Minimal impact of proprioceptive loss on implicit sensorimotor adaptation and perceived movement outcome.

Implicit sensorimotor adaptation keeps our movements well-calibrated amid changes in the body and environment. We have recently postulated that implicit adaptation is driven by a perceptual error: the difference between the desired and perceived movement outcome. According to this perceptual re-alignment model, implicit adaptation ceases when the perceived movement outcome - a multimodal percept determined by a prior belief conveying the intended action, the motor command, and feedback from proprioception and vision - is aligned with the desired movement outcome. Here, we examined the role of proprioception in implicit motor adaptation and perceived movement outcome by examining individuals who lack proprioception. We used a modified visuomotor rotation task designed to isolate implicit adaptation and probe perceived outcome throughout the experiment. Surprisingly, implicit adaptation and perceived outcome were minimally impacted by deafferentation, posing a challenge to the perceptual re-alignment model of implicit adaptation.

Spatial Response Discrimination May Elicit a Simon Effect on a Non-Complementary Task.

When paired participants are each assigned a complementary half of the Simon task, a joint Simon effect (JSE) has been observed. Co-representation, a cognitive representation of not only one's own task but also that of the co-actor, has been one of several proposed mechanisms in the JSE. Using the response-discrimination hypothesis as a framework, we tested whether it was sufficient to highlight alternative task keys in a two-person setting in which a non-complementary task was completed to elicit a Simon effect (SE). In our design, the participant's role was to perform the Go/No-Go Simon task and the co-actor's role was to initiate each trial for the participant. In one two-person setting participant group (SK group), the same task key was assigned to both the participant and the co-actor; another group (OK) was assigned spatially opposite task keys. In a third group (joint setting, TS group), the standard joint Simon task was also completed to verify that a JSE could be replicated. We hypothesized that an SE would be elicited in the OK group, since opposite task keys would uniquely promote spatial coding. We found a weak but marginally significant SE in the OK group but not in the SK group. These results suggest that, on a non-complementary task, response discrimination may contribute to the emergence of a SE in a two-person setting, while it does not have the same impact as a complementary task completed in a joint setting (TS group) that may afford more robust response representations that reveal the enhanced so-called JSE.

The effects of periodic and noisy tendon vibration on a kinesthetic targeting task.

Tendon vibration is used extensively to assess the role of peripheral mechanoreceptors in motor control, specifically, the muscle spindles. Periodic tendon vibration is known to activate muscle spindles and induce a kinesthetic illusion that the vibrated muscle is longer than it actually is. Noisy tendon vibration has been used to assess the frequency characteristics of proprioceptive reflex pathways during standing; however, it is unknown if it induces the same kinesthetic illusions as periodic vibration. The purpose of the current study was to assess the effects of both periodic and noisy tendon vibration in a kinesthetic targeting task. Participants (N = 15) made wrist extension movements to a series of visual targets without vision of the limb, while their wrist flexors were either vibrated with periodic vibration (20, 40, 60, 80, and 100 Hz), or with noisy vibration which consisted of filtered white noise with power between ~ 20 and 100 Hz. Overall, our results indicate that both periodic and noisy vibration can induce robust targeting errors during a wrist targeting task. Specifically, the vibration resulted in an undershooting error when moving to the target. The findings from this study have important implications for the use of noisy tendon vibration to assess proprioceptive reflex pathways and should be considered when designing future studies using noisy vibration.

Effects of postural threat on perceptions of lower leg somatosensory stimuli during standing.

Height-induced postural threat affects emotional state and standing balance behaviour during static, voluntary, and dynamic tasks. Facing a threat to balance also affects sensory and cortical processes during balance tasks. As sensory and cognitive functions are crucial in forming perceptions of movement, balance-related changes during threatening conditions might be associated with changes in conscious perceptions. Therefore, the purpose of this study was to examine the changes and potential mechanisms underlying conscious perceptions of balance-relevant information during height-induced postural threat. A combination of three experimental procedures utilized height-induced postural threat to manipulate emotional state, balance behavior, and/or conscious perceptions of balance-related stimuli. Experiment 1 assessed conscious perception of foot position during stance. During continuous antero-posterior pseudorandom support surface rotations, perceived foot movement was larger while actual foot movement did not change in the High (3.2 m, at the edge) compared to Low (1.1 m, away from edge) height conditions. Experiment 2 and 3 assessed somatosensory perceptual thresholds during upright stance. Perceptual thresholds for ankle rotations were elevated while foot sole vibrations thresholds remained unchanged in the High compared to Low condition. This study furthers our understanding of the relationship between emotional state, sensory perception, and balance performance. While threat can influence the perceived amplitude of above threshold ankle rotations, there is a reduction in the sensitivity of an ankle rotation without any change to foot sole sensitivity. These results highlight the effect of postural threat on neurophysiological and cognitive components of balance control and provide insight into balance assessment and intervention.

