Visuomotor control of walking in Parkinson's disease: Exploring possible links between conscious movement processing and freezing of gait.

INTRODUCTION: Changes in visual attention have been argued to influence freezing of gait (FOG) in people with Parkinson's Disease (PD). However, the specific visual search patterns of people with FOG pathology (PD + FOG) and potential underlying mechanisms are not well understood. The current study explored visual search behavior in PD + FOG while walking on a pathway featuring environmental features known to exacerbate FOG (e.g., narrow doorway and tripping hazards). Potential underpinning attentional mechanisms were also assessed, such as conscious movement processing. METHODS: Visual search behavior of twelve people with PD + FOG tested in ON-state (Mage = 74.3) and twelve age-matched healthy controls (Mage = 72.5) were analyzed during a complex walking task. The task required participants to step over an obstacle and navigate through a narrow doorway, surrounded by clutter. RESULTS: People with PD + FOG more frequently directed visual attention to ongoing and imminent steps compared to healthy controls (Mdn = 26% vs Mdn = 14%, respectively; p = 0.042). Self-reported conscious movement processing was also significantly higher in people with PD + FOG. The one participant who froze during the walking task fixated the future trip hazard (obstacle, approximately 6 steps ahead) almost exclusively during freezing trials (i.e., 60-100% of the trial). In contrast, during 'non-freeze' trials, this participant increased the duration of fixations towards ongoing and imminent steps. CONCLUSION: Results suggest that people with PD + FOG strongly monitor/control ongoing and immediately upcoming stepping movements. However, prolonged fixations towards threats to future movements might prevent people with PD + FOG from processing the visual information needed to do this, thereby provoke freezing episodes.

Is Implicit Motor Learning Preserved after Stroke? A Systematic Review with Meta-Analysis.

Many stroke patients experience difficulty with performing dual-tasks. A promising intervention to target this issue is implicit motor learning, as it should enhance patients' automaticity of movement. Yet, although it is often thought that implicit motor learning is preserved post-stroke, evidence for this claim has not been systematically analysed yet. Therefore, we systematically reviewed whether implicit motor learning is preserved post-stroke, and whether patients benefit more from implicit than from explicit motor learning. We comprehensively searched conventional (MEDLINE, Cochrane, Embase, PEDro, PsycINFO) and grey literature databases (BIOSIS, Web of Science, OpenGrey, British Library, trial registries) for relevant reports. Two independent reviewers screened reports, extracted data, and performed a risk of bias assessment. Overall, we included 20 out of the 2177 identified reports that allow for a succinct evaluation of implicit motor learning. Of these, only 1 study investigated learning on a relatively complex, whole-body (balance board) task. All 19 other studies concerned variants of the serial-reaction time paradigm, with most of these focusing on learning with the unaffected hand (N = 13) rather than the affected hand or both hands (both: N = 4). Four of the 20 studies compared explicit and implicit motor learning post-stroke. Meta-analyses suggest that patients with stroke can learn implicitly with their unaffected side (mean difference (MD) = 69 ms, 95% CI[45.1, 92.9], p < .00001), but not with their affected side (standardized MD = -.11, 95% CI[-.45, .25], p = .56). Finally, implicit motor learning seemed equally effective as explicit motor learning post-stroke (SMD = -.54, 95% CI[-1.37, .29], p = .20). However, overall, the high risk of bias, small samples, and limited clinical relevance of most studies make it impossible to draw reliable conclusions regarding the effect of implicit motor learning strategies post-stroke. High quality studies with larger samples are warranted to test implicit motor learning in clinically relevant contexts.

Focus of Attention in Children's Motor Learning: Examining the Role of Age and Working Memory.

The authors investigated the relative effectiveness of different attentional focus instructions on motor learning in primary school children. In addition, we explored whether the effect of attentional focus on motor learning was influenced by children's age and verbal working memory capacity. Novice 8-9-year old children (n = 30) and 11-12-year-old children (n = 30) practiced a golf putting task. For each age group, half the participants received instructions to focus (internally) on the swing of their arm, while the other half was instructed to focus (externally) on the swing of the club. Children's verbal working memory capacity was assessed with the Automated Working Memory Assessment. Consistent with many reports on adult's motor learning, children in the external groups demonstrated greater improvements in putting accuracy than children who practiced with an internal focus. This effect was similar across age groups. Verbal working memory capacity was not found to be predictive of motor learning, neither for children in the internal focus groups nor for children in the external focus groups. In conclusion, primary school children's motor learning is enhanced by external focus instructions compared to internal focus instructions. The purported modulatory roles of children's working memory, attentional capacity, or focus preferences require further investigation.

