Trunk bradykinesia and foveation delays during whole-body turns in spasmodic torticollis

Oculomotor functions in focal dystonias: A systematic review

Focal Dystonia (FD) is a chronic neurological disorder, which causes twisting and repetitive movements and abnormal postures induced by involuntary sustained contractions of agonist and antagonist muscles. Based on the hypothesis that several dystonia-related brain regions, including cerebellum, are implicated in oculomotor disturbances (OCD), a number of studies investigated oculomotor function in patients with dystonia. However, conceptual clarity with respect to the used assessment tools and interpretation of the findings is lacking in the literature. This is the first article to systematically review studies that assessed oculomotor function in patients with FD. In total, 329 publications, published until September 1, 2019, were identified through MEDLINE search. Twenty out of 329 studies, involving 232 subjects in total, met the inclusion criteria. Most of the studies reported oculomotor disturbances in patients with FD. Abnormalities included asymmetry in vestibulo-ocular reflex (VOR), disturbances in saccadic functions, and prolonged latencies of eye motion. Discrepancies in the results could be explained, at least partially, by the long period of time over which the reviewed studies were published, the different methods used for testing the eye movements, and the limited number of patients assessed since the majority of data derived from case reports or small-scale studies. Further prospective studies with larger subject numbers are needed, using advanced tools for the assessment of oculomotor function in focal dystonia.

Visual compensation in cervical dystonia

Introduction: Visual-spatial processing can be affected in people with cervical dystonia (CD). These impairments have almost exclusively been observed in laboratory studies, but the impact of visuo-spatial impairments on daily activities is unclear. Here, we investigated how people living with CD visually explore the environment. Method: 21 participants (10 patients and 11 controls) walked a designated course searching for targets while their eye movements were tracked by the means of eye tracking glasses. In addition, all participants performed an extensive battery of spatial neglect tests. Results: People with CD visually compensated for their lateral head position. That is, they made more eye movements towards the side opposite their lateral head position. No evidence for or against spatial asymmetries were detected in a range of behavioral measures (target detection, standard neglect battery tests) on a group level. Yet, single patients showed a neglect-like pattern. One patient with a right torticollis failed to detect most left-sided targets when walking. Conclusion: In general, participants seem to adequately compensate for the deviated head posture when judging spatial stimuli. However, the insufficient spatial exploration in one patient with a valid driving license should raise awareness in clinicians to potential visuo-spatial problems in people living with CD. An interesting question arising from our findings is whether people with CD and marked visuo-spatial deficits would benefit from scanning training, as is employed in stroke rehabilitation.

Sensorimotor Control in Dystonia

This is an overview of the sensorimotor impairments in dystonia, a syndrome characterized by sustained or intermittent aberrant movement patterns leading to abnormal movements and/or postures with or without a tremulous component. Dystonia can affect the entire body or specific body regions and results from a plethora of etiologies, including subtle changes in gray and white matter in several brain regions. Research over the last 25 years addressing topics of sensorimotor control has shown functional sensorimotor impairments related to sensorimotor integration, timing, oculomotor and head control, as well as upper and lower limb control. In the context of efforts to update the classification of dystonia, sensorimotor research is highly relevant for a better understanding of the underlying pathology, and potential mechanisms contributing to global and regional dysfunction within the central nervous system. This overview of relevant research regarding sensorimotor control in humans with idiopathic dystonia attempts to frame the dysfunction with respect to what is known regarding motor control in patients and healthy individuals. We also highlight promising avenues for the future study of neuromotor control that may help to further elucidate dystonia etiology, pathology, and functional characteristics.

Reduced vision-related quality of life in people living with dystonia

Purpose: Dystonia is a neurological movement disorder with negative impact on function and quality of life. It is currently unclear whether vision-related quality of life is affected. The aim of this study was to determine whether vision-related quality of life is reduced by dystonia.Materials and methods: A vision-related quality of life questionnaire was delivered online to probe visual function in people living with dystonia. Scores for each of six domains were compared to normative data of 819 healthy participants using one sample t-tests. Respondents were divided into two groups based on whether they had botulinum toxin injections and compared using independent samples t-tests.Results: There were 42 completed responses. There was a difference from norm for two domains; ocular symptoms (t(41) = 2.31, p = 0.026) and role performance (t(41) = 2.85, p = 0.007). There was variation in responses for all six domains. No difference in scores for the botulinum toxin injection group was found for either domain (both p > 0.74).Conclusions: Some people with dystonia experience reduced vision-related quality of life, which has potential to contribute to their disability. Health professionals should be aware of vision-related issues when managing people with dystonia and consider appropriate rehabilitative interventions to reduce disability and enhance quality of life.Implications for rehabilitationDystonia is a neurological movement disorder resulting in abnormal postures and movements.Vision-related quality of life is reduced by dystonia which may contribute to disability and reduced function.Strategies to improve vision-related quality of life should be included in rehabilitation programmes for people living with dystonia.

