Role of intracortical inhibition in selective hand muscle activation

Previous studies have shown that intracortical inhibition (ICI) plays an important role in shaping the output from primary motor cortex (M1). This study explored the muscle specificity and temporal modulation of ICI during the performance of a phasic index finger flexion task. Fifteen subjects were asked to rest their dominant hand on a computer mouse and depress the mouse button using their index finger in time with a 1-Hz auditory metronome, while keeping the rest of their hand as relaxed as possible. Responses to single- and paired-pulse transcranial magnetic stimulation were recorded from the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) muscles while subjects were at rest and during "on" and "off" phases of the task. For FDI during the on phase, motor evoked potential (MEP) amplitude and pretrigger EMG increased and ICI decreased, as expected. This pattern of modulation was also observed for APB in seven subjects. The remaining eight subjects demonstrated a decrease in MEP amplitude and increase in ICI for APB during the on phase. This was associated with significantly less APB activation during the on phase. These findings suggest that an increase in ICI and decrease in corticospinal excitability can prevent unwanted muscle activation in a muscle-specific, temporally modulated manner.     

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP responses in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. On the basis of our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle (Takahashi et al. in Clin Neurophysiol 116:2757–2764, 2005), the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Considering that the functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to the sophisticated regulation of the motor outputs even during VTC.     

Cortico-motoneuronal output to intrinsic hand muscles is differentially influenced by static changes in shoulder positions

We investigated whether shoulder position influenced the recruitment properties of the abductor digiti minimi muscle (ADM) and first dorsal interosseous muscle (FDI). ADM and FDI motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were obtained in seven healthy volunteers at two different static positions of the shoulder joint (30° adduction vs 30° abduction) while the arm was passively supported at shoulder level (90° in the horizontal plane) and the elbow joint was fixed at 90°. ADM and FDI voluntary activity was also examined during (1) externally paced finger abductions at 2 Hz in the two different shoulder positions (EMG_ADM and EMG_FDI was back-averaged time-locked to the end of finger abduction) and (2) maximal voluntary abduction of the little finger and the index finger. Maximal EMG power and force were analysed in the two shoulder positions. H-reflexes from ADM and FDI were also obtained in two subjects. The ADM stimulus-response curve to TMS showed that the slope and plateau level were significantly reduced with the shoulder at 30° abduction. In contrast, the FDI stimulus-response curve to TMS was not influenced by shoulder position. The back-averaged EMG_ADM showed a significant decrease in peak amplitude and area with the shoulder at 30° abduction, while no change in EMG_FDI was observed under the same condition. Similarly, maximal EMG_ADM and force exertion by the little finger were significantly reduced with the shoulder at 30° abduction, while no such effect was observed for FDI. ADM H-reflex, but not FDI, was also decreased with shoulder abduction. These results indicate that the corticospinal pathway to ADM is less accessible to TMS and to voluntary command when the shoulder is placed at 30° abduction. In contrast, activation of FDI, whether by TMS or by volition, is not influenced by shoulder position. This finding suggests that there are differences in the corticospinal innervation to ADM and FDI, possibly due to the different role of these muscles in hand function.     

Task-dependent changes of intracortical inhibition

 The motor-evoked potential (MEP) to transcranial magnetic stimulation (TMS) is inhibited when preceded by a subthreshold TMS stimulus at short intervals (1–6 ms; intracortical inhibition, ICI) and is facilitated when preceded by a subthreshold TMS at longer intervals (10–15 ms; intracortical facilitation, ICF). We studied changes in ICI and ICF associated with two motor tasks requiring a different selectivity in fine motor control of small hand muscles (abductor pollicis brevis muscle, APB, and fourth dorsal interosseous muscle, 4DIO). In experiment 1 (exp. 1), nine healthy subjects completed four sets (5 min duration each) of repetitive (1 Hz) thumb movements. In experiment 2 (exp. 2), the subjects produced the same number of thumb movements, but complete relaxation of 4DIO was demanded. Following free thumb movements (exp. 1), amplitudes of MEPs in response to both single and paired TMS showed a trend to increase with the number of exercise sets in both APB and 4DIO. By contrast, more focal, selective thumb movementsinvolving APB with relaxation of 4DIO (exp. 2) caused an increase in MEP amplitudes after single and paired pulses only in APB, while a marked decrease in MEPs after paired pulses, but not after single TMS, in the actively relaxed 4DIO. This effect was more prominent for the interstimulus interval (ISI) of 1–3 ms than for longer ISIs (8 ms, 10 ms, and 15 ms). F-wave amplitudes reflecting excitability of the alpha motoneuron pool were unaltered in APB and 4DIO, suggesting a supraspinal origin for the observed changes. We conclude that plastic changes of ICI and ICF within the hand representation vary according to the selective requirements of the motor program. Performance of more focal tasks may be associated with a decrease in ICI in muscles engaged in the training task, while at the same time ICI may be increased in an actively relaxed muscle, also required for a focal performance. Additionally, our data further supports the idea that ICI and ICF may be controlled independently. Received: 20 September 1996 / Accepted: 1 October 1997     

