James W. Stinear

Focal and bidirectional modulation of lower limb motor cortex using anodal transcranial direct current stimulation

BACKGROUND Because we are interested in non-invasive transcranial brain stimulation as an adjuvant to post-stroke walking therapy, we applied direct current stimulation (tDCS) preferentially to either the left or right lower limb motor cortex (M1) in two separate sessions and assessed the resulting modulation in both cortices. OBJECTIVE/HYPOTHESIS We hypothesized that tDCS applied preferentially to one lower limb M1 of healthy subjects would induce between-hemisphere opposite sign modulation. METHODS Transcranial magnetic stimulation (TMS) with the coil offset 2 cm either side of vertex was used to assess the percent change in rectified motor evoked potential (MEP) area recorded bilaterally from vastus lateralis (VL) and tibialis anterior (TA) of 10 subjects during weak tonic contraction. RESULTS ANOVA revealed an up-regulation of the target cortex and a down-regulation of the non-target cortex (p = 0.001) and no effects of hemisphere (left, right) or muscle (TA, VL). Significant modulation was evident in 78% of VL and TA muscles (all p < 0.05). Excitability increased in 60%, but decreased in 18%. For 43% when excitability increased, a simultaneous decrease in excitability was evident in homologous muscle responses providing support for our hypothesis. CONCLUSIONS The results indicate a modest effectiveness and focality of anodal tDCS when applied to lower limb M1, suggesting in a human model that the strength and depth of polarizing cortical currents induced by tDCS likely depend on inter-individual differences in the electrical properties of superficial brain structures.

A paradox: After stroke, the non-lesioned lower limb motor cortex may be maladaptive

What are the neuroplastic mechanisms that allow some stroke patients to regain high‐quality control of their paretic leg, when others do not? One theory implicates ipsilateral corticospinal pathways projecting from the non‐lesioned hemisphere. We devised a new transcranial magnetic stimulation protocol to identify ipsilateral corticospinal tract conductivity from the non‐lesioned hemisphere to the paretic limb in chronic stroke patients. We also assessed corticospinal tract degeneration by diffusion tensor imaging, and used an ankle tracking task to assess lower limb motor control. We found greater tracking error during antiphase bilateral ankle movement for patients with strong conductivity from the non‐lesioned hemisphere to the paretic ankle than for those with weak or no conductivity. These findings suggest that, instead of assisting motor control, contributions to lower limb motor control from the non‐lesioned hemisphere of some stroke survivors may be maladaptive.

Relationships between functional and structural corticospinal tract integrity and walking post stroke

Highlights ► We investigated the relationship between walking impairment after stroke and integrity of the corticospinal tract (CST). ► We used transcranial magnetic stimulation and diffusion tensor imaging to assess CST integrity. ► We demonstrate that patients with more ipsilateral connectivity between the unlesioned M1 and the affected leg had more structural damage to their CST.

Corticospinal tract integrity correlates with knee extensor weakness in chronic stroke survivors

OBJECTIVE Muscle weakness develops rapidly after stroke, adversely affecting motor performance, and contributing to reduced functional ability. While the contributions of structural and functional alterations in skeletal muscle to post-stroke weakness have been well described, the relationship between motor pathway integrity, measured using both radiological and electrophysiological techniques, and post-stroke muscle weakness is not clear. This study sought to determine the role of corticospinal tract (CST) integrity on knee extensor weakness in chronic stroke survivors. METHODS Knee extensor strength and activation testing were performed at 90° of knee flexion using an interpolated triplet technique. CST integrity was evaluated using data obtained from Diffusion Tensor Imaging and transcranial magnetic stimulation. RESULTS Recordings in nine stroke subjects indicated substantial knee extensor weakness and activation deficits in the paretic legs of the stroke survivors. Regression analysis revealed that asymmetry in CST integrity was strongly related to between-leg differences in knee strength. CONCLUSIONS The results of this study suggest a strong link between CST integrity and lower extremity strength, and add to the growing evidence of substantial knee extensor weakness and activation impairments in stroke survivors. SIGNIFICANCE The findings from this study further our understanding of the anatomical and neurophysiological contributions to motor impairments after stroke, which may benefit clinicians and researchers in the field of stroke rehabilitation.

