Rajal G Cohen

Physiological indices of cognitive processes have been the focus of considerable interest. They can be found in brain activity, facial expressions, microsaccades, and neck muscle activation. Recent work suggests that they also occur in fixational limb movements, which may include drift, tremor, and small quick movements. In the studies reported here, limb movement was measured while participants pointed to a central location and anticipated a target-directed movement. During the pauses before voluntary ...

Background
Postural deviations such as forward head posture (FHP) are associated with adverse health effects. The causes of these deviations are poorly understood. We hypothesized that anticipating target-directed movement could cause the head to get “ahead of” the body, interfering with optimal head/neck posture, and that the effect may be exacerbated by task difficulty and/or poor inhibitory control.

Method
We assessed posture in 45 healthy young adults standing quietly and when they anticipated walking to place a tray: in a simple condition and in conditions requiring that they bend low or balance an object on the tray. We defined FHP as neck angle relative to torso; we also measured head angle relative to neck and total neck length. We assessed inhibitory control using a Go/No-Go task, Stroop task, and Mindful Attention Awareness Scale (MAAS).

Results
FHP increased when participants anticipated movement, particularly for more difficult movements. Worse Stroop performance and lower MAAS scores correlated with higher FHP. False alarms on the Go/No-Go task correlated with a more extended head relative to the neck and with shortening of the neck when anticipating movement.

Conclusions
Maintaining neutral posture may require inhibition of an impulse to put the head forward of the body when anticipating target-directed movement.

Objectives: To determine feasibility and potential of Alexander technique (AT) group classes for chronic neck
pain and to assess changes in self-efficacy, posture, and neck muscle activity as potential mechanisms for pain reduction.
Design: A single-group, multiple-baseline design, with two pre-tests to control for regression toward the mean, a post-test immediately after the intervention, and another post-test five weeks later to examine retention of benefits. Participants were predominately middle-aged; all had experienced neck pain for at least six months. Intervention: Participants attended ten one-hour group classes in AT, an embodied mindful approach that may reduce habitual overactivation of muscles, including superficial neck muscles, over five weeks.
Outcome measures: (1) self-reports: Northwick Park Questionnaire (to assess neck pain and associated disability) and Pain Self-Efficacy Questionnaire; (2) superficial neck flexor activation and fatigue (assessed by electro- myography and power spectral analysis) during the cranio-cervical flexion test; (3) posture during a video game task.
Results: There were no significant changes in outcomes between pre-tests. All participants completed the in- tervention. After the intervention: (1) participants reported significantly reduced neck pain; (2) fatigue of the superficial neck flexors during the cranio-cervical flexion test was substantially lower; (3) posture was mar- ginally more upright, as compared to the second pre-intervention values. Changes in pain, self-efficacy, and neck muscle fatigue were retained at the second post-test and tended to be correlated with one another. Conclusions: Group AT classes may provide a cost-effective approach to reducing neck pain by teaching parti- cipants to decrease excessive habitual muscle contraction during everyday activity.

—Freezing of gait (FoG) in people with Parkinson's disease (PD) is an environmentally sensitive, intermittent problem that occurs most often during turning. FoG is difficult for clinicians to evaluate and treat because it can be difficult to elicit during a clinical visit. Here, we aimed to develop a clinically valid objective measure of freezing severity during a 2-min 360-degree turning-in-place. Twenty-eight subjects with PD (16 freezers, FoG+, and 12 nonfreezers, FoGÀ) in the ''off " state and 14 healthy control subjects were tested. Subjects wore three inertial sensors (one on each shin and one on the waist) while (1) turning in place for 2 min (alternating 360 degrees to the right with 360 degrees to the left) and (2) performing an Instrumented 7-m Timed Up and Go test (ITUG). Performance was videotaped, and clinical severity of FoG was independently rated by two movement disorders specialists (co-authors). Turning in place consistently resulted in FoG (13 out of 16 subjects with PD) while FoG was clinically observed in only two subjects with PD during the ITUG test. The Freezing Ratio during the turning test was significantly correlated with the clinical ratings (q = 0.7, p = 0.003) and with score on the new FoG questionnaire (q = 0.5, p = 0.03). After correcting for symptom severity (UPDRS-III), out of the four objective measures of the turning test (total number of turns, average turn peak speed and average turn smoothness), only the Freezing Ratio was significantly different across groups (p = 0.04). Freezing can be well quantified with body-worn inertial sensors during a 2-min turning-in-place protocol.