Modulation of intracortical excitability in human hand motor areas. The effect of cutaneous stimulation and its topographical arrangement

Changes in afferent input can alter the excitability of intracortical inhibitory systems. For example, using paired transcranial magnetic stimulation (TMS), both electrical digital stimulation and muscle vibration have been shown to reduce short-interval intracortical inhibition (SICI). The effects following muscle vibration are confined to the corticospinal projection to the vibrated muscles. The results following digital stimulation are less clear and the relative timing of the cutaneous stimulation and TMS is critical. Here we investigated further whether changes in SICI following digit stimulation exhibit topographic specificity. Eleven normal subjects were investigated (age 28.2±7.5 years, mean±SD). Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor digiti minimi (ADM) and abductor pollicis brevis (APB) muscles. SICI was measured, with and without preceding electrical digit II or digit V cutaneous stimulation. The interval between the digital nerve stimulus and test magnetic stimulus was independently set for each subject and established by subtracting the onset latency of the motor evoked potential (MEP) from the latency of the E2 component of the cutaneomuscular reflex. Therefore, measures of intracortical excitability were made at a time at which it is known that cutaneous input is capable of modulating cortical excitability. Single digital nerve stimuli applied to digit II significantly reduced SICI in FDI but not in ADM. Single digital nerve stimuli applied to digit V significantly reduced SICI in ADM but not in FDI or APB. There was a more generalised effect on intracortical facilitation (ICF) with both digit II and digit V stimulation significantly increasing ICF in FDI and ADM. Digital stimulation (either DII or DV) did not significantly affect SICI/ICF in APB. These findings show that appropriately timed cutaneous stimuli are capable of modulating SICI in a topographically specific manner. We suggest that the selective decrease in SICI seen with cutaneous stimulation may be important for focusing of muscle activation during motor tasks.     

Suppression of corticospinal excitability during negative motor imagery

To investigate the effect of negative motor imagery on corticospinal excitability, we performed transcranial magnetic stimulation (TMS) studies in seven healthy subjects during imagination of suppressing movements. Subjects were asked to imagine suppression of TMS-induced twitching movement of their nondominant left hands by attempting to increase the amount of relaxation after receiving an auditory NoGo cue (negative motor imagery), but to imagine squeezing hands after a Go cue (positive motor imagery). Single- and paired-pulse TMS were triggered at 2 s after Go or NoGo cues. Motor-evoked potentials (MEPs) were recorded in the first dorsal interosseus (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles of the left hand. Paired-pulse TMS with subthreshold conditioning stimuli at interstimulus intervals of 2 (short intracortical inhibition) and 15 ms (intracortical facilitation) and that with suprathreshold conditioning stimuli at interstimulus interval of 80 ms (long intracortical inhibition) were performed in both negative motor imagery and control conditions. Compared with the control state (no imagination), MEP amplitudes of FDI (but not APB and ADM) were significantly suppressed in negative motor imagery, but those from all three muscles were unchanged during positive motor imagery. F-wave responses (amplitudes and persistence) were unchanged during both negative and positive motor imagery. During negative motor imagery, resting motor threshold was significantly increased, but short and long intracortical inhibition and intracortical facilitation were unchanged. The present results demonstrate that excitatory corticospinal drive is suppressed during imagination of suppressing movements.     