The inclination for conscious motor control after stroke: validating the Movement-Specific Reinvestment Scale for use in inpatient stroke patients.

PURPOSE: Stroke survivors are inclined to consciously control their movements, a phenomenon termed "reinvestment". Preliminary evidence suggests reinvestment to impair patients' motor recovery. To investigate this hypothesis, an instrument is needed that can reliably assess reinvestment post-stroke. Therefore, this study aimed to validate the Movement-Specific Reinvestment Scale (MSRS) within inpatient stroke patients. METHOD: One-hundred inpatient stroke patients (<1 year post-stroke) and 100 healthy peers completed the MSRS, which was translated to Dutch for the study purpose. To assess structural validity, confirmatory factor analysis determined whether the scale measures two latent constructs, as previously reported in healthy adults. Construct validity was determined by testing whether patients had higher reinvestment than controls. Reliability analyses entailed assessment of retest reliability (ICC), internal consistency (Cronbach's alpha), and minimal detectable change. RESULTS: Both structural and construct validity of the MSRS were supported. Retest reliability and internal consistency indices were acceptable to good. The minimal detectable change was adequate on group level, but considerable on individual level. CONCLUSIONS: The MSRS is a valid and reliable tool and suitable to assess the relationship between reinvestment and motor recovery in the first months post-stroke. Eventually, this may help therapists to individualize motor learning interventions based on patients' reinvestment preferences. IMPLICATIONS FOR REHABILITATION: This study showed that the Movement-Specific Reinvestment Scale (MSRS) is a valid and reliable tool to objectify stroke patients' inclination for conscious motor control. The MSRS may be used to identify stroke patients who are strongly inclined to consciously control their movements, as this disposition may hinder their motor recovery. Eventually, the MSRS may enable clinicians to tailor motor learning interventions to stroke patients' motor control preferences.

Stay Focused! The Effects of Internal and External Focus of Attention on Movement Automaticity in Patients with Stroke.

Dual-task performance is often impaired after stroke. This may be resolved by enhancing patients' automaticity of movement. This study sets out to test the constrained action hypothesis, which holds that automaticity of movement is enhanced by triggering an external focus (on movement effects), rather than an internal focus (on movement execution). Thirty-nine individuals with chronic, unilateral stroke performed a one-leg-stepping task with both legs in single- and dual-task conditions. Attentional focus was manipulated with instructions. Motor performance (movement speed), movement automaticity (fluency of movement), and dual-task performance (dual-task costs) were assessed. The effects of focus on movement speed, single- and dual-task movement fluency, and dual-task costs were analysed with generalized estimating equations. Results showed that, overall, single-task performance was unaffected by focus (p = .341). Regarding movement fluency, no main effects of focus were found in single- or dual-task conditions (p's ≥ .13). However, focus by leg interactions suggested that an external focus reduced movement fluency of the paretic leg compared to an internal focus (single-task conditions: p = .068; dual-task conditions: p = .084). An external focus also tended to result in inferior dual-task performance (ß = -2.38, p = .065). Finally, a near-significant interaction (ß = 2.36, p = .055) suggested that dual-task performance was more constrained by patients' attentional capacity in external focus conditions. We conclude that, compared to an internal focus, an external focus did not result in more automated movements in chronic stroke patients. Contrary to expectations, trends were found for enhanced automaticity with an internal focus. These findings might be due to patients' strong preference to use an internal focus in daily life. Future work needs to establish the more permanent effects of learning with different attentional foci on re-automating motor control after stroke.

External attentional focus enhances movement automatization: a comprehensive test of the constrained action hypothesis.

