A block to pre-prepared movement in gait freezing, relieved by pedunculopontine nucleus stimulation

Word Repetition Paired With Startling Stimuli Decreases Aphasia and Apraxia Severity in Severe-to-Moderate Stroke: A Stratified, Single-Blind, Randomized, Phase 1 Clinical Trial

PURPOSE

This prospective, single-blinded, parallel, stratified, randomized clinical trial via telehealth aimed to investigate the impact of Startle Adjuvant Rehabilitation Therapy (START) on aphasia, apraxia of speech (AOS), and quality of life in individuals with chronic stroke. The study hypothesized that START would have a greater effect on AOS-related measures and more severe individuals.

METHOD

Forty-two participants with poststroke aphasia, AOS, or both were randomly assigned to the START or control group. Both groups received 77-dB GET READY and GO cues during a word repetition task for three 1-hr sessions on consecutive days. The START group additionally received 105-dB white noise GO cues during one third of trials. The Western Aphasia Battery-Revised, Apraxia Battery for Adults, Stroke Impact Scale, and Communication Outcomes After Stroke scale were administered at Day 1, Day 5, and 1-month follow-up.

RESULTS

START improved performance on some subtests of the Western Aphasia Battery (Comprehension, Repetition, Reading) and measures of AOS (Diadochokinetic Rate, Increasing Word Length) in individuals with moderate/severe aphasia, whereas moderate/severe controls saw no changes. Individuals with mild aphasia receiving START had improved Reading, whereas mild controls saw improved Comprehension. The START group had increased mood and perceived communication recovery by Day 5, whereas controls saw no changes in quality of life.

CONCLUSIONS

This study is the first to evaluate the impact of training with startling acoustic stimuli on clinical measures of aphasia and AOS. Our findings suggest START can enhance both nontrained speech production and receptive speech tasks in moderate/severe aphasia, possibly by reducing poststroke cortical inhibition. Our findings should be considered carefully, as our limitations include small effect sizes, within-group variability, and low completion rates for quality-of-life assessments and follow-up visits. Future studies should explore a mechanism of action, conduct larger and longer Phase 2 clinical trials, and evaluate long-term retention.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24093519.

Startle-induced rapid release of a gait initiation sequence in Parkinson's disease with freezing of gait

OBJECTIVE

Freezing of gait (FOG) in Parkinson's disease (PD) is characterized by the inability to initiate stepping, despite the intention to do so. This study used a startling acoustic stimulus paradigm to examine if the capacity to select, prepare and initiate gait under simple and choice reaction time conditions are impaired in people with PD and FOG.

METHODS

Thirty individuals (10 PD with FOG, 10 PD without FOG, and 10 controls) performed an instructed-delay gait initiation task under simple and choice reaction time conditions. In a subset of trials, a startle stimulus (124 dB) was presented 500 ms before the time of the imperative go-cue. Anticipatory postural adjustments preceding and accompanying gait initiation were quantified.

RESULTS

The presentation of a startling acoustic stimulus resulted in the rapid initiation of an anticipatory postural adjustment sequence during both the simple and choice reaction time tasks in all groups.

CONCLUSIONS

The neural capacity to prepare the spatial and temporal components of gait initiation remains intact in PD individuals with and without FOG.

SIGNIFICANCE

The retained capacity to prepare anticipatory postural adjustments in advance may explain why external sensory cues are effective in the facilitation of gait initiation in people with PD with FOG.

Novel targets in deep brain stimulation for movement disorders

The neurosurgical treatment of movement disorders, primarily via deep brain stimulation (DBS), is a rapidly expanding and evolving field. Although conventional targets including the subthalamic nucleus (STN) and internal segment of the globus pallidus (GPi) for Parkinson's disease and ventral intermediate nucleus of the thalams (VIM) for tremor provide substantial benefit in terms of both motor symptoms and quality of life, other targets for DBS have been explored in an effort to maximize clinical benefit and also avoid undesired adverse effects associated with stimulation. These novel targets primarily include the rostral zona incerta (rZI), caudal zona incerta (cZI)/posterior subthalamic area (PSA), prelemniscal radiation (Raprl), pedunculopontine nucleus (PPN), substantia nigra pars reticulata (SNr), centromedian/parafascicular (CM/PF) nucleus of the thalamus, nucleus basalis of Meynert (NBM), dentato-rubro-thalamic tract (DRTT), dentate nucleus of the cerebellum, external segment of the globus pallidus (GPe), and ventral oralis (VO) complex of the thalamus. However, reports of outcomes utilizing these targets are scattered and disparate. In order to provide a comprehensive resource for researchers and clinicians alike, we have summarized the existing literature surrounding these novel targets, including rationale for their use, neurosurgical techniques where relevant, outcomes and adverse effects of stimulation, and future directions for research.

Delays in the Reticulospinal System Are Associated With a Reduced Capacity to Learn a Simulated Feeding Task in Older Adults

Learning declines with age. Recent evidence indicates that the brainstem may play an important role in learning and motor skill acquisition. Our objective was to determine if delays in the reticular formation, measured via the startle reflex, correspond to age-related deficits in learning and retention. We hypothesized that delays in the startle reflex would be linearly correlated to learning and retention deficits in older adults. To determine if associations were unique to the reticulospinal system, we also evaluated corticospinal contributions with transcranial magnetic stimulation. Our results showed a linear relationship between startle onset latency and percent learning and retention but no relationship between active or passive motor-evoked potential onsets or peak-to-peak amplitude. These results lay the foundation for further study to evaluate if (1) the reticular formation is a subcortical facilitator of skill acquisition and (2) processing delays in the reticular formation contribute to age-related learning deficits.

Developmental neurobiology of cerebellar and Basal Ganglia connections

BACKGROUND

The high prevalence of mixed phenotypes of Early Onset Ataxia (EOA) with comorbid dystonia has shifted the pathogenetic concept from the cerebellum towards the interconnected cerebellar motor network. This paper on EOA with comorbid dystonia (EOA-dystonia) explores the conceptual relationship between the motor phenotype and the cortico-basal-ganglia-ponto-cerebellar network.

METHODS

In EOA-dystonia, we reviewed anatomic-, genetic- and biochemical-studies on the comorbidity between ataxia and dystonia.

RESULTS

In a clinical EOA cohort, the prevalence of dystonia was over 60%. Both human and animal studies converge on the underlying role for the cortico-basal-ganglia-ponto-cerebellar network. Genetic -clinical and -in silico network studies reveal underlying biological pathways for energy production and neural signal transduction.

CONCLUSIONS

EOA-dystonia phenotypes are attributable to the cortico-basal-ganglia-ponto-cerebellar network, instead of to the cerebellum, alone. The underlying anatomic and pathogenetic pathways have clinical implications for our understanding of the heterogeneous phenotype, neuro-metabolic and genetic testing and potentially also for new treatment strategies, including neuro-modulation.