Symmetric facilitation between motor cortices during contraction of ipsilateral hand muscles

Using transcranial magnetic stimulation (TMS) over the contralateral motor cortex, motor evoked potentials (MEPs) were recorded from resting abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles of eight subjects while they either rested or produced one of six levels of force with the APB ipsilateral to the TMS. F-waves were recorded from each APB at rest in response to median nerve stimulation while subjects either rested or produced one of two levels of force with their contralateral APB. Contraction of the APB ipsilateral to TMS produced facilitation of the MEPs recorded from resting APB and FDI muscles contralateral to TMS but did not modulate F-wave amplitude. Negligible asymmetries in MEP facilitation were observed between dominant and subdominant hands. These results suggest that facilitation arising from isometric contraction of ipsilateral hand muscles occurs primarily at supraspinal levels, and this occurs symmetrically between dominant and subdominant hemispheres. Electronic Publication     

Interhemispheric coupling of corticospinal excitability is suppressed during voluntary muscle activation

Motor-evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) show a trial-to-trial variation in size at rest that is positively correlated for muscles of the same, and opposite, upper limbs. To investigate the mechanisms responsible for this we have examined the effect of voluntary activation on the correlated fluctuations of MEP size. In 8 subjects TMS was concurrently applied to the motor cortex of each hemisphere using 2 figure-8 coils. MEPs (n = 50) were recorded from left and right first dorsal interosseous (FDI), abductor digiti minimi (ADM), and extensor digitorum communis. At rest, MEPs were significantly positively correlated for pairs of muscles of the same (75% of comparisons) and opposite limb (56% of comparisons). The correlation for within-limb muscle pairs was strongest for FDI and ADM. In contrast, between-limb MEP correlations showed no somatotopic organization. Voluntary activation reduced the strength of MEP correlations between limbs, even for muscle pairs that remained at rest while a remote upper limb muscle was active. In contrast, activation of a remote muscle did not affect the strength of MEP correlation for muscle pairs within the same limb that remained at rest. For within-limb comparisons, activation of one or both muscles of a pair reduced the strength of the MEP correlation, but to a lesser extent than for between-limb pairs. It is concluded that the process linking corticospinal excitability in the two hemispheres is suppressed during voluntary activation, and that different processes contribute to common fluctuations in MEP size for muscles within the same limb.     

Reduced intracortical inhibition and facilitation of corticospinal neurons in musicians

Interhemispheric inhibition between motor cortices is reduced in musicians. In the present study we have assessed intracortical inhibition (ICI) and facilitation (ICF) within ipsilateral motor cortex in 15 musicians and 15 non-musician controls. Transcranial magnetic stimulation (TMS) was used to elicit muscle evoked potentials (MEPs) from left first dorsal interosseous (FDI) muscle at rest, and during voluntary index finger abduction (0.5 N). Paired TMS with subthreshold conditioning was used to test early ICI with interstimulus intervals (ISIs) 1-5 ms, and ICF with ISIs 8-15 ms. Suprathreshold conditioning was used to test late ICI with ISIs 100-200 ms. TMS thresholds were similar in musicians and controls both at rest and with weak voluntary activation of FDI, indicating that postsynaptic excitability of corticospinal neurons was similar in both groups. ICI was less effective in musicians with FDI at rest and active, but only with an ISI of 3 ms. ICF was less effective in musicians under both rest and active conditions, and this was independent of ISI. There were no differences in late ICI between musicians and controls. We conclude that ICI and ICF circuits which are activated by weak TMS have less influence on corticospinal neuron excitability in musicians. Because of the dependence on ISI, the most likely explanation for the reduced ICI in musicians is an alteration of the interaction between the ICI circuit and neural elements responsible for the later I-waves evoked in corticospinal neurons by TMS. Excitability of the neural elements producing early and late ICI is not altered in musicians. Reduced ICF in musicians could be due to reduced excitability of neurons responsible for ICF, or an altered balance of excitatory inputs to corticospinal neurons which favours neurons that are not acted upon by the ICF circuit. The reduced influence of ICI and ICF circuits on corticospinal neuron excitability in musicians is likely to reflect a training-induced adaptation. It is not clear at present whether these differences represent an adaptive change related to their extraordinary control of finger movements, or alternatively a maladaptive change induced by "overuse" of the hands from extensive training.     

