The Strength of the Corticospinal Tract Not the Reticulospinal Tract Determines Upper-Limb Impairment Level and Capacity for Skill-Acquisition in the Sub-Acute Post-Stroke Period

Evidence of the existence of multiple modules for the stroke-caused flexion synergy from Fugl-Meyer assessment scores

Stroke-caused synergies may result from the preferential use of the reticulospinal tract (RST) due to damage to the corticospinal tract. The RST branches multiple motoneuron pools across the arm together resulting in gross motor control or abnormal synergies, and accordingly, the controllability of individual muscles decreases. However, it is not clear whether muscles involuntarily activated by abnormal synergy vary depending on the muscles voluntarily activated when motor commands descend through the RST. Studies showed that abnormal synergies may originate from the merging and reweighting of synergies in individuals without neurological deficits. This leads to a hypothesis that those abnormal synergies are still selectively excited depending on the context. In this study, we test this hypothesis, leveraging the Fugl-Meyer assessment that could characterize the neuroanatomical architecture in individuals with a wide range of impairments. We examine the ability to perform an out-of-synergy movement with the flexion synergy caused by either shoulder or elbow loading. The results reveal that about 14% [8/57, 95% confidence interval (5.0%, 23.1%)] of the participants with severe impairment (total Fugl-Meyer score <29) in the chronic phase (6 months after stroke) are able to keep the elbow extended during shoulder loading and keep the shoulder at neutral during elbow loading. Those participants underwent a different course of neural reorganization, which enhanced abnormal synergies in comparison with individuals with mild impairment (P < 0.05). These results provide evidence that separate routes and synergy modules to motoneuron pools across the arm might exist even if the motor command is mediated possibly via the RST.NEW & NOTEWORTHY We demonstrate that abnormal synergies are still selectively excited depending on the context.

Effects of task-oriented training on dexterous movements of hands in post stroke patients

Objective: The objective of the study was to determine the effect of task-oriented training on the dexterous movements of hands in Hemiplegic post-stroke patients.Methods: This study has been registered on site ClinicalTrial.gov with clinical trial number NCT05201196. The study was Randomized controlled trial, 18 patients were recruited that meet the inclusion criteria, randomly allocated to task-oriented training Group A (n = 9) and Conventional Therapy Group B (n = 9). Both exercise trainings were applied for 45 min/session, 5 times/week for 6 weeks. Fugl-Meyer Assessment Scale Motor, sensory and coordination portion, Wolf Motor Function Scale and Barthel Index were used as outcome measures, assessed patients at Baseline, after 3 weeks and 6 weeks after training. Data were analyzed by SPSS version 23.Results: The results suggested the mean Age was 60.78 ± 9.08 and 61.33 ± 6.78 for Group A and Group B, respectively. Average BMI was 23.66 ± 2.66 for Task-oriented group and 21.36 ± 2.46 for Conventional group. Fugl-Meyer scale shows significant P-value 0.03 post treatment compare to pre-treatment which was .283, Wolf Motor Function test and Barthel Index also showed significant P-values as 0.023 and 0.007, respectively, indicating that Task-oriented training shows more significant improvements than conventional group.Conclusion: Task-based training produced statistically significant as well as clinically meaningful enhancement in the dexterous hand movements of acute and subacute stroke patients than conventional therapy and ultimately improves the functional independence in their daily activities such as feeding, bathing and hygiene.

Corticomotor pathway function and recovery after stroke: a look back and a way forward

Stroke is a leading cause of adult disability that results in motor deficits and reduced independence. Regaining independence relies on motor recovery, particularly regaining function of the hand and arm. This review presents evidence from human studies that have used transcranial magnetic stimulation (TMS) to identify neurophysiological mechanisms underlying upper limb motor recovery early after stroke. TMS studies undertaken at the subacute stage after stroke have identified several neurophysiological factors that can drive motor impairment, including membrane excitability, the recruitment of corticomotor neurons, and glutamatergic and GABAergic neurotransmission. However, the inherent variability and subsequent poor reliability of measures derived from motor evoked potentials (MEPs) limit the use of TMS for prognosis at the individual patient level. Currently, prediction tools that provide the most accurate information about upper limb motor outcomes for individual patients early after stroke combine clinical measures with a simple neurophysiological biomarker based on MEP presence or absence, i.e. MEP status. Here, we propose a new compositional framework to examine MEPs across several upper limb muscles within a threshold matrix. The matrix can provide a more comprehensive view of corticomotor function and recovery after stroke by quantifying the evolution of subthreshold and suprathreshold MEPs through compositional analyses. Our contention is that subthreshold responses might be the most sensitive to reduced output of corticomotor neurons, desynchronized firing of the remaining neurons, and myelination processes that occur early after stroke. Quantifying subthreshold responses might provide new insights into post-stroke neurophysiology and improve the accuracy of prediction of upper limb motor outcomes.