The role of torque feedback in standing balance.

It has been proposed that sensory force/pressure cues are integrated within a positive feedback mechanism, which accounts for the slow dynamics of human standing behavior and helps align the body with gravity. However, experimental evidence of this mechanism remains scarce. This study tested predictions of a positive torque feedback mechanism for standing balance, specifically that differences between a "reference" torque and actual torque are self-amplified, causing the system to generate additional torque. Seventeen healthy young adults were positioned in an apparatus that permitted normal sway at the ankle until a brake on the apparatus was applied, discreetly "locking" body movement during stance. Once locked, a platform positioned under the apparatus remained in place (0 mm) or slowly translated backward (3 mm or 6 mm), tilting subjects forward. Postural behavior was characterized by two distinct responses: the center of pressure (COP) offset (i.e., change in COP elicited by the surface translation) and the COP drift (i.e., change in COP during the sustained tilt). Model simulations were performed using a linear balance control model containing torque feedback to provide a conceptual basis for the interpretation of experimental results. Holding the body in sustained tilt positions resulted in COP drifting behavior, reflecting attempts of the balance control system to restore an upright position through increases in plantar flexor torque. In line with predictions of positive torque feedback, larger COP offsets led to faster increases in COP over time. These findings provide experimental support for a positive torque feedback mechanism involved in the control of standing balance.NEW & NOTEWORTHY Using model simulations and a novel experimental approach, we tested behavioral predictions of a sensory torque feedback mechanism involved in the control of upright standing. Torque feedback is thought to reduce the effort required to stand and play a functional role in slowly aligning the body with gravity. Our results provide experimental evidence of a torque feedback mechanism and offer new and valuable insights into the sensorimotor control of human balance.

Cortical potentials time-locked to discrete postural events during quiet standing are facilitated during postural threat exposure.

During unperturbed bipedal standing, postural control is governed primarily by subcortical and spinal networks. However, it is unclear if cortical networks begin to play a greater role when stability is threatened. This study investigated how initial and repeated exposure to a height-related postural threat modulates cortical potentials time-locked to discrete centre of pressure (COP) events during standing. Twenty-seven young adults completed a series of 90-s standing trials at LOW (0.8 m above the ground, away from edge) and HIGH (3.2 m above the ground, at edge) threat conditions. Three LOW trials were completed before and after 15 consecutive HIGH trials. Participants stood on a force plate while electroencephalographic (EEG) activity was recorded. To examine changes in cortical activity in response to discrete postural events, prominent forward and backward peaks in the anterior-posterior COP time series were identified. EEG data were waveform-averaged to these events and the amplitude of event-related cortical activity was calculated. At the LOW condition, event-related potentials (ERPs) were scarcely detectable. However, once individuals stood at the HIGH condition, clear ERPs were observed, with more prominent potentials being observed for forward (edge-directed), compared to backward, COP events. Since forward COP peaks accelerate the centre of mass away from the platform edge, these results suggest there is intermittent recruitment of cortical networks that may be involved in the detection and minimization of postural sway toward a perceived threat. This altered cortical engagement appears resistant to habituation and may contribute to threat-related balance changes that persist following repeated threat exposure. KEY POINTS: While standing balance control is regulated primarily by subcortical and spinal processes, it is unclear if cortical networks play a greater role when stability is threatened. This study examined how cortical potentials time-locked to prominent peaks in the anterior-posterior centre of pressure (COP) time series were modulated by exposure to a height-related postural threat. While cortical potentials recorded over the primary sensorimotor cortices were scarcely detectable under non-threatening conditions, clear cortical potentials were observed when individuals stood under conditions of height-related threat. Cortical potentials were larger in response to COP peaks directed toward, compared to away from, the platform edge, and showed limited habituation with repeated threat exposure. Since forward COP peaks accelerate the centre of mass away from the platform edge, these findings suggest that when balance is threatened, there is intermittent recruitment of cortical networks, which may minimize the likelihood of falling in the direction of a perceived threat.

