First Dorsal Interosseus Research Articles

Paired associative stimulation increases motor cortex excitability more effectively than theta-burst stimulation

ObjectiveTo examine the effects of theta burst stimulation (TBS) and paired associative stimulation (PAS) on excitability in the human motor cortex. MethodsSixteen healthy young participants received intermittent TBS (iTBS) or PAS to the primary motor cortex on two testing occasions, at least a week apart. Ten of the participants also received iTBS or PAS after conditioning with continuous TBS on two other occasions. Cortical excitability was assessed with single TMS pulses to the motor cortex. Motor evoked potentials (MEPs) were measured from the first dorsal interosseus (FDI) muscle before TBS or PAS stimulation, and every 10min for 60min after stimulation. Changes in excitability were compared against the potential for motor learning, assessed with the rotor pursuit task. ResultsAfter the PAS protocol MEP amplitudes were significantly increased. This increase was greater than after intermittent TBS, which did not change MEPs significantly. Conditioning with continuous TBS showed no significant effect. Participants’ responses were not correlated across protocols and were not correlated with rotor pursuit learning. ConclusionsPAS was the only protocol which induced significant increases in MEP amplitude. SignificancePAS is robust in inducing excitatory cortical change. This makes it a suitable protocol for testing plasticity in healthy and patient groups.

Age-associated impairement in endpoint accuracy of goal-directed contractions performed with two fingers is due to altered activation of the synergistic muscles

The purpose of this study was to determine whether older adults compared with young adults exhibit impaired end-point accuracy during a two-finger task due to altered activation of the contributing synergistic muscles. Nine young (21.3years±1.6years, 4 men) and 9 older (73.1years±6.4years, 5 men) were instructed to accurately match the center of a target with concurrent abduction of the index and little fingers (synergistic two-finger task). The target comprised of 20% MVC and 200ms. Visual feedback of the force trajectory and target was provided 1s after each trial. Subjects completed 40 trials and the last 10 were used for analysis. Endpoint accuracy was quantified as the normalized deviation from the target in terms of peak force (peak force error), time-to-peak force (time-to-peak force error), and a combination of the two (overall error). Motor output variability was quantified as the standard deviation and coefficient of variation (CV) of peak force and time to peak force. The neural activation of the involved synergist muscles (first dorsal interosseus (FDI) and abductor digiti minimi (ADM)) was quantified with the electromyography (EMG) amplitude (root mean square) and its frequency structure (wavelet analysis). Older adults exhibited significantly greater peak force (46.7±10% vs. 24.9±3.2%) and overall endpoint error (68.5±9.7% vs. 41.7±4.3%), whereas the time to peak force error was similar for the two age groups. Older adults also exerted greater peak force variability than young adults, as quantified by the CV of peak force (34.3±3.5% vs. 24.1±2.3%). The greater peak force error in older adults was associated with changes in the activation of the ADM muscle but not the FDI. Specifically, greater peak force error was associated with greater power from 13–30Hz and lesser power from 30–60Hz. These results, therefore, suggest that older adults compared with young adults exhibit impaired endpoint force accuracy during a two finger task because of altered activation of one of the synergist muscles.

Task-dependent changes of motor cortical network excitability during precision grip compared to isolated finger contraction

The purpose of this study was to determine whether task-dependent differences in corticospinal pathway excitability occur in going from isolated contractions of the index finger to its coordinated activity with the thumb. Focal transcranial magnetic stimulation (TMS) was used to measure input-output (I/O) curves--a measure of corticospinal pathway excitability--of the contralateral first dorsal interosseus (FDI) muscle in 21 healthy subjects performing two isometric motor tasks: index abduction and precision grip. The level of FDI electromyographic (EMG) activity was kept constant across tasks. The amplitude of the FDI motor evoked potentials (MEPs) and the duration of FDI silent period (SP) were plotted against TMS stimulus intensity and fitted, respectively, to a Boltzmann sigmoidal function. The plateau level of the FDI MEP amplitude I/O curve increased by an average of 40% during the precision grip compared with index abduction. Likewise, the steepness of the curve, as measured by the value of the maximum slope, increased by nearly 70%. By contrast, all I/O curve parameters [plateau, stimulus intensity required to obtain 50% of maximum response (S(50)), and slope] of SP duration were similar between the two tasks. Short- and long-latency intracortical inhibitions (SICI and LICI, respectively) were also measured in each task. Both measures of inhibition decreased during precision grip compared with the isolated contraction. The results demonstrate that the motor cortical circuits controlling index and thumb muscles become functionally coupled when the muscles are used synergistically and this may be due, at least in part, to a decrease of intracortical inhibition and an increase of recurrent excitation.