Gait-Phase Modulates Alpha and Beta Oscillations in the Pedunculopontine Nucleus

The pedunculopontine nucleus (PPN) is a reticular collection of neurons at the junction of the midbrain and pons, playing an important role in modulating posture and locomotion. Deep brain stimulation of the PPN has been proposed as an emerging treatment for patients with Parkinson's disease (PD) or multiple system atrophy (MSA) who have gait-related atypical parkinsonian syndromes. In this study, we investigated PPN activities during gait to better understand its functional role in locomotion. Specifically, we investigated whether PPN activity is rhythmically modulated by gait cycles during locomotion. PPN local field potential (LFP) activities were recorded from PD or MSA patients with gait difficulties during stepping in place or free walking. Simultaneous measurements from force plates or accelerometers were used to determine the phase within each gait cycle at each time point. Our results showed that activities in the alpha and beta frequency bands in the PPN LFPs were rhythmically modulated by the gait phase within gait cycles, with a higher modulation index when the stepping rhythm was more regular. Meanwhile, the PPN-cortical coherence was most prominent in the alpha band. Both gait phase-related modulation in the alpha/beta power and the PPN-cortical coherence in the alpha frequency band were spatially specific to the PPN and did not extend to surrounding regions. These results suggest that alternating PPN modulation may support gait control. Whether enhancing alternating PPN modulation by stimulating in an alternating fashion could positively affect gait control remains to be tested.SIGNIFICANCE STATEMENT The therapeutic efficacy of pedunculopontine nucleus (PPN) deep brain stimulation (DBS) and the extent to which it can improve quality of life are still inconclusive. Understanding how PPN activity is modulated by stepping or walking may offer insight into how to improve the efficacy of PPN DBS in ameliorating gait difficulties. Our study shows that PPN alpha and beta activity was modulated by the gait phase, and that this was most pronounced when the stepping rhythm was regular. It remains to be tested whether enhancing alternating PPN modulation by stimulating in an alternating fashion could positively affect gait control.

Does height-induced threat modulate shortening of reaction times induced by a loud stimulus in a lateral stepping and a wrist extension task?

INTRODUCTION

The StartReact (SR) effect is the accelerated release of a prepared movement when a startling acoustic stimulus is presented at the time of the imperative stimulus (IS). SR paradigms have been used to study defective control of balance and gait in people with neurological conditions, but differences in emotional state (e.g. fear of failure) may be a potential confounder when comparing patients to healthy subjects. In this study, we aimed to gain insight in the effects of postural threat on the SR effect by manipulating surface height during a postural (lateral step) task and a non-postural (wrist extension) task.

METHODS

Eleven healthy participants performed a lateral step perpendicular to the platform edge, and 19 participants performed a wrist extension task while standing at the platform edge. Participants initiated the movement as fast as possible in response to an IS that varied in intensity across trials (80 dB to 121 dB) at both low and high platform height (3.2 m). For the lateral step task, we determined anticipatory postural adjustments (APA) and step onset latencies. For the wrist extension task, muscle onset latencies were determined. We used Wilcoxon signed-rank tests on the relative onset latencies between both heights, to identify whether the effect of height was different for IS intensities between 103 and 118 dB compared to 121 dB.

RESULTS

For both tasks, onset latencies were significantly shortened at 121 dB compared to 80 dB, regardless of height. In the lateral step task, the effect of height was larger at 112 dB compared to 121 dB. The absolute onset latencies showed that at 112 dB there was no such stimulus intensity effect at high as seen at low surface height. In the wrist extension task, no differential effects of height could be demonstrated across IS intensities.

CONCLUSIONS

Postural threat had a significant, yet modest effect on shortening of RTs induced by a loud IS, with a mere 3 dB difference between standing on high versus low surface height. Interestingly, this effect of height was specific to the postural (i.e. lateral stepping) task, as no such differences could be demonstrated in the wrist extension task. This presumably reflects more cautious execution of the lateral step task when standing on height. The present findings suggest that applying stimuli of sufficiently high intensity (≥115 dB) appears to neutralize potential differences in emotional state when studying SR effects.

Tractography patterns of pedunculopontine nucleus deep brain stimulation

Deep brain stimulation of the pedunculopontine nucleus is a promising surgical procedure for the treatment of Parkinsonian gait and balance dysfunction. It has, however, produced mixed clinical results that are poorly understood. We used tractography with the aim to rationalise this heterogeneity. A cohort of eight patients with postural instability and gait disturbance (Parkinson’s disease subtype) underwent pre-operative structural and diffusion MRI, then progressed to deep brain stimulation targeting the pedunculopontine nucleus. Pre-operative and follow-up assessments were carried out using the Gait and Falls Questionnaire, and Freezing of Gait Questionnaire. Probabilistic diffusion tensor tractography was carried out between the stimulating electrodes and both cortical and cerebellar regions of a priori interest. Cortical surface reconstructions were carried out to measure cortical thickness in relevant areas. Structural connectivity between stimulating electrode and precentral gyrus (r = 0.81, p = 0.01), Brodmann areas 1 (r = 0.78, p = 0.02) and 2 (r = 0.76, p = 0.03) were correlated with clinical improvement. A negative correlation was also observed for the superior cerebellar peduncle (r = −0.76, p = 0.03). Lower cortical thickness of the left parietal lobe and bilateral premotor cortices were associated with greater pre-operative severity of symptoms. Both motor and sensory structural connectivity of the stimulated surgical target characterises the clinical benefit, or lack thereof, from surgery. In what is a challenging region of brainstem to effectively target, these results provide insights into how this can be better achieved. The mechanisms of action are likely to have both motor and sensory components, commensurate with the probable nature of the underlying dysfunction.

Three-Year Gait and Axial Outcomes of Bilateral STN and GPi Parkinson's Disease Deep Brain Stimulation

Objective: To examine the short- and long-term clinical outcomes of the bilateral subthalamic nucleus (STN) and globus pallidus internus (GPi) deep brain stimulation (DBS) on gait and axial symptoms in Parkinson's disease (PD) patients. Available data have been inconsistent and mostly short-term regarding the effect of both brain targets on gait and axial symptoms. We aimed to identify potential target specific differences at 3-year follow-up from a large single-center experience. Methods: We retrospectively reviewed short-term (6-month follow-up) and long-term (36-month follow-up) changes in the Unified Parkinson's Disease Rating Scale (UPDRS) Part II and III total scores of 72 PD patients (53 with bilateral STN-DBS and 19 with bilateral GPi-DBS). An interdisciplinary team made target-specific decisions for each DBS patient. We analyzed changes in gait and axial subscores derived from UPDRS II and III. Results: In both the STN- and GPi-DBS cohorts, we observed no significant differences in gait and axial UPDRS derived subscores in the off-med/on stimulation state at long-term follow-up when compared to baseline. On-med axial scores remained similar in the short-term but worsened in both groups (STN, 2.23 ± 3.43, p < 0.001; GPi, 2.53 ± 2.37, p < 0.01) in the long-term possibly due to disease progression. At long-term follow-up, the UPDRS III off-med/on stimulation scores worsened but were persistently improved from baseline in both groups (-9.07 ± 13.9, p < 0.001). Conclusions: The study showed that long-term both STN- and GPi-DBS had a similar effect on gait and axial symptoms in UPDRS derived subscores at 36-month follow-up despite potential baseline differences in criteria for selection of each target. More sophisticated measures of gait and balance beyond the categorical UPDRS score will be needed for future studies.