An external focus of attention has been shown to result in superior motor performance compared to an internal focus of attention. This study investigated whether this is due to enhanced levels of movement automatization, as predicted by the constrained action hypothesis (McNevin, Shea, & Wulf, 2003). Thirty healthy participants performed a cyclic one-leg extension-flexion task with both the dominant and non-dominant leg. Focus of attention was manipulated via instructions. The degree of automatization of movement was assessed by measuring dual task costs as well as movement execution parameters (i.e., EMG activity, movement fluency, and movement regularity). Results revealed that an external focus of attention led to significantly better motor performance (i.e., shorter movement duration) than an internal focus. Although dual task costs of the motor task did not differ as a function of attentional focus, cognitive dual task costs were significantly higher when attention was directed internally. An external focus of attention resulted in more fluent and more regular movement execution than an internal focus, whereas no differences were found concerning muscular activity. These results indicate that an external focus of attention results in more automatized movements than an internal focus and, therefore, provide support for the constrained action hypothesis.

Gaze behaviour during interception in children with spastic unilateral cerebral palsy.

Anticipatory gaze behaviour during interceptive movements was investigated in children with Spastic Unilateral Cerebral Palsy (SUCP), and related to the side of the intracerebral lesion. Five children with lesions of the right hemisphere (RHL) and five children with lesions of the left hemisphere (LHL) had to walk towards and intercept a ball that moved perpendicular to the walking path. Interception accuracy and gaze patterns were measured in a no-occlusion and occlusion condition, in which the ball was occluded from view for half of its trajectory. There was a clear support for a relationship between gaze behaviour and success in interception performance, with some evidence for the presence of anticipatory gaze behaviour. There were also differences in gaze behaviour between children with RHL and children with LHL that might be related to planning, but these did not affect interception accuracy. It is concluded that gaze behaviour during interceptive movements is anticipatory, and at least partly dependent on the lesional side.

On a multistable dynamic model of behavioral and perceptual infant development.

In this theoretical work, we treat behavioral and perceptual issues on an equal footing and examine the emergence of mutually exclusive behavioral patterns and perceptual variables during infant development from the perspective of multistable competitive dynamic systems. Accordingly, behavioral modes and modes of perception compete with each other for activation. One and only one mode survives the mode-mode competition, which accounts for the incompatibility of modes being considered. However, the winning behavioral or perceptual state is not predefined. Rather, we argue that during particular stages of maturation multiple modes coexist for the same set of developmental, body-scaled, and environmental parameters or constraints. The winning behavioral or perceptual state depends on these parameters as well as on initial conditions as operationalized in terms of previously performed behaviors or utilized perceptual stimuli. We give explicit examples of our approach and address the emergence of two-handed grasping and catching movements and the emergence of monocular and binocular vision during infant development. In particular, we propose that the emergence of midline crossing movements in 3- to 6-month-old infants involves two independent but interaction control parameters: a body-scaled and a developmental one. Likewise, we argue that the onset of binocularity in infants involves two independent but interaction control parameters: a developmental and an environmental one.

Catching optical information for the regulation of timing.

Recent research almost unambiguously refutes the hypothesis that the timing of interceptive actions is solely based on the relative rate of expansion [i.e. tau(psi)]. The aim of the present experiment was to evaluated the merits of eight alternative informational variables that recently have been proposed in the literature (i.e. psi, theta, delta, tau(psi), tau(theta), tau(delta), tau(psi, theta), zeta). Participants (n=7) were required to regulate the spatio-temporal characteristics of their reach and grasp to catch a ball approaching on a constant spatial trajectory. To identify the information used to regulate the timing of the catch we examined the qualitative effects of ball speed (0.5, 1.0, 1.5, 2.0, and 2.5 m/s) and viewing (monocular versus binocular) on the kinematics of the catch. Subsequently, we directly assessed the quantitative relationship between informational variables and the timing of reach onset and hand closure. The findings raised serious doubts against the use of variables that specified the time-to-contact between the ball and the point of observation (i.e. relative rate of expansion and disparity). Further, optical variables solely confined to the trajectory of the ball (i.e. the absolute rate of expansion) did yield positive results for the timing of reach onset but not for the timing of hand closure. Only variables that were related to the closure of the gap between hand and ball were found to contribute to the timing of hand closure. These results suggest that information related to the constriction of the optical gap between end-effector and ball becomes more important with approach, whereas the contribution of the absolute rate of expansion becomes less leading.

Safety in road crossing of children with cerebral palsy.