Corticomotor excitability of arm muscles modulates according to static position and orientation of the upper limb

OBJECTIVE We investigated how multi-joint changes in static upper limb posture impact the corticomotor excitability of the posterior deltoid (PD) and biceps brachii (BIC), and evaluated whether postural variations in excitability related directly to changes in target muscle length. METHODS The amplitude of individual motor evoked potentials (MEPs) was evaluated in each of thirteen different static postures. Four functional postures were investigated that varied in shoulder and elbow angle, while the forearm was positioned in each of three orientations. Posture-related changes in muscle lengths were assessed using a biomechanical arm model. Additionally, M-waves were evoked in the BIC in each of three forearm orientations to assess the impact of posture on recorded signal characteristics. RESULTS BIC-MEP amplitudes were altered by shoulder and elbow posture, and demonstrated robust changes according to forearm orientation. Observed changes in BIC-MEP amplitudes exceeded those of the M-waves. PD-MEP amplitudes changed predominantly with shoulder posture, but were not completely independent of influence from forearm orientation. CONCLUSIONS Results provide evidence that overall corticomotor excitability can be modulated according to multi-joint upper limb posture. SIGNIFICANCE The ability to alter motor pathway excitability using static limb posture suggests the importance of posture selection during rehabilitation aimed at retraining individual muscle recruitment and/or overall coordination patterns.

The effects of paired associative stimulation on knee extensor motor excitability of individuals post-stroke: A pilot study

OBJECTIVE Paired associative stimulation (PAS) modulates bilateral distal lower limb motor pathways during walking. We assessed the effects of inhibitory PAS applied to the vastus medialis (VM) motor pathways of chronic stroke patients. METHODS PAS consisted of 120 electrical stimuli applied to the femoral nerve paired with transcranial magnetic stimulation (TMS) of the lower limb primary motor cortex so that the estimated arrival of the afferent volley occurred 8 ms after delivery of the magnetic stimulus. Stimulus pairs were delivered to the non-paretic VM motor system of 11 chronic stroke patients and the right limb motor system of 11 non-impaired subjects at 0.19 Hz. The effects of PAS on VM motor pathway excitability and muscle activity were assessed during pedaling. TMS-induced motor evoked potential (MEP) amplitudes and the percent of VM activity in the flexion phase of active pedaling (% FLEXVM) was examined before and after PAS. RESULTS Inhibitory PAS reduced VM MEP amplitudes in the target limb (p<0.05) of both groups, while post-PAS paretic VM MEP amplitudes increased for some patients and decreased for others. Group mean paretic limb % FLEXVM was not altered by inhibitory PAS. CONCLUSIONS These results indicate PAS can be used to manipulate motor cortical excitability in proximal lower limb representations, however the sign of induced modulation was unpredictable and cyclic muscle activity was not modified. SIGNIFICANCE The study has important implications for the development of therapies involving non-invasive brain stimulation to modify abnormal motor behavior following stroke.

Cortical Stimulation as an Adjuvant to Upper Limb Rehabilitation After Stroke

Recovery of upper limb function after stroke remains a clinical challenge in rehabilitation. New insights into the role of activity‐dependent motor recovery have guided clinicians to develop novel task‐oriented therapies that are effective in reducing functional limitations in hand use after stroke. A number of brain‐stimulation techniques have been examined as therapeutic adjuvants applied to enhance functional outcomes. Cortical stimulation with the use of either noninvasive techniques or implanted technology has shown some promise as an adjuvant therapy but has yet to be supported in well‐designed clinical trials. In this article, we review the physiology of neural plasticity and of cortical stimulation. Laboratory studies and early clinical trials of repetitive transcranial magnetic stimulation, transcranial direct current stimulation, and epidural cortical stimulation are reported. Cortical stimulation may have a role in facilitating motor recovery after stroke, but a better understanding of the physics of cortical stimulation, biological response to stimulation, effective stimulation protocols, and proper patient selection is needed.

Descending control to the nonparetic limb degrades the cyclic activity of paretic leg muscles

During anti-phased locomotor tasks such as cycling or walking, hemiparetic phasing of muscle activity is characterized by inappropriate early onset of activity for some paretic muscles and prolonged activity in others. Pedaling with the paretic limb alone reduces inappropriate prolonged activity, suggesting a combined influence of contralesional voluntary commands and movement-related sensory feedback. Five different non-target leg movement state conditions were performed by 15 subjects post-stroke and 15 nonimpaired controls while they pedaled with the target leg and EMG was recorded bilaterally. Voluntary engagement of the non-lesioned motor system increased prolonged paretic vastus medialis (VM) activity and increased phase-advanced rectus femoris (RF) activity. We suggest bilateral descending commands are primarily responsible for the inappropriate activity in the paretic VM during anti-phase pedaling, and contribute to the dysfunctional motor output in the paretic RF. Findings from controls suggest that even an undamaged motor system can contribute to this phenomenon.