The effects of constraining vision and eye movements on whole-body coordination during standing turns

Turning the body towards a new direction is normally achieved via a top-down synergy whereby gaze (eye direction in space) leads the upper body segments, which in turn lead the feet. These anticipatory eye movements are observable even in darkness and constraining the initial eye movements modifies the stereotyped top-down reorientation sequence. Our aim was to elucidate the relative contributions of vision and eye movements to whole-body coordination during large standing turns by observing the effects of separately removing visual information or suppressing eye movements throughout the turn. We predicted that constraining eye movements would modify the steering synergy, whereas removing vision would have little effect. We found that preventing eye movements modified both timing and spatial characteristics of axial segment and feet rotation. When gaze was fixed, gait initiation, but not axial segment rotation, was delayed in comparison to both full vision and no vision turns. When eye movements were prevented, the predictable relationship between the extent head rotation led the body and peak head angular velocity was abolished suggesting that anticipatory head movements normally subserve gaze behaviour. In addition, stepping frequency significantly reduced during the gaze fixation condition but not during the no-vision condition, suggesting that oculomotor control is linked to stepping behaviour.

Application of virtual reality head mounted display for investigation of movement: a novel effect of orientation of attention

OBJECTIVE

To date human kinematics research has relied on video processing, motion capture and magnetic search coil data acquisition techniques. However, the use of head mounted display virtual reality systems, as a novel research tool, could facilitate novel studies into human movement and movement disorders. These systems have the unique ability of presenting immersive 3D stimulus while also allowing participants to make ecologically valid movement-based responses.

APPROACH

We employed one such system (Oculus Rift DK2) in this study to present visual stimulus and acquire head-turn data from a cohort of 40 healthy adults. Participants were asked to complete head movements towards eccentrically located visual targets following valid and invalid cues. Such tasks are commonly employed for investigating the effects orientation of attention and are known as Posner cueing paradigms. Electrooculography was also recorded for a subset of 18 participants.

MAIN RESULTS

A delay was observed in onset of head movement and saccade onset during invalid trials, both at the group and single participant level. We found that participants initiated head turns 57.4 ms earlier during valid trials. A strong relationship between saccade onset and head movement onset was also observed during valid trials.

SIGNIFICANCE

This work represents the first time that the Posner cueing effect has been observed in onset of head movement in humans. The results presented here highlight the role of head-mounted display systems as a novel and practical research tool for investigations of normal and abnormal movement patterns.

Fast gaze reorientations by combined movements of the eye, head, trunk and lower extremities

Large reorientations of the line of sight, involving combined rotations of the eyes, head, trunk and lower extremities, are executed either as fast single-step or as slow multiple-step gaze transfers. In order to obtain more insight into the mechanisms of gaze and multisegmental movement control, we have investigated time-optimal gaze shifts (i.e. with the instruction to move as fast as possible) during voluntary whole-body rotations to remembered targets up to 180° eccentricity performed by standing healthy humans in darkness. Fast, accurate, single-step movement patterns occurred in approximately 70 % of trials, i.e. considerably more frequently than in previous studies with the instruction to turn at freely chosen speed (30 %). Head-in-space velocity in these cases was significantly higher than during multiple-step transfers and displayed a conspicuously regular bell-shaped profile, increasing smoothly to a peak and then decreasing slowly until realignment with the target. Head-in-space acceleration was on average not different during reorientations to the different target eccentricities. In contrast, head-in-space velocity increased with target eccentricity due to the longer duration of the acceleration phase implemented during trials to more distant targets. Eye saccade amplitude approached the eye-in-orbit mechanical limit and was unrelated to eye/head velocity, duration or target eccentricity. Overall, the combined movement was stereotyped such that the first two principal components accounted for data variance almost up to gaze shift end, suggesting that the three mechanical degrees of freedom under consideration (eye-in-orbit, head-on-trunk and trunk-in-space) are on average reduced to two kinematic degrees of freedom (i.e. eye, head-in-space). Synchronous EMG activity in the anterior tibial and gastrocnemius muscles preceded the onset of eye rotation. Since the magnitude and timing of peak head-in-space velocity were scaled with target eccentricity and because head-on-trunk and trunk-in-space displacements were on average linearly correlated, we propose a separate controller for head-in-space movement, whereas the movement of the eye-in-space may be, in contrast, governed by global, i.e. gaze feedback. The rapid progression of the line of sight can be sustained, and the reactivation of the vestibulo-ocular reflex would be postponed, until gaze error approaches zero only in association with a strong head-in-space neural control signal.

Cervical dystonia: a disorder of the midbrain network for covert attentional orienting

While the pathogenesis of cervical dystonia remains unknown, recent animal and clinical experimental studies have indicated its probable mechanisms. Abnormal temporal discrimination is a mediational endophenotype of cervical dystonia and informs new concepts of disease pathogenesis. Our hypothesis is that both abnormal temporal discrimination and cervical dystonia are due to a disorder of the midbrain network for covert attentional orienting caused by reduced gamma-aminobutyric acid (GABA) inhibition, resulting, in turn, from as yet undetermined, genetic mutations. Such disinhibition is (a) subclinically manifested by abnormal temporal discrimination due to prolonged duration firing of the visual sensory neurons in the superficial laminae of the superior colliculus and (b) clinically manifested by cervical dystonia due to disinhibited burst activity of the cephalomotor neurons of the intermediate and deep laminae of the superior colliculus. Abnormal temporal discrimination in unaffected first-degree relatives of patients with cervical dystonia represents a subclinical manifestation of defective GABA activity both within the superior colliculus and from the substantia nigra pars reticulata. A number of experiments are required to prove or disprove this hypothesis.