Pedunculopontine Nucleus Deep Brain Stimulation Improves Gait Disorder in Parkinson's Disease: A Systematic Review and Meta-analysis

Deep brain stimulation (DBS) of the pedunculopontine nucleus (PPN) has been proposed as a treatment strategy for gait disorder in patients with Parkinson's disease (PD). We thus performed a systematic review and meta-analysis of randomized and nonrandomized controlled trials to assess the effect of this treatment on gait disorder in patients with PD. We systematically searched PubMed, Cochrane, Web of Knowledge, Wan Fang and WIP for randomized and nonrandomized controlled trials (published before July 29, 2014; no language restrictions) comparing PPN-DBS with other treatments. We assessed pooled data using a random effects model and a fixed effects model. Of 130 identified studies, 14 were eligible and were included in our analysis (N = 82 participants). Compared to those presurgery, the Unified Parkinson Disease Rating Scale (UPDRS) 27-30 scores for patients were lowered by PPN-DBS [3.94 (95% confidence interval, CI = 1.23 to 6.65)]. The UPDRS 13 and 14 scores did not improve with levodopa treatment [0.43 (- 0.35 to 1.20); 0.35 (- 0.50 to 1.19)], whereas the UPDRS 27-30 scores could be improved by the therapy [1.42 (95% CI 0.34 to 2.51)]. The Gait and Falls Questionnaire and UPDRS 13 and 14 scores showed significant improvements after PPN-DBS under the medication-off (MED-OFF) status [15.44 (95% CI = 8.44 to 22.45); 1.57 (95% CI = 0.84 to 2.30); 1.34 (95% CI = 0.84 to 1.84)]. PPN-DBS is a potential therapeutic target that could improve gait and fall disorders in patients with PD. Our findings will help improve the clinical application of DBS in PD patients with gait disorder.

Long-term effect of subthalamic nucleus deep brain stimulation on freezing of gait in Parkinson's disease

OBJECTIVE

Subthalamic nucleus deep brain stimulation (STN DBS) is effective against freezing of gait (FOG) in Parkinson's disease (PD); however, whether this effect persists over the long term is debated. The aim of the current study was to investigate the long-term effect of STN DBS on FOG in patients with PD.

METHODS

Data on 52 cases in which PD patients received bilateral STN DBS were obtained from a prospective registry. The authors blindly analyzed FOG incidence and its severity from the videotapes of a 5-m walking task at the baseline and at the 1-, 2-, and 5- or 7-year follow-up visits. They also compared the axial score from the Unified Parkinson's Disease Rating Scale (UPDRS) part III, UPDRS part II (UPDRS-II) item 14, and the FOG questionnaire (FOG-Q). Postoperatively, video-based FOG analysis and the axial score were evaluated under 4 conditions (off-medication/off-stimulation, off-medication/on-stimulation, on-medication/off-stimulation, and on-medication/on-stimulation), and UPDRS-II item 14 and the FOG-Q score were evaluated under 2 conditions (off-medication/on-stimulation and on-medication/on-stimulation).

RESULTS

During the off-medication state, the on-stimulation condition improved FOG outcomes, except for video-based FOG severity, up to the last follow-up compared with the baseline. Video-based FOG outcomes and the axial score during the off-medication state were improved with the on-stimulation condition up to the last follow-up compared with the off-stimulation condition. During the on-medication state, the on-stimulation condition did not improve any FOG outcome compared with the baseline; however, it improved video-based FOG outcomes up to the 2-year follow-up and the axial score up to the last follow-up compared with the off-stimulation condition.

CONCLUSIONS

Our findings suggest that STN DBS has a long-term effect on FOG in the off-medication state. However, STN DBS did not show a long-term effect on FOG in the on-medication state, although it had a short-term effect until the 2-year follow-up.

Subthalamic nucleus oscillations correlate with vulnerability to freezing of gait in patients with Parkinson's disease

Freezing of gait (FOG) is a disabling clinical phenomenon often found in patients with advanced Parkinson's disease (PD). FOG impairs motor function, causes falls and leads to loss of independence. Whereas dual tasking that distracts patients' attention precipitates FOG, auditory or visual cues ameliorate this phenomenon. The pathophysiology of FOG remains unclear. Previous studies suggest that the basal ganglia are involved in the generation of FOG. Investigation of the modulation of neuronal activities within basal ganglia structures during walking is warranted. To this end, we recorded local field potentials (LFP) from the subthalamic nucleus (STN) while PD patients performed single-task gait (ST) or walked while dual-tasking (DT). An index of FOG (iFOG) derived from trunk accelerometry was used as an objective measure to differentiate FOG-vulnerable gait from normal gait. Two spectral activities recorded from the STN region were associated with vulnerability to freezing. Greater LFP power in the low beta (15-21 Hz) and theta (5-8 Hz) bands were noted during periods of vulnerable gait in both ST and DT states. Whereas the elevation of low beta activities was distributed across STN, the increase in theta activity was focal and found in ventral STN and/or substantia nigra (SNr) in ST. The results demonstrate that low beta and theta band oscillations within the STN area occur during gait susceptible to freezing in PD. They also add to the evidence that narrow band ~18 Hz activity may be linked to FOG.