AIM: Children with cerebral palsy (CP) are regularly confronted with physical constraints during locomotion. Because abnormalities in motor control are often related to perceptual deficits, the aim of this study was to find out whether children with CP were able to walk across a road as safely as their non-handicapped peers. METHOD: Ten children with CP and 10 non-handicapped children aged 4-14 y were asked to cross a simulated road if they felt the situation was safe. RESULTS: With respect to safety and accuracy of crossings, the behaviour of children with CP was comparable with that of non-handicapped children. However, a closer examination of children's individual crossing behaviour showed considerable differences within the CP group. In contrast to children with damage to the left hemisphere, children with damage to the right hemisphere made unsafe decisions and did not compensate for them by increasing walking speed. CONCLUSION: The differences in unsafe behaviour and in the ability to compensate for it within the group of children with CP might be related to damage to specific regions of the brain that are involved in the processing of spatial or temporal information.

Foot-targeting in reaching and grasping.

Foot positioning was investigated when right-handed (Experiment One) and left handed adults (Experiment Two) stopped walking to grasp a stationary 70 mm ball at shoulder height. In both experiments centroid location formed by the toe and heel coordinates relative to the object was highly consistent within a target-location condition, demonstrating a foot-targeting phenomenon. Centroid location in the anterior-posterior direction was uninfluenced by grasping hand but the centroid shifted right for left hand grasps and left for right hand grasps. With the target either centrally located or on the same side as the dominant hand, foot positioning brought the grasping hand closer to the target in the medial-lateral direction. When the target object was aligned with the shoulder opposite the dominant hand both groups adopted foot positions to the left of the target. Thus, neither group adopted optimal foot position when the target was located opposite their dominant hand. Foot orientation angle relative to the target was also influenced by choice of grasping hand. Collectively, the findings demonstrate a close association between grasping hand and foot position when approaching to reach and grasp an object but also suggest that foot-dominance may influence medial-lateral centroid location.

The relation of unimanual and bimanual reaching to crossing the midline.

The present study assessed the development of reaching for objects positioned in front of the contralateral shoulder. In particular, it examined how the development of crossing the midline is related to the development of bimanual reaching. Twenty infants were observed longitudinally at 12, 18, and 26 weeks of age while reaching for two balls (3 cm and 8 cm in diameter) located at three positions (ispsilateral, midline, and contralateral). The reaches were analyzed from video recordings. With age, the infants increasingly adapted the number of hands used to the size of the object. The number of reaches crossing the body midline increased with age. Furthermore, the majority of the midline crossings were part of two-handed reaches for the large ball and occurred at or after onset of bimanual reaching. Together, these findings strongly suggest that the development of crossing the body midline emerges in the context of bimanual reaching. It was concluded that the need to grasp a large ball positioned contralaterally with two hands induces midline crossing. Hence, the development of midline crossings is not exclusively dependent on organismic constraints (e.g., the maturation of hemispheric connections), but rather on their interaction with environmental constraints (e.g., object size).

Discriminating the role of binocular information in the timing of a one-handed catch. The effects of telestereoscopic viewing and ball size.

Previous work using prediction-motion tasks has indicated that time-to-contact estimates are based exclusively on binocular information when the approaching object is small. The aim of this study was to examine the influence of object size on the use of binocular information sources in specifying time-to-contact in one-handed catching. Subjects (n = 10) were required to time their grasp to catch different-sized balls (60, 80, and 100 mm in diameter) approaching with a constant spatial trajectory and constant velocity of 2.0 m/s. Binocular information was manipulated with a telestereoscope that increased the individuals' effective interocular separation. Subjects performed six trials with each of the different-sized balls in normal and telestereoscopic viewing conditions. It was found that subjects closed the hand earlier when catching the small and medium balls under telestereoscopic viewing compared with normal viewing; furthermore, that subjects closed their hand earlier for the small and medium balls compared with the large ball under telestereoscopic viewing. With regard to hand aperture, there was an effect of ball size, with the hand being opened to a wider aperture for each increase in ball size. Subjects also opened the hand to a wider aperture when catching the small and medium balls under telestereoscopic viewing. These findings are consistent with the notion of an increased reliance on the use of binocular information when the approaching object is small. Furthermore, they indicate that binocular and monocular sources of information are not used in isolation, but rather in combination to support the timing of one-handed catching.

Timing a one-handed catch. I. Effects of telestereoscopic viewing.