Activity in the pontine reticular nuclei scales with handgrip force in humans

The neural pathways that contribute to force production in humans are currently poorly understood, as the relative roles of the corticospinal tract and brainstem pathways, such as the reticulospinal tract (RST), vary substantially across species. Using functional magnetic resonance imaging (fMRI), we aimed to measure activation in the pontine reticular nuclei (PRN) during different submaximal handgrip contractions to determine the potential role of the PRN in force modulation. Thirteen neurologically intact participants (age: 28 ± 6 yr) performed unilateral handgrip contractions at 25%, 50%, 75% of maximum voluntary contraction during brain scans. We quantified the magnitude of PRN activation from the contralateral and ipsilateral sides during each of the three contraction intensities. A repeated-measures ANOVA demonstrated a significant main effect of force (P = 0.012, [Formula: see text] = 0.307) for PRN activation, independent of side (i.e., activation increased with force for both contralateral and ipsilateral nuclei). Further analyses of these data involved calculating the linear slope between the magnitude of activation and handgrip force for each region of interest (ROI) at the individual-level. One-sample t tests on the slopes revealed significant group-level scaling for the PRN bilaterally, but only the ipsilateral PRN remained significant after correcting for multiple comparisons. We show evidence of task-dependent activation in the PRN that was positively related to handgrip force. These data build on a growing body of literature that highlights the RST as a functionally relevant motor pathway for force modulation in humans.NEW & NOTEWORTHY In this study, we used a task-based functional magnetic resonance imaging (fMRI) paradigm to show that activity in the pontine reticular nuclei scales linearly with increasing force during a handgrip task. These findings directly support recently proposed hypotheses that the reticulospinal tract may play an important role in modulating force production in humans.

Moving toward elucidating alternative motor pathway structures post-stroke: the value of spinal cord neuroimaging

Stroke results in varying levels of motor and sensory disability that have been linked to the neurodegeneration and neuroinflammation that occur in the infarct and peri-infarct regions within the brain. Specifically, previous research has identified a key role of the corticospinal tract in motor dysfunction and motor recovery post-stroke. Of note, neuroimaging studies have utilized magnetic resonance imaging (MRI) of the brain to describe the timeline of neurodegeneration of the corticospinal tract in tandem with motor function following a stroke. However, research has suggested that alternate motor pathways may also underlie disease progression and the degree of functional recovery post-stroke. Here, we assert that expanding neuroimaging techniques beyond the brain could expand our knowledge of alternate motor pathway structure post-stroke. In the present work, we will highlight findings that suggest that alternate motor pathways contribute to post-stroke motor dysfunction and recovery, such as the reticulospinal and rubrospinal tract. Then we review imaging and electrophysiological techniques that evaluate alternate motor pathways in populations of stroke and other neurodegenerative disorders. We will then outline and describe spinal cord neuroimaging techniques being used in other neurodegenerative disorders that may provide insight into alternate motor pathways post-stroke.

Paired DBS and TMS Reveals Dentato-Cortical Facilitation Underlying Upper Extremity Movement in Chronic Stroke Survivors

BACKGROUND

Cerebellum shares robust di-synaptic dentato-thalamo-cortical (DTC) connections with the contralateral motor cortex. Preclinical studies have shown that DTC are excitatory in nature. Structural integrity of DTC is associated with better upper extremity (UE) motor function in people with stroke, indicating DTC are important for cerebellar influences on movement. However, there is a lack of understanding of physiologic influence of DTC in humans, largely due to difficulty in accessing the dentate nucleus.

OBJECTIVE

Characterize DTC physiology using dentate nucleus deep brain stimulation (DBS) combined with transcranial magnetic stimulation (TMS) in stroke.