Balance control systems in Parkinson's disease and the impact of pedunculopontine area stimulation

Impaired balance is a major contributor to falls and diminished quality of life in Parkinson’s disease, yet the pathophysiology is poorly understood. Here, we assessed if patients with Parkinson’s disease and severe clinical balance impairment have deficits in the intermittent and continuous control systems proposed to maintain upright stance, and furthermore, whether such deficits are potentially reversible, with the experimental therapy of pedunculopontine nucleus deep brain stimulation. Two subject groups were assessed: (i) 13 patients with Parkinson’s disease and severe clinical balance impairment, implanted with pedunculopontine nucleus deep brain stimulators; and (ii) 13 healthy control subjects. Patients were assessed in the OFF medication state and blinded to two conditions; off and on pedunculopontine nucleus stimulation. Postural sway data (deviations in centre of pressure) were collected during quiet stance using posturography. Intermittent control of sway was assessed by calculating the frequency of intermittent switching behaviour (discontinuities), derived using a wavelet-based transformation of the sway time series. Continuous control of sway was assessed with a proportional–integral–derivative (PID) controller model using ballistic reaction time as a measure of feedback delay. Clinical balance impairment was assessed using the ‘pull test’ to rate postural reflexes and by rating attempts to arise from sitting to standing. Patients with Parkinson’s disease demonstrated reduced intermittent switching of postural sway compared with healthy controls. Patients also had abnormal feedback gains in postural sway according to the PID model. Pedunculopontine nucleus stimulation improved intermittent switching of postural sway, feedback gains in the PID model and clinical balance impairment. Clinical balance impairment correlated with intermittent switching of postural sway (rho = − 0.705, P < 0.001) and feedback gains in the PID model (rho = 0.619, P = 0.011). These results suggest that dysfunctional intermittent and continuous control systems may contribute to the pathophysiology of clinical balance impairment in Parkinson’s disease. Clinical balance impairment and their related control system deficits are potentially reversible, as demonstrated by their improvement with pedunculopontine nucleus deep brain stimulation.

Focus on the pedunculopontine nucleus. Consensus review from the May 2018 brainstem society meeting in Washington, DC, USA

The pedunculopontine nucleus (PPN) is located in the mesopontine tegmentum and is best delimited by a group of large cholinergic neurons adjacent to the decussation of the superior cerebellar peduncle. This part of the brain, populated by many other neuronal groups, is a crossroads for many important functions. Good evidence relates the PPN to control of reflex reactions, sleep-wake cycles, posture and gait. However, the precise role of the PPN in all these functions has been controversial and there still are uncertainties in the functional anatomy and physiology of the nucleus. It is difficult to grasp the extent of the influence of the PPN, not only because of its varied functions and projections, but also because of the controversies arising from them. One controversy is its relationship to the mesencephalic locomotor region (MLR). In this regard, the PPN has become a new target for deep brain stimulation (DBS) for the treatment of parkinsonian gait disorders, including freezing of gait. This review is intended to indicate what is currently known, shed some light on the controversies that have arisen, and to provide a framework for future research.

The pedunculopontine region and breathing in Parkinson's disease

Objective

Respiratory abnormalities such as upper airway obstruction are common in Parkinson's disease (PD) and are an important cause of mortality and morbidity. We tested the effect of pedunculopontine region (PPNr) stimulation on respiratory maneuvers in human participants with PD, and separately recorded PPNr neural activity reflected in the local field potential (LFP) during these maneuvers.

Methods

Nine patients with deep brain stimulation electrodes in PPNr, and seven in globus pallidus interna (GPi) were studied during trials of maximal inspiration followed by forced expiration with stimulation OFF and ON. Local field potentials (LFPs) were recorded in the unstimulated condition.

Results

PEFR increased from 6.41 ± 0.63 L/sec in the OFF stimulation state to 7.5 L ± 0.65 L/sec in the ON stimulation state (z = −2.666, df = 8, P = 0.024). Percentage improvement in PEFR was strongly correlated with proximity of the stimulated electrode contact to the mesencephalic locomotor region in the rostral PPN (r = 0.814, n = 9, P = 0.008). Mean PPNr LFP power increased within the alpha band (7–11 Hz) during forced respiratory maneuvers (1.63 ± 0.16 μV²/Hz) compared to resting breathing (0.77 ± 0.16 μV²/Hz; z = −2.197, df = 6, P = 0.028). No changes in alpha activity or spirometric indices were seen with GPi recording or stimulation. Percentage improvement in PEFR was strongly positively correlated with increase in alpha power (r = 0.653, n = 14 (7 PPNr patients recorded bilaterally), P = 0.0096).

Interpretation

PPNr stimulation in PD improves indices of upper airway function. Increased alpha‐band activity is seen within the PPNr during forced respiratory maneuvers. Our findings suggest a link between the PPNr and respiratory performance in PD.

Effort to perceive the position of one visual horizontal line relative to another appearing close causes an earlier postural response to backward perturbation

Humans make an effort to stabilize the body when a stable eye position is required to improve visibility. Perceiving the position of one visual object relative to another appearing close is relatively difficult compared with perceiving those objects appearing at a distance. Thus, humans must make an effort to stabilize their body to improve visibility when they attempt to perceive the position of one object relative to another appearing close. This process may enhance the response to postural perturbation. The present study tested this hypothesis. A total of 15 healthy participants maintained a position standing over the platform and gazed at a horizontal fixation line in front of them. One of the two warning sound cues was given in each trial; one predicted the forthcoming appearance of a horizontal line 2 mm above or below the horizontal fixation line (indistinct condition), and another predicted the forthcoming appearance of the horizontal line 2 cm above or 2 cm below the horizontal fixation line (distinct condition). After the warning sound cue, the support surface moved in the forward or backward direction. A horizontal line appeared 40 ms after the onset of the support surface motion at the level above or below the horizontal fixation line with the distance from the horizontal fixation line as the warning cue predicted. The participants verbally stated the position of the horizontal line relative to the horizontal fixation line after the offset of the support surface movement. The displacement of the head and sacrum was measured with acceleration sensors. A test of the simple main effect following an analysis of variance revealed that the peak downward displacement of the pelvis in the indistinct condition was significantly earlier than that in the distinct condition. This finding indicated that the effort to stabilize the body for improvement of visibility enhances the response of the pelvis to the forward movement of the support surface. This effect may be used for improvement of the postural response in patients with problems with postural control.

Balancing between the two: Are freezing of gait and postural instability in Parkinson's disease connected?

Postural instability and freezing of gait (FoG) are key features of Parkinson's disease (PD) closely related to falls. Growing evidence suggests that co-existing postural deficits could influence the occurrence and severity of FoG. To date, the exact nature of this interrelationship remains largely unknown. We analyzed the complex interaction between postural instability and gait disturbance by comparing the findings available in the posturographic literature between patients with and without FoG. Results showed that FoG and postural instability are intertwined, can influence each other behaviorally and may coincide neurologically. The most common FoG-related postural deficits included weight-shifting impairments, and inadequate scaling and timing of postural responses most apparent at forthcoming postural changes under time constraints. Most likely, a negative cycle of combined and more severe postural deficits in people with FoG will enhance postural stability breakdown. As such, the wide brain network deficiencies involved in FoG may also concurrently influence postural stability. Future work needs to examine whether training interventions targeting both symptoms will have extra clinical benefits on fall frequency.

StartReact effects in first dorsal interosseous muscle are absent in a pinch task, but present when combined with elbow flexion

Objective

To provide a neurophysiological tool for assessing sensorimotor pathways, which may differ for those involving distal muscles in simple tasks from those involving distal muscles in a kinetic chain task, or proximal muscles in both.