Background: Freezing of gait in Parkinson's disease (PD) has been linked with deficits in inhibitory control, but causal mechanisms are not established. Freezing at gait initiation (start hesitation) is often accompanied by multiple anticipatory postural adjustments (APAs). If inhibition deficits contribute to freezing by interfering with ability to inhibit initial weight shifts in the wrong direction, then PD subjects should experience more episodes of multiple APAs than healthy controls (HCs) do. If inhibition deficits contribute to freezing by interfering with ability to release a previously inhibited step following multiple APAs, then step onset following multiple APAs should be delayed more in people with PD than in HCs. Methods: Older adults with PD and HC subjects rapidly initiated stepping in response to a light cue in blocks of simple (SRT) and choice (CRT) conditions. We recorded kinematics and ground reaction forces, and we administered the Stroop task to assess inhibitory control. Results: Multiple APAs were more common in CRT than SRT conditions but were equally common in HC and PD subjects. Step onsets were delayed in both conditions and further delayed in trials with multiple APAs, except for HC subjects in SRT trials. Poor Stroop performance correlated with many multiple APAs, late step onset, and rearward position of center of mass (COM) at cue presentation. Forward motion of the COM during the APA was higher in trials with multiple APAs than in trials with single APAs, especially in CRT trials and in PD subjects without self-reported freezing. Conclusion: Start hesitation is not caused by multiple APAs per se, but may be associated with difficulty recovering from multiple APAs, due to difficulty releasing a previously inhibited step.

Knowledge of brain correlates of postural control is limited by the technical difficulties in performing controlled experiments with currently available neuroimaging methods. Here we present a system that allows the measurement of anticipatory postural adjustment of human legs to be synchronized with the acquisition of functional magnetic resonance imaging data. The device is composed of Magnetic Resonance Imaging (MRI) compatible force sensors able to measure the level of force applied by both feet. We tested the device in a group of healthy young subjects and a group of elderly subjects with Parkinson's disease using an event-related functional MRI (fMRI) experiment design. In both groups the postural behavior inside the magnetic resonance was correlated to the behavior during gait initiation outside the scanner. The system did not produce noticeable imaging artifacts in the data. Healthy young people showed brain activation patterns coherent with movement planning. Parkinson's disease patients demonstrated an altered pattern of activation within the motor circuitry. We concluded that this force measurement system is able to index both normal and abnormal preparation for gait initiation within an fMRI experiment. Any successful voluntary movement requires that we counteract the perturbations caused by the self-induced movement. This aspect of postural control, called anticipatory postural adjustment (APA), is automatically triggered before the self-induced movement, and produces forces opposite to the biomechanical effects of the body perturbation. For instance, when we want to start walking, a combination of forces is required to move the center of mass forward. First, body weight is transferred to the support leg, followed by the withdrawal of the other heel from the floor. APAs have been thoroughly investigated in behavioral studies, and these have informed us about differences between normal and impaired gait initiation 1,2. However, understanding how this seemingly effortless coordinated movement pattern is generated by the healthy and diseased nervous system has been a challenge to conventional neuroimaging. Most studies assessing brain function related to APAs have used manual tasks 3,4 , speech articulation 5 , and ankle dorsiflexion and plantiflexion 6. Results of these studies consistently indicate a role for the supplementary motor area (SMA) in the preparation and organization of voluntary movements. It is possible that SMA also contributes to APAs during gait initiation 7. However, the involvement of SMA and other brain structures during gait initiation in humans is still not clear. Investigating gait initiation is important, since

Objectives: Aging is associated with cognitive decline, including visuomotor and memory concerns, and with motor system changes, including gait slowing and stooped posture. We investigated the associations of visuomotor performance and episodic memory with motor system characteristics in healthy older adults. Methods: Neurologically healthy older adults (N = 160, aged 50–89) completed a battery of cognitive and motor tasks. Cognitive variables were grouped by principal components analysis (PCA) into two components: visuomotor performance and verbal episodic memory. Our primary predictor variables were two aspects of motor function: timed-up-and-go (TUG) speed and neck angle. Additional predictor variables included demographic factors (age, sex and education) and indicators of physical fitness (body mass index/BMI and grip strength). All seven predictor variables were entered stepwise into a multiple regression model for each cognitive component. Results: Poor visuomotor performance was best predicted by a combination of advanced age, high BMI and slow TUG, whereas poor verbal memory performance was best predicted by a combination of advanced age, male sex, low education and acute neck angle. Conclusions: Upright posture and mobility were associated with different cognitive processes, suggesting different underlying neural mechanisms. These results provide the first evidence for a link between postural alignment and cognitive functioning in healthy older adults. Possible causal relationships are discussed.