Motor cortical functional geometry in cerebral palsy and its relationship to disability

ObjectiveTo investigate motor cortical map patterns in children with diplegic and hemiplegic cerebral palsy (CP), and the relationships between motor cortical geometry and motor function in CP. MethodsTranscranial magnetic stimulation (TMS) was used to map motor cortical representations of the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles in 13 children with CP (age 9–16years, 6 males.) The Gross Motor Function Measure (GMFM) and Melbourne upper extremity function were used to quantify motor ability. ResultsIn the hemiplegic participants (N=7), the affected (right) FDI cortical representation was mapped on the ipsilateral (N=4), contralateral (N=2), or bilateral (N=1) cortex. Participants with diplegia (N=6) showed either bilateral (N=2) or contralateral (N=4) cortical hand maps. The FDI and TA motor map center-of-gravity mediolateral location ranged from 2–8cm and 3–6cm from the midline, respectively. Among diplegics, more lateral FDI representation locations were associated with lower Melbourne scores, i.e. worse hand motor function (Spearman’s rho=−0.841, p=0.036). ConclusionsAbnormalities in TMS-derived motor maps cut across the clinical classifications of hemiplegic and diplegic CP. The lateralization of the upper and lower extremity motor representation demonstrates reorganization after insults to the affected hemispheres of both diplegic and hemiplegic children. SignificanceThe current study is a step towards defining the relationship between changes in motor maps and functional impairments in CP. These results suggest the need for further work to develop improved classification schemes that integrate clinical, radiologic, and neurophysiologic measures in CP.

Transcranial magnetic stimulation and peristimulus frequencygram

ObjectiveThe aim of our study was to use peristimulus frequencygram (PSF) constructed from single motor unit recordings to further characterise the responses evoked by low intensity TMS. MethodsTwelve healthy subjects (age 32±11 years) received single-pulse TMS over the first dorsal interosseus (FDI) motor area during weak isometric index finger abduction. Several hundred stimuli were delivered at a frequency of ∼0.3Hz and at an intensity of 79–110% of active motor threshold. FDI electromyogram (EMG) was recorded with surface and intramuscular fine wire electrodes. For single motor units, data analysis involved construction of a peristimulus time histogram (PSTH) and PSF. Surface EMG analysis involved signal averaging. Cumulative sums (CUSUMs) were calculated for SEMG, PSTH, and PSF data. ResultsForty-five single motor units were identified. The average number of stimuli per unit was 201±112. Characterisation of the response evoked by TMS differed with the use of SEMG, PSTH, and PSF CUSUMs. ConclusionsThe duration of the EMG silence that follows the MEP during voluntary contraction was longer in the PSF than SEMG and PSTH. SignificanceThese findings highlight the importance of using both probability and frequency-based analysis when determining the duration of inhibitory events in peripheral recordings.

The Long-Term Potentiation-Like Effect in the Corticomotor Area after Transcranial Direct Current Stimulation with Motor Imagery

The purpose of the present study was to clarify a long-term potentiation like effect in the corticomotor area after anodal transcranial direct current stimulation (tDCS), which had been done with motor imagery. Anodal tDCS was applied transcranially as an intervention to the left hand motor area in the right hemisphere for 15 min with the intensity of 1.0 mA during resting or motor imagery (MI) conditions. In the MI condition, subjects performed motor imagery of index finger abduction. Motor-evoked potentials (MEPs) were recorded from the first dorsal interossei (FDI) of the left hand before the intervention and at 0 min, 15 min, 30 min and 60 min after the intervention. The stimulus intensities of TMS were set at 1.05, 1.15, and 1.25 times the strength of the resting motor threshold (RMth). Increases of MEPs were detected with all intensities of TMS at 0 min and 15 min after tDCS with motor imagery. However, in the resting condition, MEP amplitudes were elevated only at 15 min after tDCS and at the highest TMS intensity of 1.25×RMth. The facilitatory effect induced by tDCS with motor imagery was significantly long-lasting and definite compared to that after tDCS without motor imagery.