Methods

We compared latencies and magnitudes of motor responses in a reaction time paradigm in a proximal (biceps brachii, BB) and a distal (first dorsal interosseous, FDI) muscle following electrical stimuli used as imperative signal (IS) delivered to the index finger. These stimuli were applied during different motor tasks: simple tasks involving either one muscle, e.g. flexing the elbow for BB (FLEX), or pinching a pen for FDI (PINCH); combined tasks engaging both muscles by pinching and flexing simultaneously (PINCH-FLEX). Stimuli were of varying intensity and occasionally elicited a startle response, and a StartReact effect.

Results

In BB, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities for non-startling trials, while for trials containing startle responses, latencies were uniformly shortened and response amplitudes similarly augmented across all IS intensities in both FLEX and PINCH-FLEX. In FDI, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities in both PINCH and PINCH-FLEX for non-startling trials, but, unlike in BB for the simple task, in PINCH for trials containing startle responses as well. In PINCH-FLEX, FDI latencies were uniformly shortened and amplitudes similarly increased across all stimulus intensities whenever startle signs were present.

Conclusions

Our results suggest the presence of different sensorimotor pathways supporting a dissociation between simple tasks that involve distal upper limb muscles (FDI in PINCH) from simple tasks involving proximal muscles (BB in FLEX), and combined tasks that engage both muscles (FDI and BB in PINCH-FLEX), all in accordance with differential importance in the control of movements by cortical and subcortical structures.

Significance

Simple assessment tools may provide useful information regarding the differential involvement of sensorimotor pathways in the control of both simple and combined tasks that engage proximal and distal muscles.

Adaptations to Postural Perturbations in Patients With Freezing of Gait

Introduction: Freezing of gait (FOG) is a powerful determinant of falls in Parkinson's disease (PD). Automatic postural reactions serve as a protective strategy to prevent falling after perturbations. However, differences in automatic postural reactions between patients with and without FOG in response to perturbation are at present unclear. Therefore, the present study aimed to compare the response patterns and neuromuscular control between PD patients with and without FOG and healthy controls (HCs) after postural perturbations.

Methods: 28 PD patients (15 FOG+, 13 FOG−) and 22 HCs were included. Participants stood on a moveable platform while random perturbations were imposed. The first anterior platform translation was retained for analysis. Center of pressure (CoP) and center of mass (CoM) trajectories and trunk, knee and ankle angles were compared between the three groups using the Statistical Parametric Mapping technique, allowing to capture changes in time. In addition, muscle activation of lower leg muscles was measured using EMG.

Results: At baseline, FOG+ stood with more trunk flexion than HCs (p = 0.005), a result not found in FOG−. Following a perturbation, FOG+ reacted with increased trunk extension (p = 0.004) in comparison to HCs, a pattern not observed in FOG−. The CoM showed greater backward displacement in FOG− and FOG+ (p = 0.008, p = 0.027). Both FOG+ and FOG− showed increased co-activation of agonist and antagonist muscles compared to HCs (p = 0.010), with no differences between FOG+ and FOG−.

Conclusions: Automatic postural reactions after a sudden perturbation are similar between PD subgroups with and without FOG but different from HCs. Reactive postural control, largely regulated by brain stem centers, seems to be modulated by different mechanisms than those governing freezing of gait. Greater differences in initial stance position, enhanced by joint stiffening, could however underlie maladaptive postural responses and increase susceptibility for balance loss in FOG+ compared to FOG−.

What is the therapeutic mechanism of pedunculopontine nucleus stimulation in Parkinson's disease?

Pedunculopontine nucleus (PPN) deep brain stimulation (DBS) is an experimental treatment for Parkinson's disease (PD) which offers a fairly circumscribed benefit for gait freezing and perhaps balance impairment. The benefit on gait freezing is variable and typically incomplete, which may reflect that the clinical application is yet to be optimised or reflect a fundamental limitation of the therapeutic mechanism. Thus, a better understanding of the therapeutic mechanism of PPN DBS may guide the further development of this therapy. The available evidence supports that the PPN is underactive in PD due to a combination of both degeneration and excessive inhibition. Low frequency PPN DBS could enhance PPN network activity, perhaps via disinhibition. A clinical implication is that in some PD patients, the PPN may be too degenerate for PPN DBS to work. Reaction time studies report that PPN DBS mediates a very specific benefit on pre-programmed movement. This seems relevant to the pathophysiology of gait freezing, which can be argued to reflect impaired release of pre-programmed adjustments to locomotion. Thus, the benefit of PPN DBS on gait freezing could be akin to that mediated by external cues. Alpha band activity is a prominent finding in local field potential recordings from PPN electrodes in PD patients. Alpha band activity is implicated in the suppression of task irrelevant processes and thus the effective allocation of attention (processing resources). Attentional deficits are prominent in patients with PD and gait freezing and PPN alpha activity has been observed to drop out prior to gait freezing episodes and to increase with levodopa. This raises the hypothesis that PPN DBS could support or emulate PPN alpha activity and consequently enhance the allocation of attention. Although PPN DBS has not been convincingly shown to increase general alertness or attention, it remains possible that PPN DBS may enhance the allocation of processing resources within the motor system, or "motor attention". For example, this could facilitate the 'switching' of motor state between continuation of pattern generated locomotion towards the intervention of pre-programmed adjustments. However, if the downstream consequence of PPN DBS on movement is limited to a circumscribed unblocking of pre-programmed movement, then this may have a similarly circumscribed degree of benefit for gait. If this is the case, then it may be possible to identify patients who may benefit most from PPN DBS. For example, those in whom pre-programmed deficits are the major contributors to gait freezing.

Increased foot strike variability in Parkinson's disease patients with freezing of gait

INTRODUCTION

Freezing of gait (FOG) is a debilitating, late motor complication of Parkinson's disease (PD) that occurs in 50-80% of patients. Gait freezing significantly worsens quality of life by decreasing mobility and increasing falls. Studies have shown that patients with episodic freezing episodes also have deficits in continuous gait. We evaluated whether there was an objective gait correlate to the increased stumbling reported by many patients with gait freezing.

METHODS

PD subjects and healthy controls (HC) were enrolled after IRB approval. Subjects with more than 1 fall/day or a Montreal Cognitive Assessment score <10 were excluded. Subjects walked at their normal pace, 8 lengths of a 20 × 4 foot pressure-sensor mat. Data was collected and analyzed using PKMAS software (Protokinetics) and statistical analysis performed using SPSS 22 (IBM).

RESULTS

72 age matched subjects (22 PD FOG, 27 PD no-FOG, and 23 HC) were enrolled. Disease duration and Hoehn & Yahr scores were not significantly different between the PD groups. Mean dimensions of foot strike were not significantly different between groups, but PD FOG subjects had increased step-to-step variability in foot strike as measured by the percent coefficient of variation (%CV) in foot strike length compared to PD no-FOG and HC, independent of stride velocity. In PD no-FOG subjects, fallers also had higher variability in foot strike length compared to non-fallers.