Hemispheric differences in use-dependent corticomotor plasticity in young and old adults

The aim of this study was to examine corticomotor excitability and plasticity following repetitive thumb abduction training in left and right hands of young and old adults. Electromyographic recordings were obtained from the abductor pollicis brevis (APB) muscle of 12 young (aged 18–27 years) and 14 old (aged 63–75 years) adults. Motor training consisted of 300 ballistic abductions of the thumb to maximize peak abduction acceleration, with each hand tested in a separate session. Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) was used to assess changes in contralateral APB motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before and after training. For young and old adults, APB MEP amplitude increased for both hands after training, which is indicative of use-dependent plasticity. However, the increase in MEP amplitude was 21% (P = 0.04) greater in the left (non-dominant) hand compared with the right (dominant) hand. This occurred despite a 40% greater improvement in peak thumb abduction acceleration (motor learning) for the right hand in young subjects compared with the left hand in young subjects (P < 0.04) and the right hand in old subjects (P < 0.01). Furthermore, no difference in use-dependent plasticity was observed between young and old adults, and SICI remained unchanged following ballistic training for both hands in all subjects. These findings suggest that there is greater strengthening of corticomotor circuits for control of the left compared with the right hand during simple ballistic thumb training and that an age-related decline in motor learning was observed only in the dominant hand. In contrast to previous studies, these data also indicate that young and old adults can demonstrate similar use-dependent corticomotor plasticity during this simple thumb-training task.     

Changes in corticomotor excitability of hand muscles in relation to static shoulder positions

We examined whether the recruitment properties of the corticospinal pathway to intrinsic hand muscles are influenced by variations of the shoulder joint angle. Abductor digiti minimi (ADM) motor evoked potentials (MEPs) in response to transcranial magnetic stimulation were examined during different static positions of the shoulder joint in the horizontal plane from 30° adduction to 30° abduction with respect to the neutral position at 0°, while elbow and wrist joints were constrained statically at 90° and 180° respectively. We found that 30° abduction of the shoulder significantly depressed MEP size and prolonged MEP latency in comparison with 30° shoulder adduction. The neutral shoulder angle position (at 0°) significantly reduced MEP size but had no effect on MEP latency in comparison with 30° shoulder abduction. The input–output relationship between MEP size and stimulus intensity was sigmoidal. The plateau value and maximum slope were significantly lower at 30° abduction than at 30° adduction of the shoulder. However, the threshold value did not differ significantly between the two positions. To differentiate excitability changes at cortical versus subcortical sites, intracortical inihibition (ICI) and intracortical facilitation (ICF) were assessed using a paired-magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30° adduction to 30° abduction. In contrast, no variation in the amount of ICI occurred in relation to the same changes in shoulder position. ADM F-waves elicited by electrical stimulation of the ulnar nerve at the wrist were significantly decreased at 30° shoulder abduction in comparison with 30° adduction. A similar pattern was observed in one subject in whom the H-reflex could be exceptionally elicited in ADM. We conclude that shoulder position influences the recruitment efficiency (gain) of the corticospinal volleys to motoneurons of intrinsic hand muscles. It is proposed that activity of peripheral receptors signalling static shoulder position influences corticomotor excitability of hand muscles both at the cortical and at the spinal level. This modulation may be functionally relevant when reaching to grasp objects.     

Change in Excitability of Corticospinal Pathway and GABA-Mediated Inhibitory Circuits of Primary Motor Cortex Induced by Contraction of Adjacent Hand Muscle

The present study examined whether the excitability of the corticospinal pathway and the GABA-mediated inhibitory circuits of the primary motor cortex that project onto the corticospinal neurons in the tonically contracting hand muscle are changed by tonic contraction of the adjacent hand muscle. The motor evoked potential (MEP) and cortical silent period (CSP) in the tonically contracting hand muscle were obtained while the adjacent hand muscle was either tonically contracting or at rest. The MEP and CSP of the first dorsal interosseous (FDI) muscle elicited across the scalp sites where the MEP is predominantly elicited in the FDI muscle were decreased by tonic contraction of the abductor digiti minimi (ADM) muscle. The centers of the area of the MEP and the duration of the CSP in the FDI muscle elicited across the sites where the MEP is predominantly elicited in the FDI muscle were lateral to those in the FDI muscle elicited across the sites where the MEP is elicited in both the FDI and ADM muscles. They were also lateral to those in the ADM muscle elicited either across the sites where the MEP is predominantly elicited in the ADM muscle, or across the sites where the MEP is elicited in both the FDI and ADM muscles. The decrease in the corticospinal excitability and the excitability of the GABA-mediated inhibitory circuits of the primary motor cortex that project onto the corticospinal neurons in the FDI muscle may be due either to (1) the interaction between the activity of the lateral area of the FDI representation and the descending drive to the ADM muscle, or (2) the decreased susceptibility of the primary motor area that predominantly projects onto the corticospinal neurons in the FDI muscle, which also plays a role in independent finger movement when both the FDI and ADM muscles act together as synergists.     