Intense training overcomes effects of the val66met BDNF polymorphism on short-term plasticity

The val(66)met polymorphism in the brain-derived neurotrophic factor (BDNF) gene impacts activity-dependent secretion of BDNF and modifies short-term cortical plasticity. The current study examined whether sustained training overcomes polymorphism effects on short-term plasticity and also examined polymorphism effects on long-term plasticity. Twenty-four subjects completed a 12-day protocol of daily training on a marble navigation task that required intense use of the first dorsal interosseus (FDI) muscle. In parallel, transcranial magnetic stimulation (TMS) mapping was used to assess serial measures of short-term cortical motor map plasticity, plus long-term cortical motor map plasticity, of the cortical FDI map. On Day 1, subjects with the polymorphism did not show significant short-term cortical motor map plasticity over 30min of FDI activity, but subjects without the polymorphism did. After 5days of intense training, a genotype-based difference in short-term cortical motor map plasticity was no longer found, as both groups showed short-term plasticity across the 30min of FDI activity. Also, across 12days of training, map area decreased significantly, in a manner that did not vary in relation to genotype. Training of sufficient intensity and duration overcomes effects that the val(66)met polymorphism has on short-term cortical motor map plasticity. The polymorphism-related differences seen with short-term plasticity are not found with long-term cortical motor map plasticity.

Interhemispheric Plasticity in Humans

Chronic unimanual motor practice increases the motor output not only in the trained but also in the nonexercised homologous muscle in the opposite limb. We examined the hypothesis that adaptations in motor cortical excitability of the nontrained primary motor cortex (iM1) and in interhemispheric inhibition from the trained to the nontrained M1 mediate this interlimb cross education. Healthy, young volunteers (n=12) performed 1000 submaximal voluntary contractions (MVC) of the right first dorsal interosseus (FDI) at 80% MVC during 20 sessions. Trained FDI's MVC increased 49.9%, and the untrained FDI's MVC increased 28.1%. Although corticospinal excitability in iM1, measured with transcranial magnetic stimulation (TMS) before and after every fifth session, increased 6% at rest, these changes, as those in intracortical inhibition and facilitation, did not correlate with cross education. When weak and strong TMS of iM1 were delivered on a background of a weak and strong muscle contraction, respectively, of the right FDI, excitability of iM1 increased dramatically after 20 sessions. Interhemispheric inhibition decreased 8.9% acutely within sessions and 30.9% chronically during 20 sessions and these chronic reductions progressively became more strongly associated with cross education. There were no changes in force or TMS measures in the trained group's left abductor minimi digiti and there were no changes in the nonexercising control group (n=8). The findings provide the first evidence for plasticity of interhemispheric connections to mediate cross education produced by a simple motor task.

Posterior Interosseus Palsy with an Incidental Froment-Rauber Nerve Presenting as a Pseudoclaw Hand

