Interhemispheric asymmetry of the human motor cortex related to handedness and gender

Most people are right-handed, preferring the right hand for skilled as well as unskilled activities, but a notable proportion are mixed-handed, preferring to use the right hand for some actions and the left hand for others. Assuming a structural/functional correlation in the motor system we tested whether asymmetries in hand performance in consistent right and left handers as well as in mixed handers are associated with anatomical asymmetries in the motor cortex. In vivo MR morphometry was used for analyzing interhemispheric asymmetry in the depth of the central sulcus in the region of cortical hand representation of 103 healthy subjects. Subjects were tested both for hand preference and hand performance. As expected, left-right differences in hand performance differed significantly between consistent right, consistent left and mixed handers and were independent on gender. Male consistent right handers showed a significant deeper central sulcus on the left hemisphere than on the right. Anatomical asymmetries decreased significantly from male consistent right over mixed to consistent left handers. Sixty two per cent of consistent left handers revealed a deeper central sulcus on the right than on the left hemisphere, but for the group as a whole this rightward asymmetry was not significant. No interhemispheric asymmetry was found in females. Thus, anatomical asymmetry was associated with handedness only in males, but not in females, suggesting sex differences in the cortical organization of hand movements.     

Visual and motor cortex excitability: a transcranial magnetic stimulation study.

OBJECTIVES: Phosphene thresholds (PTs) to transcranial magnetic stimulation over the occipital cortex and motor thresholds (MTs) have been used increasingly as measures of the excitability of the visual and motor cortex. MT has been utilized as a guide to the excitability of other, non-motor cortical areas such as dorsolateral prefrontal cortex. The aims of this study were to compare the PTs to MTs; to assess their stability across sessions; and to investigate their relation to MTs. METHODS: PTs and MTs were determined using focal transcranial magnetic stimulation over the visual and motor cortex. RESULTS: PTs were shown to be significantly higher than MTs. Both PTs and MTs were stable across sessions. No correlation between PTs and MTs could be established. CONCLUSIONS: Phosphene threshold is a stable parameter of the visual cortex excitability. MTs were not related to the excitability of non-motor cortical areas.     

Focal enhancement of motor cortex excitability during motor imagery: a transcranial magnetic stimulation study

OBJECTIVES: In order to learn more about the physiology of the motor cortex during motor imagery, we evaluated the changes in excitability of two different hand muscle representations in the primary motor cortex (M1) of both hemispheres during two imagery conditions. MATERIALS AND METHODS: We applied focal transcranial magnetic stimulation (TMS) over each M1, recording motor evoked potentials (MEPs) from the contralateral abductor pollicis brevis (APB) and first dorsal interosseus (FDI) muscles during rest, imagery of contralateral thumb abduction (C-APB), and imagery of ipsilateral thumb abduction (I-APB). We obtained measures of motor threshold (MT), MEP recruitment curve (MEP-rc) and F waves. RESULTS: Motor imagery compared with rest significantly decreased the MT and increased MEPs amplitude at stimulation intensities clearly above MT in condition C-APB, but not in condition I-APB. These effects were not significantly different between right and left hemisphere. MEPs simultaneously recorded from the FDI, which was not involved in the task, did not show facilitatory effects. There were no significant changes in F wave amplitude during motor imagery compared with rest. CONCLUSIONS: Imagery of unilateral simple movements is associated with increased excitability only of a highly specific representation in the contralateral M1 and does not differ between hemispheres.     

Transferability of cathodal tDCS effects from the primary motor to the prefrontal cortex: A multimodal TMS-EEG study

