotic arm. We found that error and adaptation increased with perturbation strength in both arm and posture. Furthermore, in both modalities, adaptation showed a significant correlation with error magnitude. Our results indicate that adaptation scales proportionally with error in the arm and near proportionally in posture. In posture only, adaptation was not sensitive to small error sizes, which were similar in size to errors experienced in unperturbed baseline movements due to inherent variability. This finding may be explained as an effect of uncertainty about the source of small errors. Our findings suggest that in rehabilitation, postural error size should be considered relative to the magnitude of inherent movement variability.

Consolidation of visuomotor adaptation memory with consistent and noisy environments

Our understanding of how we learn and retain motor behaviors is still limited. For instance, there is conflicting evidence as to whether the memory of a learned visuomotor perturbation consolidates; i.e., the motor memory becomes resistant to interference from learning a competing perturbation over time. Here, we sought to determine the factors that influence consolidation during visually guided walking. Subjects learned a novel mapping relationship, created by prism lenses, between the perceived location of two targets and the motor commands necessary to direct the feet to their positions. Subjects relearned this mapping 1 wk later. Different groups experienced protocols with or without a competing mapping (and with and without washout trials), presented either on the same day as initial learning or before relearning on day 2 We tested identical protocols under constant and noisy mapping structures. In the latter, we varied, on a trial-by-trial basis, the strength of prism lenses around a non-zero mean. We found that a novel visuomotor mapping is retained at least 1 wk after initial learning. We also found reduced foot-placement error with relearning in constant and noisy mapping groups, despite learning a competing mapping beforehand, and with the exception of one protocol, with and without washout trials. Exposure to noisy mappings led to similar performance on relearning compared with the equivalent constant mapping groups for most protocols. Overall, our results support the idea of motor memory consolidation during visually guided walking and suggest that constant and noisy practices are effective for motor learning.

NEW & NOTEWORTHY

The adaptation of movement is essential for many daily activities. To interact with targets, this often requires learning the mapping to produce appropriate motor commands based on visual input. Here, we show that a novel visuomotor mapping is retained 1 wk after initial learning in a visually guided walking task. Furthermore, we find that this motor memory consolidates (i.e., becomes more resistant to interference from learning a competing mapping) when learning in constant and noisy mapping environments.

Locomotor adaptation is modulated by observing the actions of others

Observing the motor actions of another person could facilitate compensatory motor behavior in the passive observer. Here we explored whether action observation alone can induce automatic locomotor adaptation in humans. To explore this possibility, we used the "broken escalator" paradigm. Conventionally this involves stepping upon a stationary sled after having previously experienced it actually moving (Moving trials). This history of motion produces a locomotor aftereffect when subsequently stepping onto a stationary sled. We found that viewing an actor perform the Moving trials was sufficient to generate a locomotor aftereffect in the observer, the size of which was significantly correlated with the size of the movement (postural sway) observed. Crucially, the effect is specific to watching the task being performed, as no motor adaptation occurs after simply viewing the sled move in isolation. These findings demonstrate that locomotor adaptation in humans can be driven purely by action observation, with the brain adapting motor plans in response to the size of the observed individual's motion. This mechanism may be mediated by a mirror neuron system that automatically adapts behavior to minimize movement errors and improve motor skills through social cues, although further neurophysiological studies are required to support this theory. These data suggest that merely observing the gait of another person in a challenging environment is sufficient to generate appropriate postural countermeasures, implying the existence of an automatic mechanism for adapting locomotor behavior.

Treatments for Neurological Gait and Balance Disturbance: The Use of Noninvasive Electrical Brain Stimulation

Neurological gait disorders are a common cause of falls, morbidity, and mortality, particularly amongst the elderly. Neurological gait and balance impairment has, however, proved notoriously difficult to treat. The following review discusses some of the first experiments to modulate gait and balance in healthy adults using anodal transcranial direct current stimulation (tDCS) by stimulating both cerebral hemispheres simultaneously. We review and discuss published data using this novel tDCS approach, in combination with physical therapy, to treat locomotor and balance disorders in patients with small vessel disease (leukoaraiosis) and Parkinson’s disease. Finally, we review the use of bihemispheric anodal tDCS to treat gait impairment in patients with stroke in the subacute phase. The findings of these studies suggest that noninvasive electrical stimulation techniques may be a useful adjunct to physical therapy in patients with neurological gait disorders, but further mutlicentre randomized sham-controlled studies are needed to evaluate whether experimental tDCS use can translate into mainstream clinical practice for the treatment of neurological gait disorders.