Tactile facilitation during actual and mere expectation of object reception.

During reaching and grasping movements tactile processing is typically suppressed. However, during a reception or catching task, the object can still be acquired but without suppressive processes related to movement execution. Rather, tactile information may be facilitated as the object approaches in anticipation of object contact and the utilization of tactile feedback. Therefore, the current study investigated tactile processing during a reception task. Participants sat with their upper limb still as an object travelled to and contacted their fingers. At different points along the object's trajectory and prior to contact, participants were asked to detect tactile stimuli delivered to their index finger. To understand if the expectation of object contact contributed to any modulation in tactile processing, the object stopped prematurely on 20% of trials. Compared to a pre-object movement baseline, relative perceptual thresholds were decreased throughout the object's trajectory, and even when the object stopped prematurely. Further, there was no evidence for modulation when the stimulus was presented shortly before object contact. The former results suggest that tactile processing is facilitated as an object approaches an individual's hand. As well, we purport that the expectation of tactile feedback drives this modulation. Finally, the latter results suggest that peripheral masking may have reduced/abolished any facilitation.

The effects of eccentric exercise-induced fatigue on position sense during goal-directed movement.

We investigated the impairment of position sense associated with muscle fatigue. In Experiment 1, participants performed learned eccentric extension (22°/s) movements of the elbow as the arm was pulled through the horizontal plane without vision of the arm. They opened their closed right hand when they judged it to be passing through a target. Dynamic position sense was assessed via accuracy of limb position to the target at the time of hand opening. Eccentric movements were performed against a flexion load [10% of flexion maximum voluntary contractions (MVCs)]. We investigated performance under conditions with and without biceps vibration, as well as before and after eccentric exercise. In Experiment 2, a motor was used to extend the participant's limb passively. We compared conditions with and without vibration of the lengthening but passive biceps, before and after exercise. In Experiment 1, vibration of the active biceps resulted in participants opening their hands earlier [mean, [Formula: see text] (95% confidence interval, CI) -5.52° (-7.40, -3.63)] compared with without vibration. Exercise reduced flexion MVCs by ∼44%, and participants undershot the target more [-5.51° (-9.31, -1.70)] in the post-exercise block during control trials. Exercise did not influence the persistence of the vibratory illusion. In Experiment 2, vibration resulted in greater undershooting [-2.99° (-3.99, -1.98)] compared with without vibration, before and after exercise. Although exercise reduced MVCs by ∼50%, the passive task showed no effects of exercise. We suggest that the central nervous system continues to rely on muscle spindles for limb position sense, even when they reside in a muscle exposed to fatiguing eccentric contractions.NEW & NOTEWORTHY Dynamic position errors were examined in an eccentric and a passive elbow extension proprioceptive-targeting task, before and after eccentric exercise, with and without muscle vibration. Participants actively undershot the target more when fatigued while fatigue did not exacerbate task accuracy during passive movement. Vibration caused undershoots regardless of fatigue state during active and passive movements. We propose that the central nervous system continues to rely on muscle spindles for kinesthesia, even when they reside in a fatigued muscle.

Facilitation and Habituation of Cortical and Subcortical Control of Standing Balance Following Repeated Exposure to a Height-related Postural Threat.