IntroductionAnatomic variations of peripheral nerves are well recog-nized and reported in literature [4]. It is important tounderstand these variations because they can explainunusual clinical deficits or findings. The Froment–Raubernerve is a distal continuation of the motor branch of theposterior interosseous nerve, which innervates the firstdorsal interosseus muscle. This report discusses a case ofthis rare anomalous connection that initially confused thediagnosis of a posterior interosseus palsy.Case ReportA 17-year-old left-hand dominant baseball pitcher pre-sented with an 8-month history of progressive pain in hisleft proximal forearm and elbow. His symptoms proceededto subjective weakness of the small finger that progressedradially to include the ring and middle fingers withposturing in flexion at the metacarpal−phalangeal andinterphalangeal joints. He was given the presumptivediagnosis of an ulnar palsy. Prior to the onset of handweakness, there were previous episodes of elbow pain thatresolved without sequelae. He was referred to a handsurgeon. Initial physical examination was notable forclawing of the middle, ring, and small digits (Fig. 1) withfull passive mobility. Additionally, he had subtle weaknessof the wrist and extrinsic digit extensors as well as of theextensor pollicis longis and the abductor pollicis longus. Hedenied any sensory changes but slightly decreased protec-tive sensibility was noted in the ulnar nerve distribution bySemmes–Weinstein monofilament testing (3.61), includingthe dorsal ulnar cutaneous nerve. A mildly positive Tinel’ssign over the ulnar nerve at the elbow was detected.Nonoperative treatments of splinting and nonsteroidal anti-inflammatory agents had been attempted and had failed toimprove his symptoms.Electrodiagnostic studies (nerve conduction and electro-myography (EMG)) were consistent with a posteriorinterosseous nerve (PIN) neuropathy. These findings in-cluded denervation in the extensor carpi ulnaris and firstdorsal interosseus with complex repetitive discharges in thelatter. The number of motor units in all muscles innervatedby the posterior interosseus nerve as well as in the firstdorsal interosseus were decreased. Nerve conductionstudies of the median and ulnar nerves were normal,including the amplitude of the thenar and hypothenarcompound action potentials. The amplitude of the extensorindicis proprius compound muscle action potential wassignificantly decreased. Notable is that although no neuro-physiologic evidence of ulnar nerve pathology was found atthe elbow or wrist, evidence of denervation was noted inhis first dorsal interosseus muscle on EMG. A completeneurological evaluation including magnetic resonance im-aging of the brain, cervical spine, elbow, and forearm were

The effect of pain on training-induced plasticity of the corticomotor system

Pain is thought to interfere with training-induced plasticity of corticomotor pathways. Although this implies direct interference with plastic processes, it may be explained by compromised performance in the training task during pain. Repeated finger movements can induce plasticity and change the amplitude/direction of acceleration of finger movement evoked by transcranial magnetic stimulation (TMS). We hypothesized that if pain interferes with plasticity, acceleration of finger movement would not change when the training task was painful, despite control of training task performance. TMS was applied over the optimal scalp site to evoked index finger abduction movements in nine participants. Participants then trained finger adduction with feedback of finger acceleration for three 8-min sessions, in three conditions on separate days. Conditions: first dorsal interosseus (FDI) pain and control (no-pain), with injection of 5% and 0.9% hypertonic saline, respectively, into FDI; and remote pain (5% saline injection into infrapatellar fat pad). Peak acceleration of TMS-evoked finger movement and amplitude of motor evoked potentials (MEPs) in FDI were measured at baseline, between training sessions, and at three 5-min intervals after training ceased. Plastic change was observed (reduced TMS evoked peak finger acceleration in the abduction direction) after motor training during control and FDI pain, but not during the remote pain. There was no change in FDI MEPs in any conditions. These data do not support direct effects of nociceptive input (pain) on training-induced plasticity of corticomotor pathways. Remote pain may compromise learning due to distraction from the training task or other complex central pain processes.

The interaction of respiration and visual feedback on the control of force and neural activation of the agonist muscle

The purpose of this study was to compare force variability and the neural activation of the agonist muscle during constant isometric contractions at different force levels when the amplitude of respiration and visual feedback were varied. Twenty young adults (20–32years, 10 men and 10 women) were instructed to accurately match a target force at 15% and 50% of their maximal voluntary contraction (MVC) with abduction of the index finger while controlling their respiration at different amplitudes (85%, 100% and 125% normal) in the presence and absence of visual feedback. Each trial lasted 22s and visual feedback was removed from 8–12 and 16–20s. Each subject performed three trials with each respiratory condition at each force level. Force variability was quantified as the standard deviation of the detrended force data. The neural activation of the first dorsal interosseus (FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Relative to normal respiration, force variability increased significantly only during high-amplitude respiration (∼63%). The increase in force variability from normal- to high-amplitude respiration was strongly associated with amplified force oscillations from 0 to 3Hz (R2 ranged from .68 to .84, p< .001). Furthermore, the increase in force variability was exacerbated in the presence of visual feedback at 50% MVC (vision vs. no-vision: .97 vs. .87N) and was strongly associated with amplified force oscillations from 0 to 1Hz (R2= .82) and weakly associated with greater power from 12 to 30Hz (R2= .24) in the EMG of the agonist muscle. Our findings demonstrate that high-amplitude respiration and visual feedback of force interact and amplify force variability in young adults during moderate levels of effort.