Freezing of gait (FoG) is a transient inability to initiate or maintain stepping that often accompanies advanced Parkinson's disease (PD) and significantly impairs mobility. The current study uses a multimodal neuroimaging approach to assess differences in the functional and structural locomotor neural network in PD patients with and without FoG and relates these findings to measures of FoG severity. Twenty-six PD patients and fifteen age-matched controls underwent resting-state functional magnetic resonance imaging and diffusion tensor imaging along with self-reported and clinical assessments of FoG. After stringent movement correction, fifteen PD patients and fourteen control participants were available for analysis. We assessed functional connectivity strength between the supplementary motor area (SMA) and the following locomotor hubs: 1) subthalamic nucleus (STN), 2) mesencephalic and 3) cerebellar locomotor region (MLR and CLR, respectively) within each hemisphere. Additionally, we quantified structural connectivity strength between locomotor hubs and assessed relationships with metrics of FoG. FoG+ patients showed greater functional connectivity between the SMA and bilateral MLR and between the SMA and left CLR compared to both FoG- and controls. Importantly, greater functional connectivity between the SMA and MLR was positively correlated with i) clinical, ii) self-reported and iii) objective ratings of freezing severity in FoG+, potentially reflecting a maladaptive neural compensation. The current findings demonstrate a re-organization of functional communication within the locomotor network in FoG+ patients whereby the higher-order motor cortex (SMA) responsible for gait initiation communicates with the MLR and CLR to a greater extent than in FoG- patients and controls. The observed pattern of altered connectivity in FoG+ may indicate a failed attempt by the CNS to compensate for the loss of connectivity between the STN and SMA and may reflect a loss of lower-order, automatic control of gait by the basal ganglia.

Freezing of gait is one of the most debilitating symptoms in Parkinson’s disease as it causes falls and reduces mobility and quality of life. The pedunculopontine nucleus is one of the major nuclei of the mesencephalic locomotor region and has neurons related to anticipatory postural adjustments preceding step initiation as well as to the step itself, thus it may be critical for coupling posture and gait to avoid freezing. Because freezing of gait and postural impairments have been related to frontal lesions and frontal dysfunction such as executive function, we hypothesized that freezing is associated with disrupted connectivity between midbrain locomotor regions and medial frontal cortex. We used diffusion tensor imaging to quantify structural connectivity of the pedunculopontine nucleus in patients with Parkinson’s disease with freezing of gait, without freezing, and healthy age-matched controls. We also included behavioral tasks to gauge severity of freezing of gait, quantify gait metrics, and assess executive cognitive functions to determine whether between-group differences in executive dysfunction were related to pedunculopontine nucleus structural network connectivity. Using seed regions from the pedunculopontine nucleus, we were able to delineate white matter connections between the spinal cord, cerebellum, pedunculopontine nucleus, subcortical and frontal/prefrontal cortical regions. The current study is the first to demonstrate differences in structural connectivity of the identified locomotor pathway in patients with freezing of gait. We report reduced connectivity of the pedunculopontine nucleus with the cerebellum, thalamus and multiple regions of the frontal cortex. Moreover, these structural differences were observed solely in the right hemisphere of patients with freezing of gait. Finally, we show that the more left hemisphere-lateralized the pedunculopontine nucleus tract volume, the poorer the performance on cognitive tasks requiring the initiation of appropriate actions and/or the inhibition of inappropriate actions, specifically within patients with freezing. These results support the notion that freezing of gait is strongly related to structural deficits in the right hemisphere’s locomotor network involving prefrontal cortical areas involved in executive inhibition function.