Threats to stability elicit context-specific changes in balance control; however, the underlying neural mechanisms are not fully understood. Previous work has speculated that a shift toward greater supraspinal control may contribute to threat-related balance changes. This study investigated how neural correlates of cortical and subcortical control of balance were affected by initial and repeated exposure to a height-related postural threat. Corticomuscular coherence (CMC) between EEG recorded over the sensorimotor cortex and EMG recorded from the soleus (SOL) provided an estimate of cortical control, while intermuscular coherence (IMC) between bilateral SOL provided estimates of both cortical and subcortical control. These outcomes, along with measures of psychological and arousal state and standing balance control, were examined in 28 healthy young adults during a series of 90-s quiet standing trials completed at LOW (0.8 m above ground; away from edge) and HIGH (3.2 m above ground, at edge) threat conditions. Initial exposure to threat significantly increased gamma-band CMC (31-40 Hz) and IMC at frequencies thought to be mediated by cortical (21-40 Hz) and subcortical (5-20 Hz) substrates. Following repeated threat exposure, only estimates of cortical control (gamma CMC and 21-40 Hz IMC) demonstrated significant habituation. Estimates of cortical control changed in parallel with high-frequency centre of pressure power (>0.5 Hz) and plantar-dorsiflexor coactivation, but not other threat-related balance changes which did not habituate. These results support the hypothesis that postural threat induces a shift toward more supraspinal control of balance, and suggests this altered neural control may contribute to specific threat-related balance changes.

Changes in Movement Control Processes Following Visuomotor Adaptation.

Goal-directed reaches are modified based on previous errors experienced (i.e., offline control) and current errors experienced during movement execution (i.e., online control). It is well documented that the control processes (i.e., offline and online control) underlying well learned movements change based on the time available to complete an action, such that offline control processes are engaged to a greater extent when movements are completed in a faster movement time (MT). Here, we asked if the underlying movement control processes governing newly acquired movements also change under varying MT constraints. Sixteen participants adapted their movements to a visuomotor distortion. Following reach training trials, participants reached under Long (800-1000 ms) and Short (400-500 ms) MT constraints. Results indicate that movement errors when reaching with the rotated cursor were reduced online under the Long MT constraint compared to the Short MT constraint. Thus, the contributions of offline and online movement control processes engaged in newly acquired movements can be adjusted with changes in temporal demands.

The acute effects of periodic and noisy tendon vibration on wrist muscle stretch responses.

Mechanical muscle tendon vibration activates multiple sensory receptors in the muscle and tendon. In particular, tendon vibration tends to activate the Ia afferents the strongest, but also will activate group II and Ib afferents. This activation can cause three main effects in the central nervous system: proprioceptive illusions, tonic vibration reflexes, and suppression of the stretch response. Noisy tendon vibration has been used to assess the frequency characteristics of proprioceptive reflexes and, interestingly there appeared to be no evidence for proprioceptive illusions or tonic vibration reflexes during standing [9]. However, it remains unknown if noisy vibration induces a suppression of the muscle stretch response. Therefore, the purpose of this study was to investigate the effects of noisy and periodic tendon vibration on the stretch response in the flexor carpi radialis muscle (FCR). We examined FCR stretch responses with and without periodic (20 and 100 Hz) and noisy (∼10-100 Hz) tendon vibration. We additionally had participants perform the task under the instruction set to either not respond to the perturbation or to respond as fast as possible. The key finding from this study was that both periodic and noisy vibration resulted in a reduced stretch response amplitude. Additionally, it was found that a participant's intent to respond did not modulate the amount of suppression observed. The findings from this study provide a more detailed understanding of the effects of tendon vibration on the muscle stretch response.

Learning to stand with unexpected sensorimotor delays.

Human standing balance relies on self-motion estimates that are used by the nervous system to detect unexpected movements and enable corrective responses and adaptations in control. These estimates must accommodate for inherent delays in sensory and motor pathways. Here, we used a robotic system to simulate human standing about the ankles in the anteroposterior direction and impose sensorimotor delays into the control of balance. Imposed delays destabilized standing, but through training, participants adapted and re-learned to balance with the delays. Before training, imposed delays attenuated vestibular contributions to balance and triggered perceptions of unexpected standing motion, suggesting increased uncertainty in the internal self-motion estimates. After training, vestibular contributions partially returned to baseline levels and larger delays were needed to evoke perceptions of unexpected standing motion. Through learning, the nervous system accommodates balance sensorimotor delays by causally linking whole-body sensory feedback (initially interpreted as imposed motion) to self-generated balance motor commands.