Observation-execution matching and action inhibition in human primary motor cortex during viewing of speech-related lip movements or listening to speech

One influential theory posits that language has evolved from gestural communication through observation-execution matching processes in the mirror neuron system (MNS). This theory predicts that observation of speech-related lip movements or even listening to speech would result in effector and task specific increase of the excitability of the corresponding motor representations in the primary motor cortex (M1), since actual movement execution is known be effector and task specific. In addition, effector and task specific inhibitory control mechanisms should be important to prevent overt motor activation during observation of speech-related lip movements or listening to speech. We tested these predictions by applying focal transcranial magnetic stimulation to the left M1 of 12 healthy right-handed volunteers and measuring motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) in a lip muscle, the right orbicularis oris (OO), vs. a hand muscle, the right first dorsal interosseus (FDI). We found that MEP and SICI increased only in the OO but not in the FDI during viewing of speech-related lip movements or listening to speech. These changes were highly task specific because they were absent when lip movements non-related to speech were viewed. Finally, the increase in MEP amplitude in the OO correlated inversely with accuracy of speech perception, i.e. the MEP increase was directly related to task difficulty. The MEP findings support the notion that observation-execution matching is an operating process in the putative human MNS that might have been fundamental for evolution of language. Furthermore, the SICI findings provide evidence that inhibitory mechanisms are recruited to prevent unwanted overt motor activation during action observation.

Electromyographic Activity of Hand Muscles in a Motor Coordination Game: Effect of Incentive Scheme and Its Relation with Social Capital

BackgroundA vast body of social and cognitive psychology studies in humans reports evidence that external rewards, typically monetary ones, undermine intrinsic motivation. These findings challenge the standard selfish-rationality assumption at the core of economic reasoning. In the present work we aimed at investigating whether the different modulation of a given monetary reward automatically and unconsciously affects effort and performance of participants involved in a game devoid of visual and verbal interaction and without any perspective-taking activity.Methodology/Principal FindingsTwelve pairs of participants were submitted to a simple motor coordination game while recording the electromyographic activity of First Dorsal Interosseus (FDI), the muscle mainly involved in the task. EMG data show a clear effect of alternative rewards strategies on subjects' motor behavior. Moreover, participants' stock of relevant past social experiences, measured by a specifically designed questionnaire, was significantly correlated with EMG activity, showing that only low social capital subjects responded to monetary incentives consistently with a standard rationality prediction.Conclusions/SignificanceOur findings show that the effect of extrinsic motivations on performance may arise outside social contexts involving complex cognitive processes due to conscious perspective-taking activity. More importantly, the peculiar performance of low social capital individuals, in agreement with standard economic reasoning, adds to the knowledge of the circumstances that makes the crowding out/in of intrinsic motivation likely to occur. This may help in improving the prediction and accuracy of economic models and reconcile this puzzling effect of external incentives with economic theory.