CONCLUSION

PD subjects with FOG had increased variability in foot strike suggesting that in addition to stride length variability, foot strike variability could contribute to imbalance leading to falls.

A meta-analysis of the pedunculopontine nucleus deep-brain stimulation effects on Parkinson's disease

Postural instability and gait disturbance (PIGD) are common symptoms in patients with advanced Parkinson's disease (PD). The deep-brain stimulation (DBS) of the pedunculopontine nucleus (PPN) shows a promising effect on PIGD. However, the quantitative effects of PPN DBS for PD, especially for PIGD, and the efficacy of PPN DBS combined with levodopa are controversial and ambiguous to clinical practice. We carried out a meta-analysis of original researches on PPN to PIGD/PD from the electronic databases MEDLINE and EMBASE. The scores of united Parkinson's disease rating scores (UPDRS) III were used to evaluate the improvement in motor function and the scores of UPDRS III subitems 27-30 were used to evaluate the improvement in PIGD. The outcome was expressed as the mean difference and the percentage change between the scores of baseline and end point. The quality of studies was assessed using the 'assessing risk of bias' table. Overall, 909 articles were screened and seven studies were included with a total of 45 patients. The improvement in PIGD was significant, but there were no such effects on motor function. All the differences in PIGD scores were greater in the OFF medicine state rather than the ON medicine state, especially in the comparison between postsurgery ON stimulation and presurgery. PPN DBS indeed improved PIGD in advanced PD patients. This result could inspire more researchers to focus on its clinical application.

Depression may negatively affect the change in freezing of gait following subthalamic nucleus stimulation in Parkinson's disease

OBJECTIVE

To assess the influence of preoperative depression on the change in freezing of gait (FOG) following subthalamic nucleus stimulation (STN-DBS) in patients with Parkinson's disease (PD).

METHODS

One hundred and twelve PD patients were included who received bilateral STN-DBS. Of these, 33 had no preoperative depression (PD-ND) and the other 79 had preoperative depression (PD-D). Each PD-ND patient was matched with one PD-D patient by the propensity score for which sex, age at PD onset, disease duration, UPDRS-III score during off-medication state, levodopa-equivalent daily dose, and mini mental state examination were the independent variables. We compared both a FOG-questionnaire (FOG-Q) and the axial score from UPDRS-III between the two groups over 12-month follow-up.

RESULTS

During the off-medication state, FOG-Q at 12-month was decreased with STN-DBS in both PD-ND (-52.9%, p < 0.001) and PD-D (-24.2%, p < 0.001) with a significant difference in the change of FOG in favor of PD-ND (p = 0.001). Similarly, there was an improvement in the axial score for both PD-ND (-66.1%, p < 0.001) and PD-D (-45.3%, p < 0.001) at 12-month with a significant difference between the groups. (p = 0.005). During the on-medication state, both the FOG-Q and axial score at 12-month were not improved with STN-DBS in the PD-ND and PD-D with no difference between the groups.

CONCLUSIONS

Our findings suggest that preoperative depression negatively affects the outcome of FOG following STN-DBS in the off-medication state but not in the on-medication state.

Deep Brain Stimulation in Parkinson's Disease: New and Emerging Targets for Refractory Motor and Nonmotor Symptoms

Parkinson's disease (PD) is a progressive neurodegenerative condition characterized by bradykinesia, tremor, rigidity, and postural instability (PI), in addition to numerous nonmotor manifestations. Many pharmacological therapies now exist to successfully treat PD motor symptoms; however, as the disease progresses, it often becomes challenging to treat with medications alone. Deep brain stimulation (DBS) has become a crucial player in PD treatment, particularly for patients who have disabling motor complications from medical treatment. Well-established DBS targets include the subthalamic nucleus (STN), the globus pallidus pars interna (GPi), and to a lesser degree the ventral intermediate nucleus (VIM) of the thalamus. Studies of alternative DBS targets for PD are ongoing, the majority of which have shown some clinical benefit; however, more carefully designed and controlled studies are needed. In the present review, we discuss the role of these new and emerging DBS targets in treating refractory axial motor symptoms and other motor and nonmotor symptoms (NMS).

The caudal pedunculopontine tegmental nucleus may be involved in the regulation of skeletal muscle activity by melanocortin-sympathetic pathway: a virally mediated trans-synaptic tracing study in spinally transected transgenic mice

Understanding neuroanatomical sympathetic circuitry and neuronal connections from the caudal pedunculopontine tegmental nucleus to skeletal muscle is important to the study of possible mechanisms of pedunculopontine tegmental nucleus (PPTg) and cuneiform nucleus (CnF) that are involved in the regulation of skeletal muscle activity of the sympathetic pathway. The aim of this study was to use virus PRV-614 to trace the melanocortin-sympathetic neural pathways from PPTg and CnF to a hindlimb muscle (gastrocnemius) in spinally transected MC4R-GFP transgenic mice. PRV-614 was injected into the gastrocnemius muscle after receiving a complete spinal cord transection below the L2 level. PRV-614/MC4R-GFP and PRV-614/TPH dual-labeled neurons were found in the dissipated parts of PPTg (dpPPTg), but not between the compact parts of PPTg (cpPPTg) and CnF. It is proposed that a hierarchical pathway of neurons within the caudal pedunculopontine tegmental nucleus sends projections to the RVLM, which in turn projects onto the IML sympathetic preganglionic neurons that regulate muscle blood flow through melanocortin-sympathetic signals. Our results collectively indicate that MC4Rs expressed in caudal pedunculopontine tegmental nucleus may be involved in skeletal muscle activity of melanocortin-sympathetic pathways.

Magnetic resonance diffusion tensor imaging for the pedunculopontine nucleus: proof of concept and histological correlation

The pedunculopontine nucleus (PPN) has been proposed as target for deep brain stimulation (DBS) in patients with postural instability and gait disorders due to its involvement in muscle tonus adjustments and control of locomotion. However, it is a deep-seated brainstem nucleus without clear imaging or electrophysiological markers. Some studies suggested that diffusion tensor imaging (DTI) may help guiding electrode placement in the PPN by showing the surrounding fiber bundles, but none have provided a direct histological correlation. We investigated DTI fractional anisotropy (FA) maps from in vivo and in situ post-mortem magnetic resonance images (MRI) compared to histological evaluations for improving PPN targeting in humans. A post-mortem brain was scanned in a clinical 3T MR system in situ. Thereafter, the brain was processed with a special method ideally suited for cytoarchitectonic analyses. Also, nine volunteers had in vivo brain scanning using the same MRI protocol. Images from volunteers were compared to those obtained in the post-mortem study. FA values of the volunteers were obtained from PPN, inferior colliculus, cerebellar crossing fibers and medial lemniscus using histological data and atlas information. FA values in the PPN were significantly lower than in the surrounding white matter region and higher than in areas with predominantly gray matter. In Nissl-stained histologic sections, the PPN extended for more than 10 mm in the rostro-caudal axis being closely attached to the lateral parabrachial nucleus. Our DTI analyses and the spatial correlation with histological findings proposed a location for PPN that matched the position assigned to this nucleus in the literature. Coregistration of neuroimaging and cytoarchitectonic features can add value to help establishing functional architectonics of the PPN and facilitate neurosurgical targeting of this extended nucleus.