Neurophysiological effects of transcranial direct current stimulation (tDCS) have been extensively studied over the primary motor cortex (M1). Much less is however known about its effects over non-motor areas, such as the prefrontal cortex (PFC), which is the neuronal foundation for many high-level cognitive functions and involved in neuropsychiatric disorders. In this study, we, therefore, explored the transferability of cathodal tDCS effects over M1 to the PFC. Eighteen healthy human participants (11 males and 8 females) were involved in eight randomized sessions per participant, in which four cathodal tDCS dosages, low, medium, and high, as well as sham stimulation, were applied over the left M1 and left PFC. After-effects of tDCS were evaluated via transcranial magnetic stimulation (TMS)-electroencephalography (EEG), and TMS-elicited motor evoked potentials (MEP), for the outcome parameters TMS-evoked potentials (TEP), TMS-evoked oscillations, and MEP amplitude alterations. TEPs were studied both at the regional and global scalp levels. The results indicate a regional dosage-dependent nonlinear neurophysiological effect of M1 tDCS, which is not one-to-one transferable to PFC tDCS. Low and high dosages of M1 tDCS reduced early positive TEP peaks (P30, P60), and MEP amplitudes, while an enhancement was observed for medium dosage M1 tDCS (P30). In contrast, prefrontal low, medium and high dosage tDCS uniformly reduced the early positive TEP peak amplitudes. Furthermore, for both cortical areas, regional tDCS-induced modulatory effects were not observed for late TEP peaks, nor TMS-evoked oscillations. However, at the global scalp level, widespread effects of tDCS were observed for both, TMS-evoked potentials and oscillations. This study provides the first direct physiological comparison of tDCS effects applied over different brain areas and therefore delivers crucial information for future tDCS applications.     

Kick with the finger: symbolic actions shape motor cortex excitability

A large body of research indicates that observing actions made by others is associated with corresponding motor facilitation of the observer's corticospinal system. However, it is still controversial whether this matching mechanism strictly reflects the kinematics of the observed action or its meaning. To test this issue, motor evoked potentials induced by single‐pulse transcranial magnetic stimulation were recorded from hand and leg muscles while participants observed a symbolic action carried out with the index finger, but classically performed with the leg (i.e., a soccer penalty kick). A control condition in which participants observed a similar (but not symbolic) hand movement was also included. Results showed that motor facilitation occurs both in the observer's hand (first dorsal interosseous) and leg (quadriceps femoris) muscles. The present study provides evidence that both the kinematics and the symbolic value of an observed action are able to modulate motor cortex excitability. The human motor system is thus not only involved in mirroring observed actions but is also finely tuned to their symbolic value.     

Correlates of human handedness in primary motor cortex: a review and hypothesis

A review of the research on anatomical and functional asymmetries in human primary motor cortex suggests that the area of hand representation is greater in the dominant than in the non-dominant hemisphere and that there is a greater dispersion of elementary movement representations with more profuse horizontal connections between them. The more profuse interconnections in motor cortex (M1) of the dominant hemisphere might form a neural substrate which favors the formation of experience-dependent excitatory and inhibitory interactions between elementary movement representations. Motor practice might lead to more precise spatiotemporal coordination of the activity of the elementary movement representations in M1 of the dominant than that of the non-dominant hemisphere, thus leading to more dexterous behavior of the dominant than that of the non-dominant hand.     

Visual cortex excitability in migraine before and after valproate prophylaxis: a pilot study using TMS

We examined the effect of standard migraine prophylaxis with sodium valproate on repeated measures of occipital excitability using transcranial magnetic stimulation (TMS). We predicted that, comparing pre- and post-treatment assessments, a reduction in clinical migraine parameters would be paralleled by a decrease in excitability measurements.A total of 31 migraine patients enrolled in the study, for assessment prior to and 1 month after commencement of sodium valproate prophylaxis. At each assessment, we used a standardized protocol to stimulate the occipital cortex with a 90-mm circular (coil A) and 70 mm figure-of-eight (coil B) coil. We recorded the threshold stimulation intensity at which subjects just perceived phosphenes. Subjects kept detailed records of headache parameters 1 month before and also during the study period.Valproate therapy significantly improved headache indexes, as expected. In MA subjects assessed with coil B, phosphene thresholds were significantly higher post-treatment than pre-treatment, but those for MO did not change. Modest correlations were observed in MA patients between increase in phosphene threshold and decrease in headache index. Although preliminary, the findings with coil B lend some support to the notion that effective migraine prophylaxis may be achieved through lowering cortical excitability by gamma-aminobutyric acid (GABA)-ergic intervention. Further investigation of the effect of sodium valproate or other similarly acting substances on cortical excitability in migraine is warranted.     

Exploration of motor cortex excitability in a diabetic patient with hemiballism-hemichorea.

