Direct Current Stimulation over the Primary Motor Cortex, Cerebellum, and Spinal Cord to Modulate Balance Performance: A Randomized Placebo-Controlled Trial.

OBJECTIVES: Existing applications of non-invasive brain stimulation in the modulation of balance ability are focused on the primary motor cortex (M1). It is conceivable that other brain and spinal cord areas may be comparable or more promising targets in this regard. This study compares transcranial direct current stimulation (tDCS) over (i) the M1, (ii) the cerebellum, and (iii) trans-spinal direct current stimulation (tsDCS) in the modulation of balance ability. METHODS: Forty-two sports students were randomized in this placebo-controlled study. Twenty minutes of anodal 1.5 mA t/tsDCS over (i) the M1, (ii) the cerebellum, and (iii) the spinal cord, as well as (iv) sham tDCS were applied to each subject. The Y Balance Test, Single Leg Landing Test, and Single Leg Squat Test were performed prior to and after each intervention. RESULTS: The Y Balance Test showed significant improvement after real stimulation of each region compared to sham stimulation. While tsDCS supported the balance ability of both legs, M1 and cerebellar tDCS supported right leg stand only. No significant differences were found in the Single Leg Landing Test and the Single Leg Squat Test. CONCLUSIONS: Our data encourage the application of DCS over the cerebellum and spinal cord (in addition to the M1 region) in supporting balance control. Future research should investigate and compare the effects of different stimulation protocols (anodal or cathodal direct current stimulation (DCS), alternating current stimulation (ACS), high-definition DCS/ACS, closed-loop ACS) over these regions in healthy people and examine the potential of these approaches in the neurorehabilitation.

Practice reduces the costs of producing head fakes in basketball.

Previous research indicates that performing passes with a head fake in basketball leads to increased response initiation times and errors as compared to performing a pass without a head fake. These so-called fake production costs only occurred when not given the time to mentally prepare the deceptive movement. In the current study, we investigated if extensive practice could reduce the cognitive costs of producing a pass with head fake. Twenty-four basketball novices participated in an experiment on five consecutive days. A visual cue prompted participants to play a pass with or without a head fake either to the left or right side. The cued action had to be executed after an interstimulus interval (ISI) of either 0 ms, 400 ms, 800 ms or 1200 ms, allowing for different movement preparation times. Results indicated higher response initiation times (ITs) and error rates (ERs) for passes with head fakes for the short preparation intervals (ISI 0 ms and 400 ms) on the first day but no difference for the longer preparation intervals (ISI 800 ms and 1200 ms). After only one day of practice, participants showed reduced fake production costs (for ISI 0 ms) and were even able to eliminate these cognitive costs when given time to mentally prepare the movement (for ISI 400 ms). Accordingly, physical practice can reduce the cognitive costs associated with head-fake generation. This finding is discussed against the background of the strengthening of stimulus response associations.

Less motor (re-)planning requires fewer working memory resources.

In the current study, we asked if less motor re-planning requires fewer resources in working memory (WM). To this end, participants executed a spatial WM task in parallel to different sequential motor tasks: (1) a randomised task with a high amount of motor re-planning and (2) an ordered task with a lower amount of motor re-planning. Recall performance in the spatial WM task was measured as the dependent variable. Hand posture was used to calculate the percentage of motor re-planning and, thus, to validate the experimental manipulation. The percentage of motor re-planning was lower in the ordered task, while spatial WM performance was higher. This indicates that WM resources depleted by the motor task scale with the amount of motor re-planning. Results further showed a significant recency effect (i.e. better recall of late items) in the spatial WM task. As previous studies found that recency effects in a verbal WM task are disrupted by a concurrent motor task, the presence of recency in the current study indicates a differential interference of a concurrent motor task on verbal vs. spatial recall, which has important implications for several current models of WM.

Social cues can impact complex behavior unconsciously.