Long-Lasting Contralateral Motor Cortex Excitability Is Increased by Unilateral Hand Movement That Triggers Electrical Stimulation of Opposite Homologous Muscles

Background/objective. Long-term changes in the motor cortex can be induced by practicing motor tasks with simultaneous application of peripheral nerve stimulation. This combination may augment motor rehabilitation after stroke but has been used primarily during contraction of the affected hand. The authors tested the effect of a right hand movement that electrically stimulated left hand contraction on right motor cortex excitability. Methods. Three tasks were used in 15 healthy subjects—a motor and stimulation task (MStask), stimulation only task (Stask), and motor only task (Mtask). The MStask consisted of isometric thumb abduction of the right hand that triggered paired electrical stimulation of the left abductor pollicis brevis (APB) and first dorsal interosseus (FDI) motor points. The Stask was performed 1 week later and matched the stimulation received in the MStask. The Mtask was performed as a control. Transcranial magnetic stimulation applied to the right motor cortex assessed corticospinal excitability, short latency intracortical inhibition, and intracortical facilitation of the FDI and APB before, immediately after, and 30 minutes after task performance. Results. Corticospinal excitability increased in the FDI and APB following the MStask but not following the Stask or Mtask. The increased excitability present 30 minutes after the MStask also correlated with excitability measures recorded 1 week later. Conclusion. A bilateral motor and electrical stimulation task can drive persistent adaptation within the corticospinal system. Hemiplegic subjects who have poor voluntary movement of the affected hand may be able to contract the unaffected hand to activate and train homologous movements.

Cerebellar brain inhibition is decreased in active and surround muscles at the onset of voluntary movement

Highly selective activation of the desired muscles for each movement and inhibition of adjacent muscles is attributed to surround inhibition (SI) which differentially modulates corticospinal excitability in active and surrounding muscles. Cerebellar brain inhibition (CBI) is another inhibitory neuronal network which is known to be active at rest and during tonic muscle contraction. The way in which CBI may be modulated at movement onset and its relationship with SI has not previously been investigated. We assessed motor evoked potential (MEP) size and CBI in first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles at rest and during a simple motor task where FDI was an active muscle and ADM was not involved in the movement (surround muscle). At onset of movement, MEP size in ADM was significantly suppressed, confirming the existence of SI. In contrast, CBI in both muscles was found to be significantly decreased at the onset of the movement. This was confirmed even after adjustments for changes in MEP size occurring due to onset of muscle activity in FDI and the effects of SI in ADM. Our findings fail to functionally link SI with CBI, but they do indicate a non-topographically specific modulation of CBI in association with initiation of voluntary movement.

Motor Evoked Potentials of the Upper Extremities in Healthy Children

ObjectiveTo evaluate and compare the organization of descending motor pathways to upper extremity muscles among healthy children.MethodThe healthy children were 16 males and 7 females aged 1-19 years (average, 9 years), and eight healthy adults were enrolled as the control group. Transcranial magnetic stimulation was applied to bilateral motor cortices, and motor evoked potentials (MEPs) were recorded using surface electrodes from the first dorsal interossei (FDI), the biceps brachii (BIC), and the deltoid (DEL) muscles. The onset latency, central motor conduction time (CMCT), and amplitude were obtained during a relaxed state.ResultsMEPs of FDI were obtained from subjects aged 13 months. The frequency of obtaining MEPs in proximal and distal muscles increased with age, although there was a less frequent incidence of obtaining MEPs in the proximal BIC and DEL muscles compared with those in the distal FDI muscle. MEP amplitudes increased with age, whereas latencies were relatively constant. CMCTs showed a similar pattern of maturation, and adult values were obtained by 13-years-of-age.ConclusionThese results suggest that the proximal and distal muscles of the upper extremities show different maturation and organization patterns.

Aging and movement errors when lifting and lowering light loads

The purpose was to determine the influence of movement variability and level of muscle activation on the accuracy of targeted movements performed with the index finger by young and older adults. Twelve young (27.4 ± 4.4 years) and 12 older adults (74.5 ± 8.9 years) attempted to match the end position of an index finger movement to a target position when lifting and lowering a light load (10% of the maximum). Visual feedback was provided after each trial. Movement error was calculated as the absolute distance from the target. Movement variability was quantified as the standard deviation of finger acceleration and the variability of end position across trials. The EMG activity of first dorsal interosseus (FDI) and second palmar interosseus (SPI) muscles was measured with intramuscular electrodes. Older adults exhibited greater spatial and temporal errors and greater variability in finger acceleration and end position during both the lifting and lowering tasks. Older adults lifted the load by activating FDI less but SPI the same as young adults, whereas they lowered the load by activating SPI less and FDI the same as young adults. In addition, older adults exhibited lower variability across trials in SPI activation when lifting the load and lower variability for FDI activation when lowering the load. The findings demonstrate that the decrease in spatial and temporal accuracy observed in older adults when lifting and lowering a light load to a target position was due to greater movement variability and differences in antagonistic muscle activity.

