Input-output organization of the foot motor area in humans

The use of transcranial magnetic stimulation to evaluate cortical excitability of lower limb musculature: Challenges and opportunities

Neuroplasticity is a fundamental yet relatively unexplored process that can impact rehabilitation of lower extremity (LE) movements. Transcranial magnetic stimulation (TMS) has gained widespread application as a non-invasive brain stimulation technique for evaluating neuroplasticity of the corticospinal pathway. However, a majority of TMS studies have been performed on hand muscles, with a paucity of TMS investigations focused on LE muscles. This perspective review paper proposes that there are unique methodological challenges associated with using TMS to evaluate corticospinal excitability of lower limb muscles. The challenges include: (1) the deeper location of the LE motor homunculus; (2) difficulty with targeting individual LE muscles during TMS; and (3) differences in corticospinal circuity controlling upper and lower limb muscles. We encourage future investigations that modify traditional methodological approaches to help address these challenges. Systematic TMS investigations are needed to determine the extent of overlap in corticomotor maps for different LE muscles. A simple, yet informative methodological solution involves simultaneous recordings from multiple LE muscles, which will provide the added benefit of observing how other relevant muscles co-vary in their responses during targeted TMS assessment directed toward a specific muscle. Furthermore, conventionally used TMS methods (e.g., determination of hot spot location and motor threshold) may need to be modified for TMS studies involving LE muscles. Additional investigations are necessary to determine the influence of testing posture as well as activation state of adjacent and distant LE muscles on TMS-elicited responses. An understanding of these challenges and solutions specific to LE TMS will improve the ability of neurorehabilitation clinicians to interpret TMS literature, and forge novel future directions for neuroscience research focused on elucidating neuroplasticity processes underlying locomotion and gait training.

Double-Cone Coil TMS Stimulation of the Medial Cortex Inhibits Central Pain Habituation

Objective

The aim of this study was to investigate whether Transcranial Magnetic Stimulation (TMS) applied over the medial line of the scalp affects the subjective perception of continuous pain induced by means of electric stimulation. In addition, we wanted to identify the point of stimulation where this effect was maximum.

Methods

Superficial electrical stimulation was used to induce continuous pain on the dominant hand. At the beginning of the experiment we reached a pain rating of 5 on an 11-point numeric rating scale (NRS; 0 = no pain and 10 = maximum tolerable pain) for each subject by setting individually the current intensity. The TMS (five pulses at increasing intensities) was applied on 5 equidistant points (one per session) over the medial line of the scalp in 13 healthy volunteers using a double-cone coil to stimulate underlying parts of the brain cortex. In every experimental session the painful stimulation lasted 45 minutes, during which pain and distress intensities NRS were recorded continuously. We calculated the effect of adaptation and the immediate effect of the TMS stimulation for all locations. Additionally, an ALE (Activation Likelihood Estimation) meta-analysis was performed to compare our results with the neuroimaging literature on subjective pain rating.

Results

TMS stimulation temporarily decreased the pain ratings, and pain adaptation was suppressed when applying the TMS over the FCz site on the scalp. No effect was found for distress ratings.

Conclusions

The present data suggest that the medial cortex in proximity of the cingulated gyrus has a causal role in adaptation mechanisms and in processing ongoing pain and subjective sensation of pain intensity.

Exploring the physiological effects of double-cone coil TMS over the medial frontal cortex on the anterior cingulate cortex: an H2(15)O PET study

Transcranial magnetic stimulation (TMS) using a double-cone coil over the medial frontal cortex has the potential to clarify the function of the anterior cingulate cortex (ACC) in cognition, emotion and mood disorders. Following demonstration of disruption of performance on psychological tasks closely linked to cingulate function using this TMS technique, the current study aimed to directly measure the regional distribution of physiological effects of stimulation in the brain with H2(15)O PET. Experiment 1 assessed the effect of increasing numbers of pulse trains of TMS on regional cerebral blood flow (rCBF). Experiment 2 assessed the capacity of medial frontal TMS to modulate brain activity associated with the Stroop task using medial parietal TMS as a control site of stimulation. SPM99 analyses, using the ACC as a region of interest, revealed clusters of increased rCBF during medial frontal TMS in Brodmann area 24 and reduced rCBF in more ventral ACC, the latter occurring in both experiments. In a whole-brain analysis, striking changes in rCBF were observed distal to the ACC following medial frontal TMS. Although TMS reliably affected Stroop task performance in early trials, there was no interaction between TMS and Stroop condition in rCBF. Our results suggest that medial frontal TMS using the double-cone coil can affect ACC activity. However, a number of more distal cortical areas were also affected in these experiments. These additional changes may reflect either 'downstream' effects of altered cingulate cortex activity or direct effects of the coil.