When standing, neurons in the brain send signals to skeletal muscles so we can adjust our movements to stay upright based on the requirements from the surrounding environment. The long nerves needed to connect our brain, muscles and sensors lead to considerable time delays (up to 160 milliseconds) between sensing the environment and the generation of balance-correcting motor signals. Such delays must be accounted for by the brain so it can adjust how it regulates balance and compensates for unexpected movements. Aging and neurological disorders can lead to lengthened neural delays, which may result in poorer balance. Computer modeling suggests that we cannot maintain upright balance if delays are longer than 300-340 milliseconds. Directly assessing the destabilizing effects of increased delays in human volunteers can reveal how capable the brain is at adapting to this neurological change. Using a custom-designed robotic balance simulator, Rasman et al. tested whether healthy volunteers could learn to balance with delays longer than the predicted 300-340 millisecond limit. In a series of experiments, 46 healthy participants stood on the balance simulator which recreates the physical sensations and neural signals for balancing upright based on a computer-driven virtual reality. This unique device enabled Rasman et al. to artificially impose delays by increasing the time between the generation of motor signals and resulting whole-body motion. The experiments showed that lengthening the delay between motor signals and whole-body motion destabilized upright standing, decreased sensory contributions to balance and led to perceptions of unexpected movements. Over five days of training on the robotic balance simulator, participants regained their ability to balance, which was accompanied by recovered sensory contributions and perceptions of expected standing, despite the imposed delays. When a subset of participants was tested three months later, they were still able to compensate for the increased delay. The experiments show that the human brain can learn to overcome delays up to 560 milliseconds in the control of balance. This discovery may have important implications for people who develop balance problems because of older age or neurologic diseases like multiple sclerosis. It is possible that robot-assisted training therapies, like the one in this study, could help people overcome their balance impairments.

Investigating information processing of the bimanual asymmetric cost with the response priming technique.

Constraining the degrees of freedom simplifies the coordinative challenge of bimanual asymmetric movements. This, however, comes at the cost of increased processing demands during movement preparation, referred to as the bimanual asymmetric cost. The goal of the present study was to further investigate information processing of the bimanual asymmetric cost with the response priming technique. This technique involved precuing a movement to encourage it to be preprogrammed. A different movement is occasionally cued by the go signal, which required the preprogrammed movement to be reprogrammed. In Experiment 1, 2 preprogrammed unimanual movements were reprogrammed, or integrated, into a bimanual movement. In Experiment 2, a preprogrammed bimanual movement was reprogrammed, or de-integrated, into a unimanual movement. Both experiments revealed 2 costs when integrating or de-integrating bimanual movements. One cost was likely related to aborting 1 movement and preparing another, which is the typical reprogramming cost found in response priming experiments. The second cost was likely related to constraining the degrees of freedom of bimanual asymmetric movements, which is a bimanual asymmetric cost. Integrating 2 unimanual movements into a bimanual asymmetric movement involves constraining the degrees of freedom, and de-integrating a bimanual asymmetric movement into a unimanual movement involves unconstraining the degrees of freedom. Both reprogramming and bimanual asymmetric costs occurred in 1 of the experimental conditions, and the interesting finding was that their effects were additive. Additive costs suggest that each cost affects a different stage of movement preparation. We suggest that the bimanual asymmetric cost occurs during response selection. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Attention and awareness: Representation of visuomotor space in split-brain patients.

Each cerebral hemisphere primarily controls and receives sensory input with regard to the contralateral hand. In the disconnected brain (split-brain), when the hands are uncrossed, direct visual access to each hand is available to the controlling (contralateral) hemisphere. However, when a hand crosses the midline, visual and tactile information regarding the hand are presented to different hemispheres. It is unknown how a contralateral hemisphere codes the position and orientation of a visually inaccessible hand in the disconnected brain. The present work addresses this issue. We ask how each hemisphere represents "its" hand across hand positions that span the midline in the absence of cortical input from the contralateral hemisphere. In other words, when a hand is placed across the midline and is visually inaccessible, is it represented by the controlling hemisphere: (1) in accordance with its new position with respect to the body (e.g., a left hand "becomes" a right effector when it crosses the midline), (2) with left/right position information unaltered (e.g., the left hand is represented as "left" regardless of its location), or (3) stripped of its location information altogether? The relationship between hand position and the spatial codes assigned to potential responses (an index of hand representation) was investigated in two split-brain patients using direct (Experiment 1) and orthogonal (Experiment 2) S-R compatibility paradigms. S-R compatibility effects in split-brain patients were consistent with those displayed by typical individuals. These findings suggest that position-based compatibility effects do not rely on cross-cortical connections. Rather, each hemisphere can accurately represent the full visuomotor space, a process that appears to be subserved by subcortical connections between the hemispheres.