In three experiments, we investigated the effect of unconscious social priming on human behavior in a choice reaction time task. Photographs of a basketball player passing a ball to the left/right were used as target stimuli. Participants had to respond to the pass direction either by a whole-body (complex) response or a button-press (simple) response. Visually masked stimuli, showing both a task-relevant cue (pass direction) and a task-irrelevant, social cue (gaze direction), were used as primes. Subliminal social priming was found for kinematic (center of pressure) and chronometric measures (response times): gaze direction in the primes affected responses to the pass direction in the targets. The social priming effect diminished when gaze information was unhelpful or even detrimental to the task. Social priming of a complex behavior does not require awareness or intentionality, indicating automatic processing. Nevertheless, it can be controlled by top-down, strategic processes.

Working memory load does not affect sequential motor planning.

Movement planning disrupts the recall performance in a short term memory task, indicating that both processes share common working memory (WM) resources. In the current study, we tested whether this interference was bidirectional. To this end, we combined an easy or a difficult memory task (depleting different amounts of WM resources) with a sequential motor task (opening a column of drawers). The size of the hysteresis effect in the sequential motor task was measured as a proxy for the fraction of motor plan reuse. The different WM loads created by the memory task had no effect on the fractions of motor plan reuse and motor (re-)planning, which supports the idea that motor planning has priority access to WM. A recency effect (better recall of late items) was absent in a verbal memory task but present in a spatial one. Recency is commonly attributed to the episodic buffer, a non-domain-specific storage of the central executive. The domain-specific interference of the motor task with recency indicates that the second assumption needs to be reevaluated.

Shifts of the point-of-change can be attributed to a lower mechanical cost of motor execution.

In a previous study on hand selection in a sequential reaching task, the authors showed a shift of the point-of-change (POC) to the left of the midline. This implies that participants conducted a number of contralateral reaches with their dominant, right hand. Contralateral movements have longer planning and execution times and a lower precision. In the current study, we asked whether lower mechanical costs of motor execution or lower cognitive costs of motor planning compensated for these disadvantages. Theories on hemispheric differences postulate lower mechanical costs in the dominant hemisphere and lower cognitive costs in the left hemisphere (independent of handedness). In right-handed participants, both factors act agonistically to reduce the total cost of right-handed reaches. To distinguish between the cost factors, we had left- and right-hand-dominant participants execute a sequential, unimanual reaching task. Results showed a left-shift of the POC in the right-handed and a right-shift in the left-handed group. Both shifts were similar in magnitude. These findings indicate that only the mechanical cost of motor execution compensates for the disadvantages of the contralateral reaches, while the cognitive cost of motor planning is irrelevant for the POC shift.

Is the head-fake effect in basketball robust against practice? Analyses of trial-by-trial adaptations, frequency distributions, and mixture effects to evaluate effects of practice.

Reactions to the pass of a basketball player performing a head fake are typically slower than reactions to a basketball player who passes without a head fake (i.e., head-fake effect). The present study shows that extensive practice reduces the head-fake effect in basketball. Additional analyses were conducted to explore the mechanism behind the reduced head-fake effect. First, we analyzed whether or not participants developed some control over the processing of irrelevant gaze direction, as indicated by specific trial-to-trial adaptations (i.e., congruency sequence effect). Second, we fitted the individual frequency distributions of RTs to ex-Gaussian distributions, to evaluate if practice specifically affects the Gaussian part of the distribution or the exponential part of the distribution. Third, we modeled individual RT distributions as the so-called mixture effects to examine whether the way irrelevant gaze direction impacts performance (either occasionally but massively or continuously but moderately) changes with practice. The analyses revealed that the effect of practice could not be explained with an increasing congruency-sequence effect. Also, it could not be found in the ex-Gaussian distributional analyses. The assumption that residual failure to inhibit the processing of the gaze direction in contrast to continuous failures to do so might favor mixed effects over uniform effects at later courses of practice could not be validated. The reduced head-fake effect thus is argued to source in participants' general increasing ability to inhibit the processing of the task-irrelevant gaze direction information and/or in a priority shift of gaze processing to a processing of the pass direction.

Hemispheric lateralization does not affect the cognitive and mechanical cost of a sequential motor task.

