Somatotopic organization of human somatosensory cortices for pain: a single trial fMRI study

The ability to locate pain plays a pivotal role in immediate defense and withdrawal behavior. However, how the brain localizes nociceptive information without additional information from somatotopically organized mechano-receptive pathways is not well understood. To investigate the somatotopic organization of the nociceptive system, we applied Thulium-YAG-laser evoked pain stimuli, which have no concomitant tactile component, to the dorsum of the left hand and foot in randomized order. We used single-trial functional magnetic resonance imaging (fMRI) to assess differential hemodynamic responses to hand and foot stimulation for the group and in a single subject approach. The primary somatosensory cortex (SI) shows a clear somatotopic organization ipsi- and contralaterally to painful stimulation. Furthermore, a differential representation of hand and foot stimulation appeared within the contralateral opercular--insular region of the secondary somatosensory cortex (SII). This result provides evidence that both SI and SII encode spatial information of nociceptive stimuli without additional information from the tactile system and highlights the concept of a redundant representation of basic discriminative stimulus features in human somatosensory cortices, which seems adequate in view of the evolutionary importance of pain perception.

White Matter Asymmetry in the Human Brain: A Diffusion Tensor MRI Study

Language ability and handedness are likely to be associated with asymmetry of the cerebral cortex (grey matter) and connectivity (white matter). Grey matter asymmetry, most likely linked to language has been identified with voxel-based morphometry (VBM) using T(1)-weighted images. Differences in white matter obtained with this technique are less consistent, probably due to the relative insensitivity of the T(1) contrast to the ultrastructure of white matter. Furthermore, previous VBM studies failed to find differences related to handedness in either grey or white matter. We revisited these issues and investigated two independent groups of subjects with diffusion-tensor imaging (DTI) for asymmetries in white matter composition. Using voxel-based statistical analyses an asymmetry of the arcuate fascicle was observed, with higher fractional anisotropy in the left hemisphere. In addition, we show differences related to handedness in the white matter underneath the precentral gyrus contralateral to the dominant hand. Remarkably, these findings were very robust, even when investigating small groups of subjects. This highlights the sensitivity of DTI for white matter tissue differences, making it an ideal tool to study small patient populations.

Somatotopic representation of nociceptive information in the putamen: an event-related fMRI study

The ability to locate pain plays a pivotal role in immediate defence and withdrawal behaviour. However, it is unclear to what extent nociceptive information is relayed to and processed in subcortical structures relevant for motor preparation and possibly the generation of withdrawal behaviour. We used single-trial functional magnetic resonance imaging (fMRI) to assess whether nociceptive information is represented in the putamen in a somatotopic manner. We therefore applied thulium-YAG laser-evoked pain stimuli, which had no concomitant tactile component, to the dorsum of the left hand and foot to 15 healthy subjects in a randomized order. In addition, 11 subjects were stimulated on the right body side. Differential representations of hand- and foot-related blood oxygen level dependent (BOLD) responses within the putamen were assessed using a single subject approach. Nociceptive stimuli significantly activated the putamen bilaterally. However, a somatotopic organization for hand- and foot-related responses was only present in the contralateral putamen. Here the foot was located anteriorly and medially to the hand, which parallels results from anatomical and microstimulation studies in monkeys and also human imaging data on the arrangement of movement related activity in the putamen. This result provides evidence for the hypothesis that behaviourally relevant nociceptive information without additional information from the tactile system is represented in the putamen and made available for pain related motor responses.

Motor excitability and motor behaviour after modafinil ingestion ? a double-blind placebo-controlled cross-over trial

Modafinil is a novel vigilance-enhancing agent. We were interested if modafinil would also enhance motor excitability and improve motor performance and attention in healthy subjects. Ten volunteers received either a single oral dose of placebo or 200 mg modafinil. A randomized double-blind crossover design was used. Transcranial magnetic stimulation was employed to test intracortical inhibition, intracortical facilitation, the cortical silent period and to obtain stimulus-response curves. In addition, M responses and F waves were recorded. Reaction time tasks, the nine-hole-peg test and the d2 attention test were also applied. These studies were performed prior to and 3 and 24 hours after drug ingestion. Modafinil did not change excitatory or inhibitory properties in the motor cortex. It did not alter corticospinal excitability and alpha motoneuronal excitability. In the modafinil group and in the placebo group, performance of the nine-hole-peg test and the d2 test improved to a similar extend over time. Thus, this study does not demonstrate significant differences between a single dose of modafinil and placebo in healthy subjects.

Modulation of motor cortex excitability induced by pinch grip repetition

We examined the influence of right handed pinch grips and the effect of a motor training on motor cortex excitability of the left first dorsal interosseus muscle (FDI). TMS single and paired pulses were applied over the right human motor cortex (M1) during and after right handed pinch grips with low force. In another experiment, these stimulations were performed before and after a 30-minute right handed pinch grip training.

RESULTS

MEP amplitudes in left FDI were reduced when TMS single pulses were applied during the pinch grip. Simultaneously, motor cortex excitability was enhanced but returned to baseline after the training period.

CONCLUSION

Phasic pinch grips of the right hand exert an inhibiting effect on the corticospinal excitability of the ipsilateral motor cortex and lead to an increase of intracortical excitability. These changes are distinct and independent of each other. Motor training has an interhemispheric effect on intracortical excitability.