Facilitatory I wave interaction in proximal arm and lower limb muscle representations of the human motor cortex

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits direct and indirect (I) waves in the corticospinal tract. Facilitatory I wave interaction has been demonstrated with a suprathreshold first stimulus (S1) followed by a subthreshold to threshold second stimulus (S2). Intracortical inhibition (ICI) and intracortical facilitation (ICF) can be studied by another paired TMS paradigm with a subthreshold conditioning stimulus (CS) followed by a suprathreshold test stimulus. Facilitatory I wave interaction in motor representations other than the hand area and its relationship to ICI and ICF has not been studied. We studied I wave interaction, ICI and ICF in an intrinsic hand muscle (abductor pollicis brevis, APB), in a proximal arm muscle (biceps brachii, BB) and in a lower limb muscle (tibialis anterior, TA) in 11 normal subjects. I wave facilitation was studied by paired TMS at 24 interstimulus intervals (ISIs) from 0.5 to 5.1 ms. For APB and TA, facilitation occurred in three distinct peaks at ISIs of 0.9-1.7, 2. 5-3.5, and 4.1-5.1 ms. For BB, facilitation was significant for the first two peaks. The latencies of the peaks were similar among different muscles, but the magnitude of facilitation was much greater for APB and TA compared with BB. For all three muscles, changing the S2 to transcranial electrical stimulation (TES) resulted in much less facilitation of the first peak. For APB, there was significant I wave facilitation with S2 at 72% motor threshold (MT). The same stimulus used as the CS did not elicit ICF at ISI of 15 ms, suggesting that the threshold for eliciting I wave facilitation is lower than that for ICF. For BB and TA, there was no I wave facilitation with S2 at 90% of APB MT, and the same stimulus used as CS led to ICI. Thus in BB and TA the threshold for eliciting ICI is lower than that for I wave facilitation. We conclude that the circuits that mediate I wave interactions are present in the proximal arm and lower limb representations of the motor cortex. I wave facilitation occurs predominately in the cortex and may be primarily related to the monosynaptic corticomotoneuronal (CM) system. The reduced I wave facilitation for BB compared with APB and TA may be related to less extensive CM projection and involvement of other polysynaptic descending pathways. I wave facilitation, ICI, and ICF appears to be mediated by different neuronal circuits.     

Stability of corticospinal excitability and grip force in intrinsic hand muscles in man over a 24-h period

The maximum voluntary muscle force can vary throughout the day; typically being low in the morning and high in the evening. The nature of this possible variation has been investigated with respect to corticospinal excitability. Six healthy subjects were studied. Maximum voluntary contraction (MVC) in the thenar muscles was measured. In addition, we monitored several indices of corticospinal excitability using electromyographic (EMG) recording and transcranial magnetic stimulation (TMS) of the motor cortex. Motor evoked potentials (MEPs) were recorded while relaxed and at 10% MVC when the silent period was assessed as an index of corticospinal inhibition. Readings were taken every 3 h for 24 h. MVC of the thenar muscles did not change significantly over the 24 h. The mean areas, latencies and durations of MEPs did not show significant changes over the 24-h test period with the muscle relaxed or contracted; however, MEP area did vary between sessions at all stimulus intensities suggesting non-time-of-day-dependent changes in corticospinal excitability. Furthermore, the extent and duration of the silent period seen after the MEP in the contracted muscle did not change significantly over the 24 h of the experiment at any stimulus intensity. These results provide evidence that the MVC force of the thenar muscles and their responses to TMS are stable throughout the course of the day and suggest that, in hand muscles, corticospinal excitability may not be subject to circadian variation.     