P36-20 Motor learning with augmented feedback enhance motor performance and motor cortical excitability

Methods: Nine young healthy individuals completed the study. They were instructed to grip a force transducer with their non-dominant thumb and index finger. Subjects were instructed to reach the target force as accurate and as fast as possible. Feedback about the target force and the time taken to complete each set of exercise was indicated on the computer screen. Subjects were requested to complete 15 sets of exercises, which took about 20 minutes. Outcome measures consisted of purdue pegboard test that recorded the number of pins to be inserted in the pegboard in 30 seconds, tapping test that recorded the number of letter ‘b’ typed on a computer keyboard for 30 seconds, and maximum pinch grip force exerted by thumb and index finger. For the electrophysiological test, transcranial magnetic stimulation (TMS) was used to elicit single pulse to the hemisphere contralateral to stimulate first dorsal interossei (FDI). Motor evoked potential recruitment curve was obtained from stimulator output of 100% to 160%, and the peak MEP amplitude was also recorded.

Enhanced human brain associative plasticity in Costello syndrome

Costello syndrome (CS) is a rare multiple congenital anomaly disorder which is caused by germline mutations in the v-Ha-ras Harvey rat sarcoma viral oncogene homologue (HRAS) proto-oncogene. Experimental data suggest perturbing effects of the mutated protein on the functional and structural organization of networks of cerebral cortex and on the activity-dependent strengthening of synaptic transmission known as long term potentiation (LTP). In five patients with molecularly proven diagnosis of CS and in a group of 13 age-matched control subjects we investigated activity-dependent synaptic plasticity. To this end, we used a paired associative stimulation (PAS) protocol, in which left ulnar nerve stimuli were followed by transcranial magnetic stimulation (TMS) pulses to right cortical hand area, and recorded motor evoked potentials (MEPs) by single pulse TMS from left first dorsal interosseus (FDI) muscle before and after PAS. In 4 out of 5 CS patients and in a subgroup of nine control subjects we also evaluated the time course and the topographical specificity of PAS after-effects. In these two subgroups, MEPs were measured before, immediately after and 30 min after PAS in the left FDI and left abductor pollicis brevis (APB). While the PAS protocol led to a 65% increase of the FDI MEP amplitude in controls, the LTP-like phenomenon was significantly more pronounced in CS patients, with motor responses increased by 230%. In addition, CS patients showed a similar MEP increase in both muscles while control subjects showed a slight increase in APB and only immediately after PAS. We hypothesize that the extremely enhanced PAS after-effects could be due to the influence of HRAS activity on the susceptibility of synapses to undergo LTP.

Motor Resonance May Originate From Sensorimotor Experience

In humans, the motor system can be activated by passive observation of actions or static pictures with implied action. The origin of this facilitation is of major interest to the field of motor control. Recently it has been shown that sensorimotor learning can reconfigure the motor system during action observation. Here we tested directly the hypothesis that motor resonance arises from sensorimotor contingencies by measuring corticospinal excitability in response to abstract non-action cues previously associated with an action. Motor evoked potentials were measured from the first dorsal interosseus (FDI) while human subjects observed colored stimuli that had been visually or motorically associated with a finger movement (index or little finger abduction). Corticospinal excitability was higher during the observation of a colored cue that preceded a movement involving the recorded muscle than during the observation of a different colored cue that preceded a movement involving a different muscle. Crucially this facilitation was only observed when the cue was associated with an executed movement but not when it was associated with an observed movement. Our findings provide solid evidence in support of the sensorimotor hypothesis of action observation and further suggest that the physical nature of the observed stimulus mediating this phenomenon may in fact be irrelevant.