Attention modulates adaptive motor learning in the 'broken escalator' paradigm

The physical stumble caused by stepping onto a stationary (broken) escalator represents a locomotor aftereffect (LAE) that attests to a process of adaptive motor learning. Whether such learning is primarily explicit (requiring attention resources) or implicit (independent of attention) is unknown. To address this question, we diverted attention in the adaptation (MOVING) and aftereffect (AFTER) phases of the LAE by loading these phases with a secondary cognitive task (sequential naming of a vegetable, fruit and a colour). Thirty-six healthy adults were randomly assigned to 3 equally sized groups. They performed 5 trials stepping onto a stationary sled (BEFORE), 5 with the sled moving (MOVING) and 5 with the sled stationary again (AFTER). A 'Dual-Task-MOVING (DTM)' group performed the dual-task in the MOVING phase and the 'Dual-Task-AFTEREFFECT (DTAE)' group in the AFTER phase. The 'control' group performed no dual task. We recorded trunk displacement, gait velocity and gastrocnemius muscle EMG of the left (leading) leg. The DTM, but not the DTAE group, had larger trunk displacement during the MOVING phase, and a smaller trunk displacement aftereffect compared with controls. Gait velocity was unaffected by the secondary cognitive task in either group. Thus, adaptive locomotor learning involves explicit learning, whereas the expression of the aftereffect is automatic (implicit). During rehabilitation, patients should be actively encouraged to maintain maximal attention when learning new or challenging locomotor tasks.

Changes in task parameters during walking prism adaptation influence the subsequent generalization pattern

An understanding of the transfer (or generalization) of motor adaptations between legs and across tasks during walking has remained elusive due to limited research and mixed results. Here, we asked whether stepping sequences or task constraints introduced during walking prism-adaptation tasks influence generalization patterns. Forty subjects adapted to prism glasses in precision-walking or obstacle-avoidance tasks that required a specific stepping sequence to the center of two/three targets or laterally over an obstacle. We then tested for generalization, reflected by aftereffects in the nonadapted task. Our previous study using these tasks found that only one leg generalized. Here, we reversed the stepping sequence and found that only the opposite leg generalized in the subject group that adapted in a precision-walking task. The combination of stepping sequence and direction of prism shift caused subjects in two groups to collide with the obstacle early during adaptation, thus making the step prior to going over the obstacle more important. Both legs subsequently generalized. A fourth subject group experienced a three-target, precision-walking task, resulting in a balanced, right-left, left-right stepping sequence, meant to induce bilateral generalization. While only one leg generalized, foot placement aftereffects before stepping over the obstacle would have caused subjects to collide with it. Together with our previous study, the results suggest a contribution of stepping sequence during the adapted task on generalization patterns, likely driven by proprioceptive feedback. The results also support the idea that negative consequences during adaptation and/or perceived threat can influence generalization.

Accessing escalators: a central vestibular disorder after posterior fossa tumor removal

About 20% of childhood tumors originate within the central nervous system. Progress in assessment and treatment of these lesions has led to improved survival rates. We describe a patient with a posterior fossa ependymoma who despite a remarkable recovery following treatment has been frustrated by difficulty in using escalators. Such symptom selectivity is explained by specific vertical visuomotor and high-frequency vestibular deficits disrupting the execution of this complex motor act.

Enhanced locomotor adaptation aftereffect in the "broken escalator" phenomenon using anodal tDCS

The everyday experience of stepping onto a stationary escalator causes a stumble, despite our full awareness that the escalator is broken. In the laboratory, this "broken escalator" phenomenon is reproduced when subjects step onto an obviously stationary platform (AFTER trials) that was previously experienced as moving (MOVING trials) and attests to a process of motor adaptation. Given the critical role of M1 in upper limb motor adaptation and the potential for transcranial direct current stimulation (tDCS) to increase cortical excitability, we hypothesized that anodal tDCS over leg M1 and premotor cortices would increase the size and duration of the locomotor aftereffect. Thirty healthy volunteers received either sham or real tDCS (anodal bihemispheric tDCS; 2 mA for 15 min at rest) to induce excitatory effects over the primary motor and premotor cortex before walking onto the moving platform. The real tDCS group, compared with sham, displayed larger trunk sway and increased gait velocity in the first AFTER trial and a persistence of the trunk sway aftereffect into the second AFTER trial. We also used transcranial magnetic stimulation to probe changes in cortical leg excitability using different electrode montages and eyeblink conditioning, before and after tDCS, as well as simulating the current flow of tDCS on the human brain using a computational model of these different tDCS montages. Our data show that anodal tDCS induces excitability changes in lower limb motor cortex with resultant enhancement of locomotor adaptation aftereffects. These findings might encourage the use of tDCS over leg motor and premotor regions to improve locomotor control in patients with neurological gait disorders.

Natural error patterns enable transfer of motor learning to novel contexts

Successful behavior demands motor learning to be transferable in some cases (e.g., adjusting walking patterns as we develop and age) and context specific in others (e.g., learning to walk in high heels). Here we investigated differences in motor learning transfer in people learning a new walking pattern on a split-belt treadmill, where the legs move at different speeds. We hypothesized that transfer of the newly acquired walking pattern on the treadmill to natural over ground walking might depend on the pattern of errors experienced during learning. Error patterns within a person's natural range might be experienced as endogenous (i.e., produced by the body), encouraging general adjustments that transfer across contexts. On the other hand, larger errors might be experienced as exogenous (i.e., produced by the environment), indicating unusual conditions requiring context-specific learning. To test this, we manipulated the distribution of errors experienced during learning to lie either within or outside the normal distribution of walking errors. We found that restriction of errors to the natural range produced transfer of the new walking pattern from the treadmill to natural walking off the treadmill, while larger errors prevented transfer. This result helps explain how transfer of motor learning is controlled, and it offers an important strategy for clinical rehabilitation, where transfer of motor learning to other contexts is essential.