METHODS

Nine chronic stroke survivors with moderate-to-severe UE impairment (Fugl-Meyer 13-38) underwent a paired DBS-TMS experiment before receiving experimental dentate nucleus DBS in our first-in-human phase I trial (Baker et al., 2023, Nature Medicine). Conditioning DBS pulses were given to dentate nucleus 1 to 10 ms prior to supra-threshold TMS pulses given to ipsilesional motor cortex. Effects were assessed on motor evoked potentials (MEPs). Size of DBS-conditioned MEPs was expressed relative to TMS MEPs, where values >1 indicate facilitation.

RESULTS

Dentate nucleus DBS led to facilitation of MEPs at short-latency intervals (3.5 and 5 ms, P = .049 and .021, respectively), a phenomenon we have termed dentato-cortical facilitation (DCF). Higher DCF was observed among patients with more severe UE impairment. Diffusion tensor imaging revealed microstructure of thalamo-cortical portion of DTC was related to higher corticomotor excitability.

CONCLUSIONS

Our in vivo investigation reveals for the first time in humans the intrinsic excitatory properties of DTC, which can serve as a novel therapeutic target for post-stroke motor recovery.

Brain computer interface training with motor imagery and functional electrical stimulation for patients with severe upper limb paresis after stroke: a randomized controlled pilot trial

BACKGROUND

Restorative Brain-Computer Interfaces (BCI) that combine motor imagery with visual feedback and functional electrical stimulation (FES) may offer much-needed treatment alternatives for patients with severely impaired upper limb (UL) function after a stroke.

OBJECTIVES

This study aimed to examine if BCI-based training, combining motor imagery with FES targeting finger/wrist extensors, is more effective in improving severely impaired UL motor function than conventional therapy in the subacute phase after stroke, and if patients with preserved cortical-spinal tract (CST) integrity benefit more from BCI training.

METHODS

Forty patients with severe UL paresis (< 13 on Action Research Arm Test (ARAT) were randomized to either a 12-session BCI training as part of their rehabilitation or conventional UL rehabilitation. BCI sessions were conducted 3-4 times weekly for 3-4 weeks. At baseline, Transcranial Magnetic Stimulation (TMS) was performed to examine CST integrity. The main endpoint was the ARAT at 3 months post-stroke. A binominal logistic regression was conducted to examine the effect of treatment group and CST integrity on achieving meaningful improvement. In the BCI group, electroencephalographic (EEG) data were analyzed to investigate changes in event-related desynchronization (ERD) during the course of therapy.

RESULTS

Data from 35 patients (15 in the BCI group and 20 in the control group) were analyzed at 3-month follow-up. Few patients (10/35) improved above the minimally clinically important difference of 6 points on ARAT, 5/15 in the BCI group, 5/20 in control. An independent-samples Mann-Whitney U test revealed no differences between the two groups, p = 0.382. In the logistic regression only CST integrity was a significant predictor for improving UL motor function, p = 0.007. The EEG analysis showed significant changes in ERD of the affected hemisphere and its lateralization only during unaffected UL motor imagery at the end of the therapy.

CONCLUSION

This is the first RCT examining BCI training in the subacute phase where only patients with severe UL paresis were included. Though more patients in the BCI group improved relative to the group size, the difference between the groups was not significant. In the present study, preserved CTS integrity was much more vital for UL improvement than which type of intervention the patients received. Larger studies including only patients with some preserved CST integrity should be attempted.

POTENTIATION OF CORTICO-SPINAL OUTPUT VIA TARGETED ELECTRICAL STIMULATION OF THE MOTOR THALAMUS

Cerebral white matter lesions prevent cortico-spinal descending inputs from effectively activating spinal motoneurons, leading to loss of motor control. However, in most cases, the damage to cortico-spinal axons is incomplete offering a potential target for new therapies aimed at improving volitional muscle activation. Here we hypothesized that, by engaging direct excitatory connections to cortico-spinal motoneurons, stimulation of the motor thalamus could facilitate activation of surviving cortico-spinal fibers thereby potentiating motor output. To test this hypothesis, we identified optimal thalamic targets and stimulation parameters that enhanced upper-limb motor evoked potentials and grip forces in anesthetized monkeys. This potentiation persisted after white matter lesions. We replicated these results in humans during intra-operative testing. We then designed a stimulation protocol that immediately improved voluntary grip force control in a patient with a chronic white matter lesion. Our results show that electrical stimulation targeting surviving neural pathways can improve motor control after white matter lesions.