In sequential, repetitive tasks, we often partially reuse former motor plans. This causes a persistence of an earlier adopted posture (termed motor hysteresis). The cost-optimization hypothesis states that a partial reuse reduces the cognitive cost of a movement, while the persistence in a former posture increases its mechanical cost. An optimal fraction of reuse, which depends on the relative cognitive and mechanical cost, minimizes the total movement cost. Several studies postulate differences in mechanical or cognitive cost as a result of hemispheric lateralization. In the current study, we asked whether these differences would result in different fractions of motor plan reuse. To this end, left- and right-handed dominant participants executed a sequential motor task (opening a column of drawers) with their dominant and non-dominant hand. The size of the motor hysteresis effect was measured as a proxy for the fraction of plan reuse. Participants used similar postures and exhibited a similar hysteresis effect, irrespective of hand and handedness. This finding indicates that either the cognitive and mechanical costs of a motor task are unaffected by hemispheric differences or that their effect on motor planning is negligible.

A Simple Model to Estimate the Percentage of Motor Plan Reuse From Hysteresis Effect Size.

In sequential tasks, a partial reuse of former motor plans results in a persistence in the former posture (termed hysteresis). The cost-optimization hypothesis states that the percentage of reuse depends on the relative cognitive and mechanical cost of each movement. These costs should be constant across all drawers, yet previous studies found a larger hysteresis effect at the central drawers and declining effects toward the periphery. In the current study, we show that a simple mathematical model that assumes a sigmoid optimal grasp angle function and a fixed percentage of motor plan reuse explains the posture variance in a randomized and an ordered sequential drawer opening task. This finding indicates that (1) the optimal pro/supination angle is a sigmoid function of drawer height, (2) the percentage of motor plan reuse is constant across drawers, and (3) a constant percentage of reuse results in a larger hysteresis effect at the central drawers. Based on the model, the percentage of motor plan reuse in future studies can be estimated from the size of the motor hysteresis effect.

Motor hysteresis in a sequential grasping and pointing task is absent in task-critical joints.

In a prior study (Schütz et al. in Exp Brain Res 2016. doi: 10.1007/s00221-016-4608-6 ), we demonstrated that the cognitive cost of motor planning did not differ in a vertical pointing and grasping task. It was unclear whether the similar cost implied that both tasks required the same number of independent degrees of freedom (IDOFs) or that the number of IDOFs did not affect motor planning. To differentiate between both cases, a reanalysis of the prior data was conducted. The number of IDOFs in the pointing and grasping tasks was computed by factor analysis. In both tasks, two IDOFs were used, which was the minimum number required for position control. This indicates that hand alignment in the grasping task did not require an additional IDOF. No conclusions regarding the link between the cognitive cost of motor planning and the number of IDOFs could be drawn. A subset of task-critical joint angles was not affected by motor hysteresis. This indicates that a joint's susceptibility to motor hysteresis depends on its relevance to the task goal. In task-critical joints, planning cost minimization by motor plan reuse is suppressed in favor of the task goal.

Cognitive costs of motor planning do not differ between pointing and grasping in a sequential task.

Neurophysiologic studies have shown differences in brain activation between pointing and grasping movements. We asked whether these two movement types would differ in their cognitive costs of motor planning. To this end, we designed a sequential, continuous posture selection task, suitable to investigate pointing and grasping movements to identical target locations. Participants had to open a column of drawers or point to a column of targets in ascending and descending progression. The global hand pro/supination at the moment of drawer/target contact was measured. The size of the motor hysteresis effect, i.e., the persistence to a former posture, was used as a proxy for the cognitive cost of motor planning. A larger hysteresis effect equals higher cognitive cost. Both motor tasks had similar costs of motor planning, but a larger range of motion was found for the grasping movements.

Movement plans for posture selection do not transfer across hands.

In a sequential task, the grasp postures people select depend on their movement history. This motor hysteresis effect results from the reuse of former movement plans and reduces the cognitive cost of movement planning. Movement plans for hand trajectories not only transfer across successive trials, but also across hands. We therefore asked whether such a transfer would also be found in movement plans for hand postures. To this end, we designed a sequential, continuous posture selection task. Participants had to open a column of drawers with cylindrical knobs in ascending and descending sequences. A hand switch was required in each sequence. Hand pro/supination was analyzed directly before and after the hand switch. Results showed that hysteresis effects were present directly before, but absent directly after the hand switch. This indicates that, in the current study, movement plans for hand postures only transfer across trials, but not across hands.