Influence of kinesthetic motor imagery and effector specificity on the long-latency stretch response.

The long-latency "reflexive" response (LLR) following an upper limb mechanical perturbation is generated by neural circuitry shared with voluntary control. This feedback response supports many task-dependent behaviors and permits the expression of goal-directed corrections at latencies shorter than voluntary reaction time. An extensive body of literature has demonstrated that the LLR shows flexibility akin to voluntary control, but it has not yet been tested whether instruction-dependent LLR changes can also occur in the absence of an overt voluntary response. The present study used kinesthetic motor imagery (experiment 1) and instructed participants to execute movement with the unperturbed contralateral limb (experiment 2) to explore the relationship between the overt production of a voluntary response and LLR facilitation. Activity in stretched right wrist flexors were compared with standard "do not-intervene" and "compensate" conditions. Our findings revealed that on ~40% of imagery and ~50% of contralateral trials, a response occurred during the voluntary epoch in the stretched right wrist flexors. On these "leaked" trials, the early portion of the LLR (R2) was facilitated and displayed a similar increase to compensate trials. The latter half of the LLR (R3) showed further modulation, mirroring the patterns of voluntary epoch activity. By contrast, the LLR on "non-leaked" imagery and contralateral trials did not modulate. We suggest that even though a hastened voluntary response cannot account for all instruction-dependent LLR modulation, the overt execution of a response during the voluntary epoch in the same muscle(s) as the LLR is a prerequisite for instruction-dependent facilitation of this feedback response.NEW & NOTEWORTHY Using motor imagery and contralateral responses, we provide novel evidence that facilitation of the long-latency reflex (LLR) requires the execution of a response during the voluntary epoch. A high proportion of overt response "leaks" were found where the mentally simulated or mirrored response appeared in stretched muscle. The first half of the LLR was categorically sensitive to the appearance of leaks, whereas the latter half displayed characteristics closely resembling activity in the ensuing voluntary period.

The effect of response complexity on simple reaction time occurs even with a highly predictable imperative stimulus.

It is well known that increasing the complexity of the required response results in a corresponding increase in simple reaction time (RT). This "response complexity effect" has typically been attributed to increased time required to prepare some aspect of the response; however, most studies examining the response complexity effect have used an unpredictable foreperiod, which does not allow for optimal preparation to occur. Thus, it is conceivable that response complexity effects are influenced by an inability to predict the occurrence of the go-signal. In order to examine this possibility, participants (N = 36) were randomly assigned to one of four groups that differed in predictability of the go signal: 1) 2500-3500 ms random foreperiod; 2) 3000 ms constant foreperiod; 3) 1000 ms constant foreperiod; 4) 3000 ms constant foreperiod with a 1000 ms countdown timer. Participants performed one of three different key-press responses in a simple RT paradigm: 1) single key-press; 2) three key-presses with an equal/isochronous time interval between presses; 3) three key-presses with an unequal/non-isochronous time interval between presses. Results confirmed that while the countdown timer group had an overall reduced RT, response complexity effects were present and of similar magnitude for all groups in all testing blocks. This confirms that predictability of the go signal does not affect the response complexity effect.

Going offline: differences in the contributions of movement control processes when reaching in a typical versus novel environment.

Human movements are remarkably adaptive. We are capable of completing movements in a novel visuomotor environment with similar accuracy to those performed in a typical environment. In the current study, we examined if the control processes underlying movements under typical conditions were different from those underlying novel visuomotor conditions. 16 participants were divided into two groups, one receiving continuous visual feedback during all reaches (CF), and the other receiving terminal feedback regarding movement endpoint (TF). Participants trained in a virtual environment by completing 150 reaches to three targets when (1) a cursor accurately represented their hand motion (i.e., typical environment) and (2) a cursor was rotated 45° clockwise relative to their hand motion (i.e., novel environment). Analyses of within-trial measures across 150 reaching trials revealed that participants were able to demonstrate similar movement outcomes (i.e., movement time and angular errors) regardless of visual feedback or reaching environment by the end of reach training. Furthermore, a reduction in variability across several measures (i.e., reaction time, movement time, time after peak velocity, and jerk score) over time showed that participants improved the consistency of their movements in both reaching environments. However, participants took more time and were less consistent in the timing of initiating their movements when reaching in a novel environment compared to reaching in a typical environment, even at the end of training. As well, angular error variability at different proportions of the movement trajectory was consistently greater when reaching in a novel environment across trials and within a trial. Together, the results suggest a greater contribution of offline control processes and less effective online corrective processes when reaching in a novel environment compared to when reaching in a typical environment.