The Evolution of Hand Proprioceptive and Motor Impairments in the Sub-Acute Phase After Stroke

BACKGROUND

Hand proprioception is essential for fine movements and therefore many activities of daily living. Although frequently impaired after stroke, it is unclear how hand proprioception evolves in the sub-acute phase and whether it follows a similar pattern of changes as motor impairments.

OBJECTIVE

This work investigates whether there is a corresponding pattern of changes over time in hand proprioception and motor function as comprehensively quantified by a combination of robotic, clinical, and neurophysiological assessments.

METHODS

Finger proprioception (position sense) and motor function (force, velocity, range of motion) were evaluated using robotic assessments at baseline (<3 months after stroke) and up to 4 weeks later (discharge). Clinical assessments (among others, Box & Block Test [BBT]) as well as Somatosensory/Motor Evoked Potentials (SSEP/MEP) were additionally performed.

RESULTS

Complete datasets from 45 participants post-stroke were obtained. For 42% of all study participants proprioception and motor function had a dissociated pattern of changes (only 1 function considerably improved). This dissociation was either due to the absence of a measurable impairment in 1 modality at baseline, or due to a severe lesion of central somatosensory or motor tracts (absent SSEP/MEP). Better baseline BBT correlated with proprioceptive gains, while proprioceptive impairment at baseline did not correlate with change in BBT.

CONCLUSIONS

Proprioception and motor function frequently followed a dissociated pattern of changes in sub-acute stroke. This highlights the importance of monitoring both functions, which could help to further personalize therapies.

Identifying the role of the reticulospinal tract for strength and motor recovery: A scoping review of nonhuman and human studies

In addition to the established postural control role of the reticulospinal tract (RST), there has been an increasing interest on its involvement in strength, motor recovery, and other gross motor functions. However, there are no reviews that have systematically assessed the overall motor function of the RST. Therefore, we aimed to determine the role of the RST underpinning motor function and recovery. We performed a literature search using Ovid Medline, Embase, CINAHL Plus, and Scopus to retrieve papers using key words for RST, strength, and motor recovery. Human and animal studies which assessed the role of RST were included. Studies were screened and 32 eligible studies were included for the final analysis. Of these, 21 of them were human studies while the remaining were on monkeys and rats. Seven experimental animal studies and four human studies provided evidence for the involvement of the RST in motor recovery, while two experimental animal studies and eight human studies provided evidence for strength gain. The RST influenced gross motor function in two experimental animal studies and five human studies. Overall, the RST has an important role for motor recovery, gross motor function and at least in part, underpins strength gain. The role of RST for strength gain in healthy people and its involvement in spasticity in a clinical population has been limitedly described. Further studies are required to ascertain the role of the RST's role in enhancing strength and its contribution to the development of spasticity.

Force oscillations underlying precision grip in humans with lesioned corticospinal tracts

Stability of precision grip depends on the ability to regulate forces applied by the digits. Increased frequency composition and temporal irregularity of oscillations in the force signal are associated with enhanced force stability, which is thought to result from increased voluntary drive along the corticospinal tract (CST). There is limited knowledge of how these oscillations in force output are regulated in the context of dexterous hand movements like precision grip, which are often impaired by CST damage due to stroke. The extent of residual CST volume descending from primary motor cortex may help explain the ability to modulate force oscillations at higher frequencies. Here, stroke survivors with longstanding hand impairment (n = 17) and neurologically-intact controls (n = 14) performed a precision grip task requiring dynamic and isometric muscle contractions to scale and stabilize forces exerted on a sensor by the index finger and thumb. Diffusion spectrum imaging was used to quantify total white matter volume within the residual and intact CSTs of stroke survivors (n = 12) and CSTs of controls (n = 14). White matter volumes within the infarct region and an analogous portion of overlap with the CST, mirrored onto the intact side, were also quantified in stroke survivors. We found reduced ability to stabilize force and more restricted frequency ranges in force oscillations of stroke survivors relative to controls; though, more broadband, irregular output was strongly related to force-stabilizing ability in both groups. The frequency composition and temporal irregularity of force oscillations observed in stroke survivors did not correlate with maximal precision grip force, suggesting that it is not directly related to impaired force-generating capacity. The ratio of residual to intact CST volumes contained within infarct and mirrored compartments was associated with more broadband, irregular force oscillations in stroke survivors. Our findings provide insight into granular aspects of dexterity altered by corticospinal damage and supply preliminary evidence to support that the ability to modulate force oscillations at higher frequencies is explained, at least in part, by residual CST volume in stroke survivors.