Functional demanded excitability changes of human hand motor area

The present study was performed to examine if there are functional differences between the first dorsal interosseous (FDI) and the abductor digit minimi (ADM) muscles during different muscle contractions, namely dynamic and static contractions of the index and little finger abductions. It was also examined whether these functional differences occur at the cortical level. The motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and force curves, during the muscle contractions, were simultaneously recorded. Rest motor thresholds (RMTs) and active motor thresholds (AMTs), during dynamic and static contractions, were determined in the two muscles. In all trials, the background EMGs (B.EMGs) were kept at the same level in each muscle. Results showed that the target matching errors of dynamic contractions were statistically smaller in the FDI muscle than those in the ADM. In the FDI muscle, the AMT during dynamic contractions was significantly lower than during static ones and the MEPs elicited by TMS were larger during dynamic contractions than those during static ones. However, such results were not found in the ADM muscle. In order to investigate whether the differences were caused by the excitability changes that occurred in the cortical level, the responses elicited by subcortical stimulations were recorded using the same procedures as the experiment of TMS. Responses to subcortical stimulations during dynamic contractions were similar to those during static ones in either muscle. It is concluded that there are differences in the task-dependent MEP facilitations between the FDI and ADM muscles. And the differences are due to the functional demanded excitability changes accompanied by the cortical activation.     

Mapping of direction and muscle representation in the human primary motor cortex controlling thumb movements

Larger body parts are somatotopically represented in the primary motor cortex (M1), while smaller body parts, such as the fingers, have partially overlapping representations. The principles that govern the overlapping organization of M1 remain unclear. We used transcranial magnetic stimulation (TMS) to examine the cortical encoding of thumb movements in M1 of healthy humans. We performed M1 mapping of the probability of inducing a thumb movement in a particular direction and used low intensity TMS to disturb a voluntary thumb movement in the same direction during a reaction time task. With both techniques we found spatially segregated representations of the direction of TMS-induced thumb movements, thumb flexion and extension being best separated. Furthermore, the cortical regions corresponding to activation of a thumb muscle differ, depending on whether the muscle functions as agonist or as antagonist for flexion or extension. In addition, we found in the reaction time experiment that the direction of a movement is processed in M1 before the muscles participating in it are activated. It thus appears that one of the organizing principles for the human corticospinal motor system is based on a spatially segregated representation of movement directions and that the representation of individual somatic structures, such as the hand muscles, overlap.     

Covariation between human intrinsic hand muscles of the silent periods and compound muscle action potentials evoked by magnetic brain stimulation: evidence for common inhibitory connections

Transcranial magnetic stimuli at different stimulus intensities were applied in six healthy subjects to test the hypothesis that, in different intrinsic hand muscles, the duration of the resultant cortically evoked silent periods (C-SPs) from each stimulus would be positively correlated between muscles, indicating a common inhibitory mechanism. A figure-of-eight coil discharging through a Magstim 200 stimulator delivered 25 stimuli at each stimulus intensity at a minimum of five intensities ranging from 55% to 160% of the individual resting motor threshold. In each subject, simultaneous surface recordings from pairs of muscles were made from the first dorsal interosseous (FDI), opponents pollicis (OP), abductor pollicis brevis (APB) and abductor digiti minimi (ADM). The C-SP durations within all three muscle pairs tested were highly correlated (P<0.001). The amplitude of the preceding compound-muscle action potentials (CMAPs) was positively correlated between FDI and OP, but not between APB and ADM or FDI and ADM. C-SP duration was linearly related to stimulus intensity, but did not correlate with the latency or amplitude of the preceding CMAP. SPs elicited by peripheral nerve stimuli in pairs of hand muscles did not co-vary significantly. The results provide evidence that inhibitory influences of cortical origin are distributed widely to intrinsic hand muscles. In contrast, covariation of excitatory effects only appears between muscles synergistically involved in a motor task. Received: 16 February 1998 / Accepted: 20 May 1998     

Reduced muscle selectivity during individuated finger movements in humans after damage to the motor cortex or corticospinal tract