Lesion-induced and training-induced brain reorganization

INTRODUCTION

A stroke may modulate motor cortex excitability. We examined if distinct ischemic brain lesions are associated with a specific pattern of excitability changes. We also investigated the effects of a rehabilitative therapy on motor excitability.

METHODS

In stroke patients, the consequences of a) a lesion in the central somatosensory system, b) a cerebellar lesion and c) a two week period of Constraint-induced movement therapy (CIMT), on motor cortex excitability were studied. Transcranial magnetic stimulation techniques and functional magnetic resonance imaging (fMRI) were employed.

RESULTS

Patients with a lesion in the primary somatosensory cortex or in the ventroposterolateral nucleus of the thalamus had a decreased intracortical inhibition on the affected side. Patients with lesions in the territory of the superior cerebellar artery had a loss of intracortical facilitation and an increase of intracortical inhibition. Patients with cortical lesions undergoing CIMT had a loss of intracortical inhibition prior to therapy. After CIMT, changes of ICI were stronger in the lesioned than in the non-lesioned hemisphere but could result either in an increase of ICI or a reduction of ICI. In three patients fMRI results showed that cortical activation was less post CIMT as compared to pre-treatment activation. In parallel, ICI was reduced after treatment.

CONCLUSIONS

Our results suggest that, physiologically, central somatosensory influence on the motor cortex is inhibitory. In contrast, the cerebellum normally exerts a facilitatory influence on the motor cortex. CIMT induces changes of intracortical excitability mainly in the affected hemisphere.

Interhemispheric effects of high and low frequency rTMS in healthy humans

OBJECTIVE

We investigated whether repetitive transcranial magnetic stimulation (rTMS) applied to the right motor cortex modified the excitability of the unstimulated left motor cortex.

METHODS

Interhemispheric effects of 0.5 and 5 Hz subthreshold rTMS over the right motor cortex were examined by single pulse and paired pulse TMS and by transcranial electrical stimulation (TES) applied to the unstimulated left motor cortex. The effects of (a) 1800 pulses real and sham rTMS with 5 Hz, (b) 180 pulses real and sham rTMS with 0.5 Hz and (c) 1800 pulses real rTMS with 0.5 Hz were studied.

RESULTS

Following 5 Hz right motor rTMS motor evoked potential (MEP) amplitudes induced by single pulse TMS over the left motor cortex increased significantly. Intracortical inhibition (ICI) and facilitation (ICF) and MEP amplitudes evoked by TES were unchanged. Sham stimulation had no influence on motor cortex excitability. After 180 pulses right motor cortex rTMS with 0.5 Hz a significant decrease of left motor ICF, but no change in single pulse MEP amplitudes was found. A similar trend was observed with 1800 pulses rTMS with 0.5 Hz.

CONCLUSIONS

High frequency right motor rTMS can increase left motor cortex excitability whereas low frequency right motor rTMS can decrease it. These effects outlast the rTMS by several minutes. The underlying mechanisms mediating interhemispheric excitability changes are likely to be frequency dependent.

Motor system abnormalities in hereditary spastic paraparesis type 4 (SPG4) depend on the type of mutation in the spastin gene

BACKGROUND

Hereditary spastic paraparesis (HSP) denotes a group of inherited neurological disorders with progressive lower limb spasticity as their clinical hallmark; a large proportion of autosomal dominant HSP belongs to HSP type 4, which has been linked to the SPG4 locus on chromosome 2. A variety of mutations have been identified within the SPG4 gene product, spastin.

OBJECTIVE

Correlation of genotype and electrophysiological phenotype.

MATERIAL

Two large families with HSP linked to the SPG4 locus with a very similar disease with respect to age of onset, progression, and severity of symptoms.

METHODS

Mutation analysis was performed by PCR from genomic DNA and cDNA, and direct sequencing. The motor system was evaluated using transcranial magnetic stimulation.

RESULTS

Patients differ in several categories depending on the type of mutation present.

CONCLUSIONS

For the first time in hereditary spastic paraparesis, a phenotypic correlate of a given genetic change in the spastin gene has been shown.

Motor excitability in a patient with a somatosensory cortex lesion

OBJECTIVE

We report a patient with an ischemic lesion in right somatosensory cortex who developed dystonic posturing and pseudo-athetotic involuntary left-sided finger movements during voluntary muscle contractions.

METHODS

Motor excitability was assessed using transcranial magnetic stimulation techniques and electrical peripheral nerve stimulation. Results obtained from abductor digiti minimi muscles of both hands were compared.

RESULTS

On the affected side, silent period duration and intracortical inhibition were reduced, indicating a loss of inhibitory properties. Intracortical facilitation was enhanced. Stimulus-response curves showed a smaller increase of motor evoked potential amplitudes when recorded during muscle relaxation, but not during voluntary muscle activation.

CONCLUSIONS

The results suggest that, under normal conditions, somatosensory cortex modifies inhibitory as well as excitatory properties in the motor system.

Contralateral coding of imagined body parts in the superior parietal lobe

In monkeys, neurons in the superior parietal lobe (area 5) code for spatial position of contralateral body parts by combining visual and somatosensory signals. Using a modified version of the classical mental rotation task, we were able to demonstrate that in humans activation in the contralateral superior parietal lobe could be evoked when mental rotation was combined with motor imagery of hands. These findings show that even in the absence of visual and somatosensory input, information provided by motor imagery suffices to induce contralateral superior parietal lobe monitoring of the imagined limb configuration. This constitutes an important prerequisite for effective imagined motor practice that can be used to improve actual motor performance.