Hemiballism-hemichorea in older patients with hyperglycemia, associated with high signal intensity in the contralateral striatum on T1-weighted magnetic resonance scans, is now an accepted clinical entity. We present an additional patient with this disorder. Using transcranial magnetic stimulation, we show that intracortical inhibition in the motor cortex contralateral to hemiballism-hemichorea is increased. This finding is discussed in the context of current models of basal ganglia-thalamo-cortical connectivity.     

Cyclical excitability of the motor cortex in patients with catamenial epilepsy: A transcranial magnetic stimulation study

PURPOSE: The pathophysiology of catamenial epilepsy is still unclear. Therefore, we investigated the cortical excitability of women with catamenial epilepsy during different phases of the menstrual cycle. METHODS: Using transcranial magnetic stimulation, six patients suffering from catamenial epilepsy were investigated during ovulatory cycles. On days 8, -14, -7 and 2 of the cycle (day 1 being the first day of menstrual bleeding), resting motor threshold (RMT), cortical silent period (CSP), intracortical inhibition (ICI) and intracortical facilitation (ICF) were investigated. The non-parametric Friedman-test for multiple comparisons and Wilcoxon signed rank test were used for statistical analysis. RESULTS: Five patients suffered from focal epilepsy (three right hemispheric, one bitemporal, one unknown origin) and one patient had idiopathic generalized epilepsy. All patients experienced perimenstrual seizure clustering and two also showed an increased seizure frequency during the luteal phase. In the right hemispheres there was a significant change of CSP duration in the course of the menstrual cycle (chi(2)=8.3, P=0.041), due to a shorter CSP during the luteal phase (Z=-2.0, P=0.043) and menstruation (Z=-2.2, P=0.028) as compared to the follicular phase. There was no significant variation of CSP in the left hemispheres. RMT, ICI and ICF showed no significant changes in the course of the menstrual cycle. CONCLUSIONS: The CSP changes suggest a decreased inhibition involving GABA-ergic neurotransmission during the luteal phase and menstruation. These TMS alterations correlated with the clinical course of the epilepsies and were found in the hemispheres containing the majority of the epileptogenic zones.     

Effects of citalopram on the excitability of the human motor cortex: a paired magnetic stimulation study

Several recent reports suggest the possibility of monitoring pharmacological effects on brain excitability through transcranial magnetic stimulation (TMS). Different drugs have been studied using paired magnetic stimulation in normal subjects and patients. In particular, it has been suggested that antidepressant drugs may have an appreciable effect on motor excitability. The aim of the present study was to investigate motor area excitability in normal subjects after oral administration of a single dose of citalopram, a selective serotonin reuptake inhibitor (SSRI) antidepressant. Motor cortex excitability was studied by single and paired transcranial magnetic stimulation before and 2.5 and 36 (t1/2=36 h) h after oral administration of 30 mg of citalopram. Cortical excitability was measured using different transcranial magnetic stimulation parameters: motor threshold (MT), motor-evoked potential (MEP) amplitude and latency, motor recruitment, duration of cortical silent period (CSP), intracortical inhibition and intracortical facilitation. Spinal excitability and peripheral nerve conduction were measured by F response and M wave. Temporary but significant increases in motor threshold, motor-evoked potentials, silent period and intracortical inhibition were observed 2.5 h after drug administration, without any significant changes in motor-evoked potential amplitude and latency and spinal excitability parameters. Our findings suggest that a single oral dose of citalopram can induce significant but transitory suppression of motor cortex excitability in normal subjects.     

Modulating cortical excitability in acute stroke: a repetitive TMS study.

OBJECTIVE: Changes in cerebral cortex excitability have been demonstrated after a stroke and are considered relevant for recovery. Repetitive transcranial magnetic stimulation (rTMS) of the brain can modulate cerebral cortex excitability and, when rTMS is given as theta burst stimulation (TBS), LTP- or LTD-like changes can be induced. The aim of present study was to evaluate the effects of TBS on cortical excitability in acute stroke. METHODS: In 12 acute stroke patients, we explored the effects of facilitatory TBS of the affected hemisphere and of inhibitory TBS of the unaffected hemisphere on cortical excitability to single-pulse transcranial magnetic stimulation (TMS) on both sides. The effects produced by TBS in patients were compared with those observed in a control group of age-matched healthy individuals. RESULTS: In patients, both the facilitatory TBS of the affected motor cortex and the inhibitory TBS of the unaffected motor cortex produced a significant increase of the amplitude of MEPs evoked by stimulation of the affected hemisphere. The effects observed in patients were comparable to those observed in controls. CONCLUSIONS: Facilitatory TBS over the stroke hemisphere and inhibitory TBS over the intact hemisphere in acute phase enhance the excitability of the lesioned motor cortex. SIGNIFICANCE: TBS might be useful to promote cortical plasticity in stroke patients.     