We investigated how damage to the motor cortex or corticospinal tract affects the selective activation of finger muscles in humans. We hypothesized that damage relatively restricted to the motor cortex or corticospinal tract would result in unselective muscle activations during an individuated finger movement task. People with pure motor hemiparesis attributed to ischemic cerebrovascular accident were tested. Pure motor hemiparetic and control subjects were studied making flexion/extension and then abduction/adduction finger movements. During the abduction/adduction movements, we recorded muscle activity from 3 intrinsic finger muscles: the abductor pollicis brevis, the first dorsal interosseus, and the abductor digit quinti. Each of these muscles acts as an agonist for only one of the abduction/adduction movements and might therefore be expected to be active in a highly selective manner. Motor cortex or corticospinal tract damage in people with pure motor hemiparesis reduced the selectivity of finger muscle activation during individuated abduction/adduction finger movements, resulting in reduced independence of these movements. Abduction/adduction movements showed a nonsignificant trend toward being less independent than flexion/extension movements in the affected hands of hemiparetic subjects. These changes in the selectivity of muscle activation and the consequent decrease in individuation of movement were correlated with decreased hand function. Our findings imply that, in humans, spared cerebral motor areas and descending pathways that remain might activate finger muscles, but cannot fully compensate for the highly selective control provided by the primary motor cortex and the crossed corticospinal system.     

Motor facilitation while observing hand actions: specificity of the effect and role of observer's orientation

Action observation enhances cortico-spinal excitability. Here we tested the specificity of this effect and the role played by the orientation of the observer. Ten normal subjects observed video clips of right hand performing three different finger movements (thumb ab-/adduction, index ab-/adduction, index extens-/flexion) in two different orientations (Away, i.e., natural hand-orientation facing out from the observer; or Toward, i.e., unnatural hand-orientation facing toward the observer). Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) were recorded from the abductor pollicis brevis (APB) and the first dorsal interosseus (FDI) muscles. Movement direction of the index finger was recorded using force transducers. Facilitation of MEP size was significantly greater for APB during observation of thumb movements and for FDI during observation of index finger movements. Facilitation of MEP size was significantly greater when the hand presented on screen was facing out from and corresponding to that of the observer (Away orientation). The direction of the index finger movement evoked by TMS shifted toward extension/flexion versus ab-/adduction matching the observed movement. Our results give further evidence that observation of a movement enhances motor output to the muscles involved in the movement and facilitates the observed action. In addition, we provide novel evidence about the high degree of specificity of this observation-induced motor cortical modulation. The degree of modulation depends on hand orientation. The modulation is maximal when the observed action corresponds to the orientation of the observer.     

Asymmetry of motor cortex excitability during a simple motor task: relationships with handedness and manual performance

Transcranial magnetic stimulation (TMS) was used to assess the relative contribution of the corticospinal (CS) pathway in activating the first dorsal interosseous (FDI) muscle in each hand of 16 right- (RH) and 16 left-handed (LH) subjects with varied degrees of hand preference. It was hypothesised that asymmetry in corticospinal activation of the two hands may be related to hand preference and interlimb differences in manual performance. Subjects performed isometric index finger abduction at force levels of 0.5 N, 1 N and 2 N while TMS was applied at resting threshold intensity (T), 0.9T, or 0.8T. The amount of contraction-induced facilitation of the muscle evoked potential (MEP) was used as an estimate of corticospinal involvement in the task. Patterns of MEP facilitation in each hand were compared with measures of manual performance (finger tapping speed, Purdue pegboard, maximal FDI strength). Threshold TMS intensities for an MEP in FDI at rest were similar in LH and RH subjects, and did not vary between hands. Facilitation of the MEP with voluntary activation was larger overall on the left side (P<0.05), but the asymmetry was dependent on the degree of lateralisation of hand preference. For subjects with consistent hand preference (either LH or RH), MEP facilitation in active FDI was larger for the left hand. For non-consistent RH subjects, contraction-induced MEP facilitation was larger in the right FDI muscle than the left. Asymmetry of MEP facilitation was not correlated with differences between hands in finger tapping speed or performance in the pegboard task, but was associated with relative differences in FDI strength. MEP facilitation tended to be larger in the stronger FDI muscle of the pair. We conclude that corticospinal involvement in the command for index finger abduction is generally greater when the left hand is used, although in RH subjects the asymmetry is influenced by the degree of lateralisation of hand preference. The corticospinal asymmetry is not related to speed or dexterity of finger movements, but the association with muscle strength suggests that it may be influenced in part by preferential use of one hand for tasks which strengthen the FDI muscle.