Habitual vs non-habitual manual actions: an ERP study on overt movement execution.

This study explored the neurophysiological mechanisms underlying the planning and execution of an overt goal-related handle rotation task. More specifically, we studied the neural basis of motor actions concerning the influence of the grasp choice. The aim of the present study was to differentiate cerebral activity between grips executed in a habitual and a non-habitual mode, and between specified and free grip choices. To our knowledge, this is the first study to differentiate cerebral activity underlying overt goal-related actions executed with a focus on the habitual mode. In a handle rotation task, participants had to use thumb-toward (habitual) or thumb-away (non-habitual) grips to rotate a handle to a given target position. Reaction and reach times were shorter for the habitual compared to the non-habitual mode indicating that the habitual mode requires less cognitive processing effort than the non-habitual mode. Neural processes for action execution (measured by event-related potentials (ERPs)) differed between habitual and non-habitual conditions. We found differential activity between habitual and non-habitual conditions in left and right frontal areas from -600 to 200 ms time-locked to reaching the target position. No differential neural activity could be traced for the specification of the grip. The results suggested that the frontal negativity reflected increased difficulty in movement precision control in the non-habitual mode compared to the habitual mode during the homing in phase of grasp and rotation actions.

Influence of mechanical load on sequential effects.

Almost two decades ago, sequential effects of human grasping behaviour were described for the first time: In a sequential task, participants persisted in using the previous grasp type. According to the plan-modification hypothesis, such sequential effects reduce the movement planning costs and occur within a limited range of indifference. In the current study, we asked whether the anticipated mechanical costs of a movement would compete with the movement planning costs and, thus, reduce the magnitude of the sequential effect. To this end, participants were tested in a sequential, perceptual-motor task (opening a column of drawers), which offered a continuous range of posture solutions for each trial. In a pre-/post-test design, the magnitude of the sequential effect was measured before and after a manipulation phase with increased mechanical costs. Participants displayed a sequential effect for the majority of drawers in the pre-test, which was significantly reduced in the post-test. This finding indicates that each executed movement is a weighted function of both its cognitive and mechanical costs. The result also implies that sequential effects do not result solely from dynamical properties of the motor system, but instead reflect computational features of the movement selection process.

Motor primitives of pointing movements in a three-dimensional workspace.

A central question of motor control is how the motor system deals with redundant degrees of freedom. Redundancy can be reduced by coupling multiple degrees of freedom into a single motor primitive. Previous studies measuring motor primitives in aimed limb movements were restricted to two-dimensional target planes. We asked whether a limited number of motor primitives would also be sufficient to capture the posture variance of aimed limb movements in a three-dimensional target volume. To this end, participants had to point towards virtual targets uniformly spaced in a three-dimensional workspace. Results showed that three motor primitives captured 89.7 ± 2.8 % of the data variance of unrestrained pointing movements. Each motor primitive corresponded to a valid movement of the arm. The findings imply that complex postures in a three-dimensional target volume can be reduced to three motor primitives. The reduction results in a unique mapping of target position and posture and, thus, solves the redundancy problem. The reduction further indicates that, in a pointing task, the motor system does not control hand rotation independent of hand translation.

Prospective and retrospective effects in a virtual pointing task.

Over two decades ago prospective and retrospective effects of posture selection in a sequential task were described for the first time. Since then, both effects have been reproduced in a number of reaching studies. We asked (1) whether retrospective effects would also be found in a sequential pointing task and (2) whether pro/retrospective effects of posture selection would transfer to the end-effector position in the absence of haptic feedback. To this end, we created a sequential, perceptual-motor task in a virtual environment. Participants had to point to a row of targets in the frontal plane in sequential order. In a control experiment, physical targets were placed at the same locations. Results showed that kinematic parameters were similar in the virtual and real environment. Retrospective effects of posture/position were found in neither environment, indicating that pointing movements require lower cognitive planning costs than reaching movements. Prospective effects of posture were found both in the virtual and real environment. Prospective effects of position, on the other hand, were present in the virtual but not in the real environment, indicating that the absence of haptic feedback may result in unconscious shifts of the end-effector position.

Individual differences in motor planning during a multi-segment object manipulation task.