Epidural stimulation of the cervical spinal cord for post-stroke upper-limb paresis

Cerebral strokes can disrupt descending commands from motor cortical areas to the spinal cord, which can result in permanent motor deficits of the arm and hand. However, below the lesion, the spinal circuits that control movement remain intact and could be targeted by neurotechnologies to restore movement. Here we report results from two participants in a first-in-human study using electrical stimulation of cervical spinal circuits to facilitate arm and hand motor control in chronic post-stroke hemiparesis ( NCT04512690 ). Participants were implanted for 29 d with two linear leads in the dorsolateral epidural space targeting spinal roots C3 to T1 to increase excitation of arm and hand motoneurons. We found that continuous stimulation through selected contacts improved strength (for example, grip force +40% SCS01; +108% SCS02), kinematics (for example, +30% to +40% speed) and functional movements, thereby enabling participants to perform movements that they could not perform without spinal cord stimulation. Both participants retained some of these improvements even without stimulation and no serious adverse events were reported. While we cannot conclusively evaluate safety and efficacy from two participants, our data provide promising, albeit preliminary, evidence that spinal cord stimulation could be an assistive as well as a restorative approach for upper-limb recovery after stroke.

Home-based portable fNIRS-derived cortical laterality correlates with impairment and function in chronic stroke

Introduction

Improved understanding of the relationship between post-stroke rehabilitation interventions and functional motor outcomes could result in improvements in the efficacy of post-stroke physical rehabilitation. The laterality of motor cortex activity (M1-LAT) during paretic upper-extremity movement has been documented as a useful biomarker of post-stroke motor recovery. However, the expensive, labor intensive, and laboratory-based equipment required to take measurements of M1-LAT limit its potential clinical utility in improving post-stroke physical rehabilitation. The present study tested the ability of a mobile functional near-infrared spectroscopy (fNIRS) system (designed to enable independent measurement by stroke survivors) to measure cerebral hemodynamics at the motor cortex in the homes of chronic stroke survivors.

Methods

Eleven chronic stroke survivors, ranging widely in their level of upper-extremity motor deficit, used their stroke-affected upper-extremity to perform a simple unilateral movement protocol in their homes while a wireless prototype fNIRS headband took measurements at the motor cortex. Measures of participants' upper-extremity impairment and function were taken.

Results

Participants demonstrated either a typically lateralized response, with an increase in contralateral relative oxyhemoglobin (ΔHbO), or response showing a bilateral pattern of increase in ΔHbO during the motor task. During the simple unilateral task, M1-LAT correlated significantly with measures of both upper-extremity impairment and function, indicating that participants with more severe motor deficits had more a more atypical (i.e., bilateral) pattern of lateralization.

Discussion

These results indicate it is feasible to gain M1-LAT measures from stroke survivors in their homes using fNIRS. These findings represent a preliminary step toward the goals of using ergonomic functional neuroimaging to improve post-stroke rehabilitative care, via the capture of neural biomarkers of post-stroke motor recovery, and/or via use as part of an accessible rehabilitation brain-computer-interface.

Flexor carpi radialis H-reflex in different body positions in patients with post-stroke

Background

Spinal stretch reflex (SSR) hyperexcitability reflected by the H-reflex has been reported in more strongly affected extremities after stroke. The H-reflex in the lower extremities is modulated by body position normally and alternatively modulated post-stroke.

Objective

This study aimed to preliminarily explore how upper extremity (UE) H-reflexes are modulated by body position after stroke, which remains unknown.

Materials and methods

Three patients after stroke with hemiparesis/hemiplegia were included. Bilateral flexor carpi radialis (FCR) H-reflexes were examined in the supine position while standing. Other clinical evaluations include the modified Ashworth scale (MAS) and postural stability measurement.

Results

The three cases herein showed that (1) SSR excitability was higher in more strongly affected UEs than less-affected UEs, (2) down-modulation of SSR excitability occurred in less-affected UEs in static standing compared with the supine position, but modulation of SSR excitability in more-affected UEs varied, and (3) bilateral UE SSR excitability in case 3 was down-modulated the most. Moreover, case 3 showed no difference in muscle tone of the more affected UE between supine and standing positions, and case 3 showed the best postural stability.