The present study investigated whether postural responses are influenced by the stability constraint of a voluntary, manual task. We also examined how task constraint and first experience (the condition with which the participants started the experiment) influence the kinematic strategies used to simultaneously accomplish a postural response and a voluntary task. Twelve healthy, older adults were perturbed during standing, while holding a tray with a cylinder placed with the flat side down (low constraint, LC) or with the rolling, round side down (high constraint, HC). Central set changed according to the task constraint, as shown by a higher magnitude of both the gastrocnemius and tibialis anterior muscle activation bursts in the HC than in the LC condition. This increase in muscle activation was not reflected, however, in changes in the center of pressure or center of mass displacement. Task constraint influenced the peak shoulder flexion for the voluntary tray task but not the peak hip flexion for the postural task. In contrast, first experience influenced the peak hip flexion but not the peak shoulder flexion. These results suggest an interaction between two separate control mechanisms for automatic postural responses and voluntary stabilization tasks.

This study assessed the effects of stability constraints of a voluntary task on postural responses to an external perturbation in subjects with Parkinson's disease (PD) and healthy elderly participants. Eleven subjects with PD and 12 control subjects were perturbed with backward surface translations while standing and performing two versions of a voluntary task: holding a tray with a cylinder placed with the flat side down (low constraint - LC) or with the rolling, round side down (high constraint - HC). Participants performed alternating blocks of low and high constraint trials. Parkinson's disease participants accomplished the voluntary task as well as controls, showing slower tray velocity in the high, compared with the low, constraint condition. However, the latency of postural responses was longer in the high constraint condition only for control subjects. Control subjects presented different patterns of hip-shoulder coordination as a function of task constraint whereas PD subjects had a relatively invariant pattern. Initiating the experiment with the high constraint task led to (a) decreased postural stability in PD subjects only, and (b) reduced peak hip flexion in control subjects only. These results suggest that Parkinson's disease impairs the capacity to adapt postural responses to constraints imposed by a voluntary task.

Timing is central to many coordinated actions, and the temporal accuracy of CNS commands presents an important limit to skilled performance. Using target-oriented throwing in a virtual environment as an example task, this study presents a novel analysis that quantifies contributions of timing accuracy and shaping of hand trajectories to performance. Task analysis reveals that the result of a throw is fully determined by the projectile’s position and velocity at release; zero error can be achieved by a manifold of position and velocity combinations (solution manifold).

Four predictions were tested: (1) Performers learn to release the projectile closer to the optimal moment for a given arm trajectory, achieving timing accuracy levels similar to those reported in other timing tasks (9 ms). (2) Performers develop a hand trajectory that follows the solution manifold such that zero error can be achieved without perfect timing. (3) Skilled performers exploit both routes to improvement more than unskilled performers. (4) Long-term improvement in skilled performance relies on continued optimization of the arm trajectory as timing limits are reached.

Average and skilled subjects practiced for 6 and 15 days, respectively. In 6 days, both timing and trajectory alignment improved for all subjects, and skilled subjects showed an advantage in timing. With extended practice, performance continued to improve due to continued shaping of the trajectory, while timing accuracy reached an asymptote at 9 ms.

We conclude that skilled subjects first maximize timing accuracy, then optimize trajectory shaping to compensate for intrinsic limitations of timing accuracy.

"Background
Many patients with Parkinson's disease (PD) develop freezing of gait (FoG), which may manifest as a hesitation or “getting stuck” when they attempt to pass through a doorway. In two experiments, we asked whether FoG is associated with (1) a deficit in internal representation of one's body size with respect to a doorway and (2) a mismatch between imagined and actual walking times when passing through a doorway.

Methods
23 subjects with PD (11 with and 13 without FoG) and 10 control subjects of similar age completed two experiments. In the Passability experiment, subjects judged the passability of doorways with different apertures scaled to their body widths. We compared passability estimates across groups. In the Imagery experiment, subjects timed themselves while: (1) imagining walking through doorways of different apertures and from different distances and (2) actually walking in the same conditions they had just imagined. We compared imagined and actual walking durations across groups and conditions.

Results
In the Passability experiment, the estimated just-passable doorway was wider, relative to body width, in PD subjects than in control subjects, but there was no difference between PD subjects with and without FoG. In the Imagery experiment, subjects in all groups walked more slowly through narrow doorways than though wide doorways, and subjects with FoG walked much more slowly through the narrowest doorways. PD subjects with FoG showed a large discrepancy between actual and imagined time to pass through narrow doorways, unlike PD subjects without FoG and control subjects.