Identifying brain regions for integrative sensorimotor processing with ankle movements

The objective of this study was to define cortical and subcortical structures activated during both active and passive movements of the ankle, which have a fundamental role in the physiology of locomotion, to improve our understanding of brain sensorimotor integration. Sixteen healthy subjects, all right-foot dominant, performed a dorsi-plantar flexion task of the foot using a custom-made wooden manipulandum, which enabled measurements of the movement amplitude. All subjects underwent a training session, which included surface electromyography, and were able to relax completely during passive movements. Patterns of activation during active and passive movements and differences between functional MRI (fMRI) responses for the two types of movement were assessed. Regions of common activation during the active and passive movements were identified by conjunction analysis. We found that passive movements activated cortical regions that were usually similar in location to those activated by active movements, although the extent of the activations was more limited with passive movements. Active movements of both feet generated greater activation than passive movements in some regions (such as the ipsilateral primary motor cortex) identified in previous studies as being important for motor planning. Common activations during active and passive movements were found not only in the contralateral primary motor and sensory cortices, but also in the premotor cortical regions (such as the bilateral rolandic operculum and contralateral supplementary motor area), and in the subcortical regions (such as the ipsilateral cerebellum and contralateral putamen), suggesting that these regions participate in sensorimotor integration for ankle movements. In future, similar fMRI studies using passive movements have potential to elucidate abnormalities of sensorimotor integration in central nervous system diseases that affect motor function.

Identifying brain regions for integrative sensorimotor processing with ankle movements

The objective of this study was to define cortical and subcortical structures activated during both active and passive movements of the ankle, which have a fundamental role in the physiology of locomotion, to improve our understanding of brain sensorimotor integration. Sixteen healthy subjects, all right-foot dominant, performed a dorsi-plantar flexion task of the foot using a custom-made wooden manipulandum, which enabled measurements of the movement amplitude. All subjects underwent a training session, which included surface electromyography, and were able to relax completely during passive movements. Patterns of activation during active and passive movements and differences between functional MRI (fMRI) responses for the two types of movement were assessed. Regions of common activation during the active and passive movements were identified by conjunction analysis. We found that passive movements activated cortical regions that were usually similar in location to those activated by active movements, although the extent of the activations was more limited with passive movements. Active movements of both feet generated greater activation than passive movements in some regions (such as the ipsilateral primary motor cortex) identified in previous studies as being important for motor planning. Common activations during active and passive movements were found not only in the contralateral primary motor and sensory cortices, but also in the premotor cortical regions (such as the bilateral rolandic operculum and contralateral supplementary motor area), and in the subcortical regions (such as the ipsilateral cerebellum and contralateral putamen), suggesting that these regions participate in sensorimotor integration for ankle movements. In future, similar fMRI studies using passive movements have potential to elucidate abnormalities of sensorimotor integration in central nervous system diseases that affect motor function.

Towards an understanding of gait control: brain activation during the anticipation, preparation and execution of foot movements

While a detailed understanding of brain activity with hand movements has developed, less is known about the functional anatomy of motor control for foot movements. Here we have used fMRI to define brain activity associated with unilateral foot extension and flexion, component movements of gait. We studied brain responses to visually cued active and passive movements and periods of either preparation (before active movement) or anticipation (before passive movement) with a pseudo-randomized block design. A mixed-effects (n = 12) contrast of the active movement condition vs. rest identified brain activation in regions including the medial wall of the primary sensorimotor cortex, consistent with expected somatotopy. Medial wall activation during passive movement vs. rest was less intense and localized to the same region. Frontal and association cortices were more active during preparation or anticipation periods than during the movements themselves. A contrast of preparation to move vs. active movement showed significant activation in the medial frontal and frontopolar gyri and the precuneus. Contrast of the anticipation of movement with the passive movement condition revealed activation in the dorsal premotor cortex and precuneus. Our study thus provides evidence for somatotopy in multiple functional regions in the motor control network. The anterior prefrontal activity is involved in the preparation for cued movement with distinct regions of the medial motor cortex (including SMA and CMA) preferentially involved in motor program planning and execution. This direct characterization of brain activation patterns associated with foot movements promises use of fMRI for the functional analysis of pathologies of gait.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