Conclusion

Spinal stretch reflex hyperexcitability in strongly affected UEs could commonly occur in different phases of recovery after stroke. Down-modulation of SSR excitability could occur in less-affected UEs in the standing position compared with the supine position, while modulation of SSR excitability might be altered in strongly affected UEs and vary in different phases of recovery. There could be some correlation between postural control and UE SSR hyperexcitability. The H-reflex may help to offer a new perspective on rehabilitation evaluation and interventions to promote UE motor control after stroke.

Poststroke arm and hand paresis: should we target the cervical spinal cord?

Despite advances in understanding of corticospinal motor control and stroke pathophysiology, current rehabilitation therapies for poststroke upper limb paresis have limited efficacy at the level of impairment. To address this problem, we make the conceptual case for a new treatment approach. We first summarize current understanding of motor control deficits in the arm and hand after stroke and their shared physiological mechanisms with spinal cord injury (SCI). We then review studies of spinal cord stimulation (SCS) for recovery of locomotion after SCI, which provide convincing evidence for enhancement of residual corticospinal function. By extrapolation, we argue for using cervical SCS to restore upper limb motor control after stroke.

Upper Limbs Muscle Co-contraction Changes Correlated With the Impairment of the Corticospinal Tract in Stroke Survivors: Preliminary Evidence From Electromyography and Motor-Evoked Potential

Objective

Increased muscle co-contraction of the agonist and antagonist muscles during voluntary movement is commonly observed in the upper limbs of stroke survivors. Much remain to be understood about the underlying mechanism. The aim of the study is to investigate the correlation between increased muscle co-contraction and the function of the corticospinal tract (CST).

Methods

Nine stroke survivors and nine age-matched healthy individuals were recruited. All the participants were instructed to perform isometric maximal voluntary contraction (MVC) and horizontal task which consist of sponge grasp, horizontal transportation, and sponge release. We recorded electromyography (EMG) activities from four muscle groups during the MVC test and horizontal task in the upper limbs of stroke survivors. The muscle groups consist of extensor digitorum (ED), flexor digitorum (FD), triceps brachii (TRI), and biceps brachii (BIC). The root mean square (RMS) of EMG was applied to assess the muscle activation during horizontal task. We adopted a co-contraction index (CI) to evaluate the degree of muscle co-contraction. CST function was evaluated by the motor-evoked potential (MEP) parameters, including resting motor threshold, amplitude, latency, and central motor conduction time. We employed correlation analysis to probe the association between CI and MEP parameters.

Results

The RMS, CI, and MEP parameters on the affected side showed significant difference compared with the unaffected side of stroke survivors and the healthy group. The result of correlation analysis showed that CI was significantly correlated with MEP parameters in stroke survivors.

Conclusion

There existed increased muscle co-contraction and impairment in CST functionality on the affected side of stroke survivors. The increased muscle co-contraction was correlated with the impairment of the CST. Intervention that could improve the excitability of the CST may contribute to the recovery of muscle discoordination in the upper limbs of stroke survivors.

Shared and distinct voxel-based lesion-symptom mappings for spasticity and impaired movement in the hemiparetic upper limb

Hemiparesis and spasticity are common co-occurring manifestations of hemispheric stroke. The relationship between impaired precision and force in voluntary movement (hemiparesis) and the increment in muscle tone that stems from dysregulated activity of the stretch reflex (spasticity) is far from clear. Here we aimed to elucidate whether variation in lesion topography affects hemiparesis and spasticity in a similar or dis-similar manner. Voxel-based lesion-symptom mapping (VLSM) was used to assess the impact of lesion topography on (a) upper limb paresis, as reflected by the Fugl-Meyer Assessment scale for the upper limb and (b) elbow flexor spasticity, as reflected by the Tonic Stretch Reflex Threshold, in 41 patients with first-ever stroke. Hemiparesis and spasticity were affected by damage to peri-Sylvian cortical and subcortical regions and the putamen. Hemiparesis (but not spasticity) was affected by damage to the corticospinal tract at corona-radiata and capsular levels, and by damage to white-matter association tracts and additional regions in the temporal cortex and pallidum. VLSM conjunction analysis showed only a minor overlap of brain voxels where the existence of damage affected both hemiparesis and spasticity, suggesting that control of voluntary movement and regulation of muscle tone at rest involve largely separate parts of the motor network.