The aim of this study was to examine the role of binocular and monocular information sources in specifying time-to-contact. More specifically, it was investigated whether the timing of the one-handed catch is consistent with a binocular tau-function strategy. Subjects (n=8) were required to time their grasp to catch a ball approaching with a constant spatial trajectory. The ball approached at three different constant velocities (1.5, 2.0 and 2.5 m/s). Vergence and disparity were manipulated through subjects wearing a telestereoscope to increase the effective interocular separation, under both binocular and monocular viewing. Subjects performed 24 trials in each of the four conditions. Subjects' started the opening of the hand earlier in the binocular telestereoscope condition when a ball approached with velocity of 1. 5 m/s. They then closed the hand earlier in the binocular telestereoscope condition at all ball approach velocities. There were no effects of telestereoscope on the timing of hand opening and closing under monocular viewing. This finding suggests the use of the binocular information in timing the grasp. However, there were effects of approach velocity under all conditions of monocular and binocular viewing. Subjects' closed the hand earlier as a function of increasing approach velocity. Together, the effects of the telestereoscope and approach velocity indicate that timing of the one-handed catch is not consistent with the use of a binocular "tau-function" variable. Rather, it is concluded that multiple sources of monocular and binocular information contribute to the regulation of timing.

Timing a one-handed catch. II. Adaptation to telestereoscopic viewing.

A pre-exposure, exposure, post-exposure design was used to assess the adaptation of the timing of a one-handed catch during telestereoscopic viewing. More specifically, it was examined whether the adaptation involved: (1) ignoring binocular sources of information and selecting other information, or (2) a recalibration of the coupling between the effected binocular information and the catching movement, and (3), if it is recalibration, whether it is restricted to the manipulated binocular information. To test these hypotheses, subjects (n=16) were assigned to one of two groups, each group performing three blocks of 15 trials in the dark with only the ball visible. In the exposure condition, both groups were required to catch balls under binocular telestereoscopic viewing. In the pre-exposure and post-exposure conditions, subjects performed under binocular and monocular viewing, respectively. Kinematics of the grasping movement were recorded. It was predicted that, in the case of a selection process, no after effects would occur in the post-exposure condition, whereas, in the case of recalibration, aftereffects would occur. Moreover, if the recalibration is restricted to the manipulated information, only the group that was provided with binocular vision during the pre- exposure and post-exposure conditions would show aftereffects. Significant condition (pre-exposure, exposure, post-exposure) by block (first three trials, last three trials) effects were found for the moments of grasp onset, peak opening velocity and hand closure, indicating that the hand was opened and closed earlier in the first three trials of telestereoscopic viewing. This coincided with an increase in catching failures. In addition, for the moments of hand closure and peak closing velocity, negative aftereffects were found in the post-exposure condition. The hand was closed later in the first three trials after removal of telestereoscope. With respect to the presence of the aftereffects, no differences were found between the groups. It was concluded that adaptation to telestereoscopic viewing in the timing of a one-handed catch is due to the recalibration of the coupling between information and movement, rather than a selection of another source of information. Moreover, it is likely that the recalibration was not restricted to the single, manipulated information. Rather, the recalibration involves multiple binocular and monocular optical and oculomotor sources of information.

The kinematics of eating with a spoon: bringing the food to the mouth, or the mouth to the food?

In traditional reach-to-point and reach-to-grasp studies an increase in accuracy demands typically results in a lengthening of the deceleration phase of the reach and a freezing of the more distal joints. The purpose of the present experiment was to examine whether similar changes in the reach kinematics could be observed during a tool-using skill, as would be predicted from an effector independence perspective. Five subjects were required to eat two substances (i.e. a solid and a liquid one) that imposed different requirements on the accuracy of the movement. The subjects transported the substances from the plate into the mouth. A prolonged movement duration was found for the liquid as compared to the solid substance. However, rather than being exclusively due to a lengthening of deceleration phase, the larger movement duration resulted from a slowing down of the whole movement. Therefore, the skewed velocity profiles found in the traditional reach-to-grasp studies may well be the result of the accuracy demands only impinging on the final part of the movement trajectory, rather than being a consequence of central, effector-independent, organising principles. In addition, under increased accuracy demands subjects were shown to redistribute their movement in a proximodistal direction. Movements of the distal components were reduced to a minimum and the involvement of trunk and head movement increased.