Postural Threat Modulates Perceptions of Balance-Related Movement During Support Surface Rotations.

Postural threat decreases center of pressure displacements yet increases the magnitude of movement-related conscious sway perception during quiet standing. It is unknown how these changes influence perception of whole body movement during dynamic stance. The aim of this study was to examine how postural threat influences whole-body movements and conscious perception of these movements during continuous pseudo-random support surface perturbations to stance. Sixteen healthy young adults stood on a moveable platform with their eyes closed for 7 min in a low threat (1.1 m above ground, away from edge) then high threat (3.2 m above ground, near edge) condition. Continuous pseudorandom roll platform rotations (± 4.5°, < 0.5 Hz) evoked large amplitude sway in the medio-lateral (ML) direction. Participants were asked to remain upright and avoid a fall at all times while tracking their ML body movements using a hand-held rotary encoder. Kinematic data was recorded using three markers placed on the upper trunk. Questionnaires assessed anxiety, fear and confidence. Electrodermal activity (EDA) was recorded as an indicator of arousal. Height-induced threat increased fear, anxiety and EDA, and decreased confidence. Trunk sway amplitude remained constant, while tracked movement amplitude increased at height. The gain for perceived to trunk movement was significantly increased at height across frequencies. Threat-related increases in sensitivity of sensory systems related to postural control and changes in cognitive and attention processes may lead to misperceptions of actual movement amplitudes, which may be important when examining increased fall risk in those with a fear of falling.

Whose turn is it anyway? The moderating role of response-execution certainty on the joint Simon effect.

When a two-choice "Simon task" is distributed between two people, performance in the shared go/no-go task resembles performance in the whole task alone. This finding has been described as the joint Simon effect (JSE). Unlike the individual go/no-go task, not only is the typical joint Simon task shared with another person, but also the imperative stimuli dictate whose turn it is to respond. Therefore, in the current study, we asked whether removing the agent discrimination component of the joint Simon task influences co-representation. Participants performed the typical joint Simon task, which was compared to two turn-taking versions of the task. For these turn-taking tasks, pairs predictably alternated turns on consecutive trials, with their respective imperative stimulus presented either on 100% of their turns (fully predictable group) or on 83% of their turns (response-uncertainty group, 17% no-go catch trials). The JSE was absent in the fully predictable, turn-taking task, but emerged similarly under the response-uncertainty condition and the typical joint Simon task condition where there is both turn and response-execution-related uncertainty. These results demonstrate that conflict related to agent discrimination is likely not a critical factor driving the JSE, whereas conflict surrounding the need to execute a response (and hence the degree of preparation) appears fundamental to co-representation.

Preparation of timing structure involves two independent sub-processes.

The current study examined the processes involved in the preparation of sequencing and timing initiation for multi-component responses. In two experiments, participants performed a reaction time (RT) task involving a three key-press sequence with either a simple (isochronous) or complex (non-isochronous) timing structure. Conditions involved a precue that provided information about all features of the movement (simple RT), no features of the movement (choice RT), sequencing only, or timing structure only. When sequencing was precued, RT decreased significantly as compared to choice RT, indicative of advance preparation of sequencing. When timing was precued, RT decreased significantly compared to choice RT when the timing structure was simple, suggesting that some aspect of timing preparation can occur prior to the go stimulus. However, even when the timing structure was known in advance, RT was still affected by timing complexity, confirming that some aspect of timing preparation cannot occur until after the onset of the stimulus and thus occurs during the RT interval. To explain these findings, we propose a two-component model of preparation for the timing initiation structure in which timing selection occurs in advance but timing implementation must occur following the go signal. These results support and extend previous findings regarding the independence of the processes associated with response sequencing and timing initiation.