Effects of a photic input on the human cortico-motoneuron connection

OBJECTIVES

Disease manifestations such as photic cortical reflex myoclonus or myoclonus due to intermittent light stimulation rely on a pathologic interaction between non-structured visual inputs and the corticospinal system. We wanted to assess the normal interaction, if any, between a prior photic input and the output of the cortico-motoneuron connection.

METHODS

In 9 consenting healthy subjects we quantified the changes exerted by a sudden, unexpected bright light flash on (i) the motor potentials (MEPs) evoked in the right first dorsal interosseous muscle (FDI) by transcranial magnetic or electrical stimulation (TMS/TES) of the primary motor cortex, (ii) the FDI F-waves and (iii) the soleus H-wave. Separately, we measured the simple reaction times to the flash itself. All determinations were repeated twice with an interval of 2-24 months.

RESULTS

When the flash preceded TMS by 55-70 ms, the MEP size was reduced, while at interstimulus intervals (ISIs) of 90-130 ms it was enlarged. Statistical significance (P<0.05) emerged at ISIs of 55, 70, 100, 105 and 120 ms. Conversely, the MEP latency was prolonged at ISIs of 55-70 ms and shortened at ISIs of 90-130 ms (P<0.05 at ISIs of 55, 110 and 130 ms). Electrical MEPs were enhanced at an ISI of 120 ms. The F-wave size showed a non-significant trend of enhancement at ISIs of 90-130 ms. The soleus H-wave showed significant enlargement at ISIs of 90-130 ms (P<0.05 at ISIs of 100 and 105 ms). The minimum reaction time was on average 120 ms.

CONCLUSIONS

An unexpected photic input, to which no reaction is planned, can cause an early inhibition of the responses to TMS. We think its origin lies within the primary motor cortex, since it is not associated with changes in spinal excitability or electrical MEPs. A later facilitation persists using TES and has a temporal relationship with an enlargement of the soleus H-wave. Thus, it likely results from activation of descending (possibly reticulospinal) fibers that excite the spinal motor nucleus.

Predominant activation of I1-waves from the leg motor area by transcranial magnetic stimulation

We performed transcranial magnetic stimulation (TMS) to elucidate the D- and I-wave components comprising the motor evoked potentials (MEPs) elicited from the leg motor area, especially at near-threshold intensity. Recordings were made from the tibialis anterior muscle using needle electrodes. A figure-of-eight coil was placed so as to induce current in the brain in eight different directions, starting from the posterior-to-anterior direction and rotating it in 45 degrees steps. The latencies were compared with those evoked by transcranial electrical stimulation (TES) and TMS using a double cone coil. Although the latencies of MEPs ranged from D to I3 waves, the most prominent component evoked by TMS at near-threshold intensity represented the I1 wave. With the double cone coil, the elicited peaks always represented I1 waves, and D waves were evoked only at very high stimulus intensities, suggesting a high effectiveness of this coil in inducing I1 waves. Using the figure-of-eight coil, current flowing anteriorly or toward the hemisphere contralateral to the recorded muscle was more effective in eliciting large responses than current flowing posteriorly or toward the ipsilateral hemisphere. The effective directions induced I1 waves with the lowest threshold, whereas the less effective directions elicited I1 and I2 waves with a similar frequency. Higher stimulus intensities resulted in concomitant activation of D through I3 waves with increasing amount of D waves, but still the predominance of I1 waves was apparent. The amount of I waves, especially of I1 waves, was greater than predicted by the hypothesis that TMS over the leg motor area activates the output cells directly, but rather suggests predominant transsynaptic activation. The results accord with those of recent human epidural recordings.