Functional neurological disorder: Extending the diagnosis to other disorders, and proposing an alternate disease term—Attentionally-modifiable disorder

BACKGROUND: The term “functional neurological disorder,” or “FND,” applies to disorders whose occurrence of neurological symptoms fluctuate with the patient’s attention to them. However, many other disorders that are not called “FND” nonetheless can also follow this pattern. Consequently, guidelines are unclear for diagnosing “FND.” OBJECTIVE: To review the neurological conditions that follow this pattern, but which have not so far been termed “FND,” to understand their overlap with conditions that have been termed “FND,” and to discuss the rationale for why FND has not been diagnosed for them. METHOD: A systematic review of the PubMed literature registry using the terms “fluctuation,” “inconsistency,” or “attention” did not yield much in the way of these candidate disorders. Consequently, this review instead relied on the author’s personal library of peer-reviewed studies of disorders that have resembled FND but which were not termed this way, due to his longstanding interest in this problem. Consequently, this approach was not systematic and was subjective regarding disease inclusion. RESULTS: This review identified numerous, diverse conditions that generally involve fluctuating neurological symptoms that can vary with the person’s attention to them, but which have not been called “FND.” The literature was unclear for reasons for not referring to “FND” in these instances. CONCLUSION: Most likely because of historical biases, the use of the term “FND” has been unnecessarily restricted. Because at its core FND is an attentionally-influenced disorder that can respond well to behavioral treatments, the field of neurological rehabilitation could benefit by extending the range of conditions that could be considered as “FND” and referred for similar behavioral treatments. Because the term “FND” has been viewed unfavorably by some patients and clinical practitioners and whose treatment is not implied, the alternative term attentionally-modifiable disorder is proposed.

Temporal Profile of Descending Cortical Modulation of Spinal Excitability: Group and Individual-Specific Effects

Sensorimotor control is modulated through complex interactions between descending corticomotor pathways and ascending sensory inputs. Pairing sub-threshold transcranial magnetic stimulation (TMS) with peripheral nerve stimulation (PNS) modulates the Hoffmann’s reflex (H-reflex), providing a neurophysiologic probe into the influence of descending cortical drive on spinal segmental circuits. However, individual variability in the timing and magnitude of H-reflex modulation is poorly understood. Here, we varied the inter-stimulus interval (ISI) between TMS and PNS to systematically manipulate the relative timing of convergence of descending TMS-induced volleys with respect to ascending PNS-induced afferent volleys in the spinal cord to: (1) characterize effective connectivity between the primary motor cortex (M1) and spinal circuits, mediated by both direct, fastest-conducting, and indirect, slower-conducting descending pathways; and (2) compare the effect of individual-specific vs. standard ISIs. Unconditioned and TMS-conditioned H-reflexes (24 different ISIs ranging from −6 to 12 ms) were recorded from the soleus muscle in 10 able-bodied individuals. The magnitude of H-reflex modulation at individualized ISIs (earliest facilitation delay or EFD and individual-specific peak facilitation) was compared with standard ISIs. Our results revealed a significant effect of ISI on H-reflex modulation. ISIs eliciting earliest-onset facilitation (EFD 0 ms) ranged from −3 to −5 ms across individuals. No difference in the magnitude of facilitation was observed at EFD 0 ms vs. a standardized short-interval ISI of −1.5 ms. Peak facilitation occurred at longer ISIs, ranging from +3 to +11 ms. The magnitude of H-reflex facilitation derived using an individual-specific peak facilitation was significantly larger than facilitation observed at a standardized longer-interval ISI of +10 ms. Our results suggest that unique insights can be provided with individual-specific measures of top-down effective connectivity mediated by direct and/or fastest-conducting pathways (indicated by the magnitude of facilitation observed at EFD 0 ms) and other descending pathways that encompass relatively slower and/or indirect connections from M1 to spinal circuits (indicated by peak facilitation and facilitation at longer ISIs). By comprehensively characterizing the temporal profile and inter-individual variability of descending modulation of spinal reflexes, our findings provide methodological guidelines and normative reference values to inform future studies on neurophysiological correlates of the complex array of descending neural connections between M1 and spinal circuits.