Conclusions
The equivalent passability judgments in PD subjects with and without FoG indicate that FoG is not specifically associated with a deficit in ability to internally represent space with reference to body size. However, the large difference in duration between actual and imagined walking through narrow doorways in subjects with FoG suggests that PD subjects with FoG did not know how much they would slow down to pass through narrow doorways. The observed discrepancy between imagined and actual walking times may point to a specific problem that contributes to the occurrence of FoG. These results also suggest that caution should be used when interpreting brain imaging results from locomotor imagery studies with PD subjects who have FoG."

People pick up objects in ways that reflect prospective as well as retrospective control. Prospective control is indicated by planning for end-state comfort such that people grasp a cylinder to be rotated or translated with a hand orientation or at a height that affords a comfortable final posture. Retrospective control is indicated when people reuse a remembered grasp rather than using a new grasp that would ensure end-state comfort. Here, we asked whether these manifestations of prospective and retrospective control co-occur. We did so by having healthy young-adult participants grasp a cylinder to rotate and translate it between a horizontal position and a vertical position at each of five heights. We found that participants planned for comfortable final hand orientations for first moves but relied on recall for subsequent hand orientations. The results suggest that motor planning is sensitive to computational as well as physical demands and that object rotation and translation are not dissociable features of motor control, at least as reflected in their contributions to grasp selection. The latter result is consistent with the hypothesis that movements constitute holistic body changes between successive goal postures.

Background. This study asked whether older subjects were more likely than younger subjects to err in the initial direction of their anticipatory postural adjustment (APA) prior to a step (indicating a motor program error), whether initial motor program errors accounted for reaction time differences for step initiation, and whether initial motor program errors were linked to inhibitory failure.

Methods. In a stepping task with choice reaction time (CRT) and simple reaction time (SRT) conditions, we measured forces under the feet to quantify APA onset and step latency, and we used body kinematics to quantify forward movement of center of mass and length of first step.

Results. Trials with APA errors were almost three times as common for older adults as for younger adults, and they were nine times more likely in CRT trials than in SRT trials. In trials with APA errors, step latency was delayed, correlation between APA onset and step latency was diminished, and forward motion of the center of mass prior to the step was increased. Subjects with more APA errors tended to have worse Stroop interference scores, regardless of age.

Conclusions. The results support the hypothesis that findings of slow CRT step initiation in older subjects are attributable to inclusion of trials with incorrect initial motor preparation, and that these errors are caused by deficits in response inhibition. By extension, the results also suggest that mixing of trials with correct and incorrect initial motor preparation might explain apparent CRT slowing with age in upper limb tasks.

Variability in motor performance decreases with practice but is never entirely eliminated, due in part to inherent motor noise. The present study develops a method that quantifies how performers can shape their performance to minimize the effects of motor noise on the result of the movement. Adopting a statistical approach on sets of data, the method quantifies three components of variability (tolerance, noise, and covariation) as costs with respect to optimal performance. T-Cost quantifies how much the result could be improved if the location of the data were optimal, N-Cost compares actual results to results with optimal dispersion at the same location, and C-Cost represents how much improvement stands to be gained if the data covaried optimally. The TNC-Cost analysis is applied to examine the learning of a throwing task that participants practiced for 6 or 15 days. Using a virtual set-up, 15 participants threw a pendular projectile in a simulated concentric force field to hit a target. Two variables, angle and velocity at release, fully determined the projectile’s trajectory and thereby the accuracy of the throw. The task is redundant and the successful solutions define a nonlinear manifold. Analysis of experimental results indicated that all three components were present and that all three decreased across practice. Changes in T-Cost were considerable at the beginning of practice; C-Cost and N-Cost diminished more slowly, with N-Cost remaining the highest. These results showed that performance variability can be reduced by three routes: by tuning tolerance, covariation and noise in execution. We speculate that by exploiting T-Cost and C-Cost, participants minimize the effects of inevitable intrinsic noise.