Functional Connectivity of the Pedunculopontine Nucleus and Surrounding Region in Parkinson's Disease

Deep brain stimulation of the pedunculopontine nucleus and surrounding region (PPNR) is a novel treatment strategy for gait freezing in Parkinson's disease (PD). However, clinical results have been variable, in part because of the paucity of functional information that might help guide selection of the optimal surgical target. In this study, we use simultaneous magnetoencephalography and local field recordings from the PPNR in seven PD patients, to characterize functional connectivity with distant brain areas at rest. The PPNR was preferentially coupled to brainstem and cingulate regions in the alpha frequency (8-12 Hz) band and to the medial motor strip and neighboring areas in the beta (18-33 Hz) band. The distribution of coupling also depended on the vertical distance of the electrode from the pontomesencephalic line: most effects being greatest in the middle PPNR, which may correspond to the caudal pars dissipata of the pedunculopontine nucleus. These observations confirm the crucial position of the PPNR as a functional node between cortical areas such as the cingulate/ medial motor strip and other brainstem nuclei, particularly in the dorsal pons. In particular they suggest a special role for the middle PPNR as this has the greatest functional connectivity with other brain regions.

Reward and Behavioral Factors Contributing to the Tonic Activity of Monkey Pedunculopontine Tegmental Nucleus Neurons during Saccade Tasks

The pedunculopontine tegmental nucleus (PPTg) in the brainstem plays a role in controlling reinforcement learning and executing conditioned behavior. We previously examined the activity of PPTg neurons in monkeys during a reward-conditioned, visually guided saccade task, and reported that a population of these neurons exhibited tonic responses throughout the task period. These tonic responses might depend on prediction of the upcoming reward, successful execution of the task, or both. Here, we sought to further distinguish these factors and to investigate how each contributes to the tonic neuronal activity of the PPTg. In our normal visually guided saccade task, the monkey initially fixated on the central fixation target (FT), then made saccades to the peripheral saccade target and received a juice reward after the saccade target disappeared. Most of the tonic activity terminated shortly after the reward delivery, when the monkey broke fixation. To distinguish between reward and behavioral epochs, we then changed the task sequence for a block of trials, such that the saccade target remained visible after the reward delivery. Under these visible conditions, the monkeys tended to continue fixating on the saccade target even after the reward delivery. Therefore, the prediction of the upcoming reward and the end of an individual trial were separated in time. Regardless of the task conditions, half of the tonically active PPTg neurons terminated their activity around the time of the reward delivery, consistent with the view that PPTg neurons might send reward prediction signals until the time of reward delivery, which is essential for computing reward prediction error in reinforcement learning. On the other hand, the other half of the tonically active PPTg neurons changed their activity dependent on the task condition. In the normal condition, the tonic responses terminated around the time of the reward delivery, while in the visible condition, the activity continued until the disappearance of the saccade target (ST) after reward delivery. Thus, for these neurons, the tonic activity might be related to maintaining attention to complete fixation behavior. These results suggest that, in addition to the reward value information, some PPTg neurons might contribute to the execution of conditioned task behavior.

The pedunculopontine tegmental nucleus-A functional hypothesis from the comparative literature

We present data from animal studies showing that the pedunculopontine tegmental nucleus-conserved through evolution, compartmentalized, and with a complex pattern of inputs and outputs-has functions that involve formation and updates of action-outcome associations, attention, and rapid decision making. This is in contrast to previous hypotheses about pedunculopontine function, which has served as a basis for clinical interest in the pedunculopontine in movement disorders. Current animal literature points to it being neither a specifically motor structure nor a master switch for sleep regulation. The pedunculopontine is connected to basal ganglia circuitry but also has primary sensory input across modalities and descending connections to pontomedullary, cerebellar, and spinal motor and autonomic control systems. Functional and anatomical studies in animals suggest strongly that, in addition to the pedunculopontine being an input and output station for the basal ganglia and key regulator of thalamic (and consequently cortical) activity, an additional major function is participation in the generation of actions on the basis of a first-pass analysis of incoming sensory data. Such a function-rapid decision making-has very high adaptive value for any vertebrate. We argue that in developing clinical strategies for treating basal ganglia disorders, it is necessary to take an account of the normal functions of the pedunculopontine. We believe that it is possible to use our hypothesis to explain why pedunculopontine deep brain stimulation used clinically has had variable outcomes in the treatment of parkinsonism motor symptoms and effects on cognitive processing. © 2016 International Parkinson and Movement Disorder Society.

Subcortical evoked activity and motor enhancement in Parkinson's disease

Enhancements in motor performance have been demonstrated in response to intense stimuli both in healthy subjects and in the form of 'paradoxical kinesis' in patients with Parkinson's disease. Here we identify a mid-latency evoked potential in local field potential recordings from the region of the subthalamic nucleus, which scales in amplitude with both the intensity of the stimulus delivered and corresponding enhancements in biomechanical measures of maximal handgrips, independent of the dopaminergic state of our subjects with Parkinson's disease. Recordings of a similar evoked potential in the related pedunculopontine nucleus - a key component of the reticular activating system - provide support for this neural signature in the subthalmic nucleus being a novel correlate of ascending arousal, propagated from the reticular activating system to exert an 'energizing' influence on motor circuitry. Future manipulation of this system linking arousal and motor performance may provide a novel approach for the non-dopaminergic enhancement of motor performance in patients with hypokinetic disorders such as Parkinson's disease.

Duration of step initiation predicts freezing in Parkinson's disease

OBJECTIVES

In some individuals with idiopathic Parkinson's disease (PD), freezing of gait episodes develops as the disease progresses. The neural mechanism underlying freezing in PD is poorly understood. Here, we report a 2-year follow-up on the novel discovery of prolonged step initiation duration as a potential marker of impending freezing.

METHODS

Non-freezing PD participants in stages 2.5-4 of the Hoehn and Yahr disease severity scale were recruited from an earlier study which determined the effect of semi-virtual cues on walking. Responders were those who completed the first step faster in the presence of the virtual cues while non-responders either did not change or took longer to complete the first step. Both groups of participants were interviewed 2 years later to determine who had developed freezing of gait.