Research has demonstrated that people will adopt initially awkward grasps if they afford more comfortable postures at the end of the movement. This end-state comfort effect provides evidence that humans represent future posture states and select appropriate grasps in anticipation of these postures. The purpose of the study was to examine to what extent the final action goal of a task influences motor planning of preceding segments, and whether grasp postures are planned to optimize end-state comfort during a three-segment action sequence in which two objects are manipulated, and participants can select from a continuous range of possible grasp postures. In the current experiment, participants opened a drawer, grasped an object from inside the drawer, and placed it on a table in one of the three target orientations (0°, 90°, or 180° object rotation required). Grasp postures during the initial movement segment (drawer opening) were not influenced by the final action goal (i.e., required target orientation). In contrast, both the intermediate (i.e., object grasping) and the final movement segment (i.e., object placing) were influenced by target orientation. In addition, participants adopted different strategies to achieve the action goal when the object required 180° rotation, with 42 % of participants prioritizing intermediate-state comfort and 58 % prioritizing end-state comfort. The results indicate that individuals optimize task performance by selecting lower level constraints that allow for successful completion of the action goal and that the selection of these constraints is dependent upon contextual, environmental, and internal influences.

Effects of stimulus cueing on bimanual grasp posture planning.

The purpose of the present study was to investigate whether difficulties in bimanual grasp posture planning arise from conflicts in response selection. Forty-five participants were assigned to one of three groups (symbolic cueing, semi-symbolic cueing, and direct cueing) and instructed to reach for, grasp, and place two objects on a board in various end-orientations, depending on condition. In general, the tendency to adopt initial grasps that resulted in end-state comfort was significantly higher for the semi-symbolic, than that for the other two groups. There were, however, noticeable individual differences in grip behavior in the symbolic and direct cueing groups. Although the majority of participants performed the task in a similar fashion to the semi-symbolic group, there was a subset of participants (40% in each group) who grasped the two objects using an overhand grip in virtually all trials, regardless of condition. It is hypothesized that the observed individual differences in grasp posture strategy arise from differences in motor planning abilities, or the strategies participants employ in order to comply with task demands. A secondary finding is that the degree of interlimb coupling was larger for congruent, than incongruent, conditions irrespective of stimulus cueing. This finding indicates that the interference in the execution of bimanual grasping and placing tasks arises from interference during the specification of movement parameters specific to planning and execution of bimanual movements, or neuronal cross-talk in efferent pathways, rather than response selection conflicts.

Active tactile exploration for adaptive locomotion in the stick insect.

Insects carry a pair of actively movable feelers that supply the animal with a range of multimodal information. The antennae of the stick insect Carausius morosus are straight and of nearly the same length as the legs, making them ideal probes for near-range exploration. Indeed, stick insects, like many other insects, use antennal contact information for the adaptive control of locomotion, for example, in climbing. Moreover, the active exploratory movement pattern of the antennae is context-dependent. The first objective of the present study is to reveal the significance of antennal contact information for the efficient initiation of climbing. This is done by means of kinematic analysis of freely walking animals as they undergo a tactually elicited transition from walking to climbing. The main findings are that fast, tactually elicited re-targeting movements may occur during an ongoing swing movement, and that the height of the last antennal contact prior to leg contact largely predicts the height of the first leg contact. The second objective is to understand the context-dependent adaptation of the antennal movement pattern in response to tactile contact. We show that the cycle frequency of both antennal joints increases after obstacle contact. Furthermore, inter-joint coupling switches distinctly upon tactile contact, revealing a simple mechanism for context-dependent adaptation.

Motor control strategies in a continuous task space.

Previous studies on sequential effects of human grasping behavior were restricted to binary grasp type selection. We asked whether two established motor control strategies, the end-state comfort effect and the hysteresis effect, would hold for sequential motor tasks with continuous solutions. To this end, participants were tested in a sequential (predictable) and a randomized (nonpredictable) perceptual-motor task, which offered a continuous range of posture solutions for each movement trial. Both the end-state comfort effect and the hysteresis effect were reproduced under predictable, continuous conditions, but only the end-state comfort effect was present under nonpredictable conditions. Experimental results further revealed a work range restriction effect, which was reproduced for the dominant and the nondominant hand.