Moving around obstacles requires balancing the need to avoid collisions with the need to minimize biomechanical costs. We investigated this tradeoff by studying the effects of visual uncertainty, motor noise, and practice on clearance over obstacles in a manual positioning task. Participants moved a manipulandum back and forth over a stationary obstacle. We varied visual uncertainty by placing the obstacle at different heights relative to participants’ eyes, and we varied motor noise by having participants hold the object to be moved at different positions relative to the range of motion of the arm joints. Clearance was larger in conditions of higher visual uncertainty than in conditions of lower visual uncertainty, larger in the higher motor noise conditions than in the lower motor noise conditions, and larger early in practice than late in practice. The results indicate that spatial accuracy and biomechanical costs are both taken into account during reaching over obstacles, but to differing degrees across practice.

Trial-to-trial fluctuations in discrete goal-directed tasks are always present, even in highly skilled and motivated performance. Due to such fluctuations humans are unable to perform identical movements in sequence. Yet, performance of identical actions in sequence is important in many situations including in games, such as bowling.

Taking a cue from recent discoveries of directional bias in microsaccades during visual fixation, we investigated directional bias in tremor during manual pointing. Subjects memorized and then performed patterns of alternating postures and voluntary movements. The directions of the tiny movements occurring during periods of intended stillness were predictive of subsequent targetdirected movements such that in the horizontal axis, relative to baseline, the frequency of tremor decreased and the amplitude of tremor increased before horizontal movements, but not before vertical movements. This effect was less pronounced in the finger than in the arm, forearm, and hand. Possible explanations of the effect are based on eyelimb coupling, decreasing stiffness in the axis of forthcoming movement, and release of inhibition. The discovery of directionally specific preparatory activity suggests that the simple task of holding still before moving may provide a new window into the processes that allow for the translation of intentions into actions.

Previous studies of object manipulation have suggested that when participants return an object to the place from which they just carried it, they tend to grasp the object for the target-back-to-home trips close to where they just grasped it for the home-to-target trips [Exp Brain Res 157(4):486–495, 2004; Psychon Bull Rev, 2006]. What was unclear from these previous studies was whether participants recalled postures or locations. According to the posture hypothesis, they remembered what body positions they adopted when they last held the object. According to the location hypothesis, they remembered where they held the
object and then took hold of it there or nearby again. To distinguish between these possibilities, we had participants mount or dismount a platform after home-to-target moves and before target-back-to-home moves. In the control condition, they did not change their vertical position relative to the shelf containing the home and target platforms (they merely stepped sideways). We found that participants grasped the object at nearly the same place along its length as they had before, even if this meant adopting very different postures than before. This outcome is consistent with the location-recall account and is inconsistent with the posture-recall account. The implications for motor planning are discussed.

Physiological indices of cognitive processes have been the focus of considerable interest. They can be found in brain activity, facial expressions, microsaccades, and neck muscle activation. Recent work suggests that they also occur in fixational limb movements, which may include drift, tremor, and small quick movements. In the studies reported here, limb movement was measured while participants pointed to a central location and anticipated a target-directed movement. During the pauses before voluntary movement, the amplitude of small "fixational" movement increased in the axis of forthcoming target-directed movement relative to the orthogonal axis, while frequency decreased in the axis of forthcoming target-directed movement relative to the orthogonal axis. These fixational movement biases may be online indicators of attention or some other cognitive process. Alternatively or additionally, they may reflect some functional aspect of motor preparation. This project investigates the phenomenon of fixational limb movement biases and seeks to elucidate whether, and in what manner, they reflect mental and physiological aspects of readiness.

Despite a great deal of research on the time to move the hand to targets of varying distances and widths, it is unclear whether target distance and width are both represented in movement plans prior to movement initiation. We addressed this question by studying performance in an object manipulation task. Our participants reached out and took hold of a familiar object (a bath-room plunger), to move it to wide or narrow targets of varying heights. Grasp heights on the plunger were additively affected by target height and target width, suggesting that both factors were taken into account prior to movement of the plunger from its initial position. Another factor that was manipulated in the experiment was the width of the base from which the plunger was lifted on its way to its next position. This factor also affected grasp heights, but no more than target widths. The latter result contradicts the view that movement starts are planned in more detail than movement ends, as might be expected from the fact that movement starts come sooner. Together, the results suggest that forthcoming movements are planned in considerable detail. A surprising methodological implication of this study is that recording how people prepare to move can reveal as much, or in some cases more, about what they have planned than recording their subsequent movements.