RESULTS

Participants in the responder group had a 13-fold risk of developing freezing of gait within 2 years following the cueing study (OR=13.3, 95% CI=1.1-167). A cutoff score of -2.6% (i.e., a decrease in the duration of the first step with visual cues by 2.6% relative to no cues) gave a sensitivity and specificity of 100% and 89%, respectively.

CONCLUSIONS

To the best of our knowledge, this is the first novel discovery of a physical predictor of freezing in PD. The time to complete the first step is a simple test to administer in the clinic or at home and may therefore be easily incorporated into a fall prevention training program for PD before the inception of freezing.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.

StartReact effects support different pathophysiological mechanisms underlying freezing of gait and postural instability in Parkinson's disease

Introduction

The pathophysiology underlying postural instability in Parkinson’s disease is poorly understood. The frequent co-existence with freezing of gait raises the possibility of shared pathophysiology. There is evidence that dysfunction of brainstem structures contribute to freezing of gait. Here, we evaluated whether dysfunction of these structures contributes to postural instability as well. Brainstem function was assessed by studying the StartReact effect (acceleration of latencies by a startling acoustic stimulus (SAS)).

Methods

We included 25 patients, divided in two different ways: 1) those with postural instability (HY = 3, n = 11) versus those without (HY<3, n = 14); and 2) those with freezing (n = 11) versus those without freezing (n = 14). We also tested 15 matched healthy controls. We tested postural responses by translating a balance platform in the forward direction, resulting in backward balance perturbations. In 25% of trials, the start of the balance perturbation was accompanied by a SAS.

Results

The amplitude of automatic postural responses and length of the first balance correcting step were smaller in patients with postural instability compared to patients without postural instability, but did not differ between freezers and non-freezers. In contrast, the StartReact effect was intact in patients with postural instability but was attenuated in freezers.

Discussion

We suggest that the mechanisms underlying freezing of gait and postural instability in Parkinson’s disease are at least partly different. Underscaling of automatic postural responses and balance-correcting steps both contribute to postural instability. The attenuated StartReact effect was seen only in freezers and likely reflects inadequate representation of motor programs at upper brainstem level.

Cortical involvement in the StartReact effect

The rapid release of prepared movements by a loud acoustic stimulus capable of eliciting a startle response has been termed the StartReact effect (Valls-Solé et al., 1999), and premotor reaction times (PMTs) of <70 ms are often observed. Two explanations have been given for these short latency responses. The subcortical storage and triggering hypothesis suggests movements that can be prepared in advance of a "go" signal are stored and triggered from subcortical areas by a startling acoustic stimulus (SAS) without cortical involvement. Alternatively, it has been hypothesized that the SAS can trigger movements from cortical areas through a faster pathway ascending from subcortical structures. Two experiments were designed to examine the possible role of the primary motor cortex in the StartReact effect. In Experiment 1, we used suprathreshold transcranial magnetic stimulation (TMS) during the reaction time (RT) interval to induce a cortical silent period in the contralateral primary motor cortex (M1). Thirteen participants performed 20° wrist extension movements as fast as possible in response to either a control stimulus (82 dB) or SAS (124 dB). PMTs for startle trials were faster than for control trials, while TMS significantly delayed movement onset compared to No TMS or Sham TMS conditions. In Experiment 2, we examined the StartReact effect in a highly cortically represented action involving speech of a consonant-vowel (CV) syllable. Similar to previous work examining limb movements, a robust StartReact effect was found. Collectively, these experiments provide evidence for cortical (M1) involvement in the StartReact effect.

StartReact restores reaction time in HSP: evidence for subcortical release of a motor program

Startling acoustic stimuli (SAS) can accelerate reaction times ("StartReact" effect), but the underlying mechanism remains unclear. Both direct release of a subcortically stored motor program and a subcortically mediated trigger for a cortically stored motor program have been hypothesized. To distinguish between these hypotheses, we examined the StartReact effect in humans with pure hereditary spastic paraplegia (HSP). Delayed reaction times in HSP patients in trials both with and without a SAS would argue in favor of a cortically stored response. We instructed 12 HSP patients and 12 matched controls to respond as rapidly as possible to a visual imperative stimulus, in two different conditions: dorsiflexion of the dominant ankle; or flexion of the dominant wrist. In 25% of trials, a SAS was delivered simultaneously with the imperative stimulus. Before these tests, subjects received five SAS while standing to verify normal function of the reticulospinal tract in HSP. Latencies of startle responses in sternocleidomastoid and tibialis anterior muscles were comparable between patients and controls. During the ankle dorsiflexion task, HSP patients had an average 19 ms delay in reaction times compared with controls. Administration of a SAS accelerated ankle dorsiflexion in both groups, but more so in the patients, which completely normalized their latencies. The wrist flexion task yielded no differences in onset latencies between HSP patients and controls. The reticulospinal tract seems unaffected in HSP patients, because startle reflex onsets were normal. The corticospinal tract was affected, as reflected by delayed ankle dorsiflexion reaction times. These delayed onsets in HSP were normalized when the imperative stimulus was combined with a SAS, presumably through release of a subcortically stored motor program conveyed by the preserved reticulospinal tract.

Neural plasticity in human brain connectivity: the effects of long term deep brain stimulation of the subthalamic nucleus in Parkinson's disease

BACKGROUND

Positive clinical outcomes are now well established for deep brain stimulation, but little is known about the effects of long-term deep brain stimulation on brain structural and functional connectivity. Here, we used the rare opportunity to acquire pre- and postoperative diffusion tensor imaging in a patient undergoing deep brain stimulation in bilateral subthalamic nuclei for Parkinson's Disease. This allowed us to analyse the differences in structural connectivity before and after deep brain stimulation. Further, a computational model of spontaneous brain activity was used to estimate the changes in functional connectivity arising from the specific changes in structural connectivity.

RESULTS

We found significant localised structural changes as a result of long-term deep brain stimulation. These changes were found in sensory-motor, prefrontal/limbic, and olfactory brain regions which are known to be affected in Parkinson's Disease. The nature of these changes was an increase of nodal efficiency in most areas and a decrease of nodal efficiency in the precentral sensory-motor area. Importantly, the computational model clearly shows the impact of deep brain stimulation-induced structural alterations on functional brain changes, which is to shift the neural dynamics back towards a healthy regime. The results demonstrate that deep brain stimulation in Parkinson's Disease leads to a topological reorganisation towards healthy bifurcation of the functional networks measured in controls, which suggests a potential neural mechanism for the alleviation of symptoms.

CONCLUSIONS

The findings suggest that long-term deep brain stimulation has not only restorative effects on the structural connectivity, but also affects the functional connectivity at a global level. Overall, our results support causal changes in human neural plasticity after long-term deep brain stimulation and may help to identify the underlying mechanisms of deep brain stimulation.