Medial prefrontal cortex reacts to unfairness if this damages the self: a tDCS study

Neural correlates of unfairness perception depend on who is the target of the unfair treatment. These previous findings suggest that the activation of medial prefrontal cortex (MPFC) is related to unfairness perception only when the subject of the measurement is also the person affected by the unfair treatment. We aim at demonstrating the specificity of MPFC involvement using transcranial direct current stimulation (tDCS), a technique that induces cortical excitability changes in the targeted region. We use a modified version of the Ultimatum Game, in which responders play both for themselves (myself—MS condition) and on behalf of an unknown third-party (TP condition), where they respond to unfairness without being the target of it. We find that the application of cathodal tDCS over MPFC decreases the probability of rejecting unfair offers in MS, but not in TP; conversely, the same stimulation increases the probability of rejecting fair offers in TP, but not in MS. We confirm the hypothesis that MPFC is specifically related to processing unfairness when the self is involved, and discuss possible explanations for the opposite effect of the stimulation in TP.     

帕金森病患者运动皮质兴奋性的经颅磁刺激研究

目的:本研究拟应用低频重复性经颅磁刺激(rTMS)分别刺激帕金森病(PD)患者M1手代表区(M1Hand)及运动前区(PMC),探讨不同干预手段对运动皮质兴奋性的影响,以及M1与PMC间的联系。方法:对18名确诊PD患者先后进行4种不同干预,即口服美多芭、低频rTMS刺激M1Hand(0.5Hz,100%静息阈值,共1600次脉冲)、低频rTMS刺激PMC(0.5Hz,100%静息阈值,共1600次脉冲)以及假刺激。于每次干预前后各进行临床评价并测定运动诱发电位(MEP)相关指标。结果:①口服美多芭后UPDRSⅢ(P=0.001)以及其中有关僵直(P=0.001)、运动迟缓(P<0.001)的评分均较服药前显著改善。三种不同磁刺激干预产生结果不同,M1Hand组UPDRSⅢ减低(P=0.015),僵直(P=0.010)、运动迟缓(P=0.004)亦有所改善;PMC组UPDRSⅢ较干预前减低(P=0.046),僵直评分亦减低,但无显著性意义(P=0.163);②口服美多芭1h后MEP120减低(P=0.002),CSP延长(P=0.006);M1Hand组MEP120无著变,而CSP延长(P=0.015);PMC组MEP120减低(P=0.004),而CSP无著变;假刺激组则均无显著性改变。结论:低频rTMS对不同脑区产生的效应不同:刺激M1可使CSP延长;而刺激PMC可使MEP波幅减低。     

Different strategies do not moderate primary motor cortex involvement in mental rotation: a TMS study

Background  

Regions of the dorsal visual stream are known to play an essential role during the process of mental rotation. The functional role of the primary motor cortex (M1) in mental rotation is however less clear. It has been suggested that the strategy used to mentally rotate objects determines M1 involvement. Based on the strategy hypothesis that distinguishes between an internal and an external strategy, our study was designed to specifically test the relation between strategy and M1 activity.     

EEG alpha asymmetry in musicians and non-musicians: a study of hemispheric specialization

Forty subjects, 10 male musicians, 10 female musicians, 10 male non-musicians and 10 female non-musicians, were presented with 2 min tape recorded segments of linguistic, atonal and musical stimuli. Bilateral EEG recordings were made and alpha asymmetry was computed for each type of stimulus for each subject. Musicians and non-musicians differed significantly (P < 0·005) only for the musical stimuli. Right hemispheric activation accompanied music among non-musicians but was absent for musicians. These data suggest that hemispheric specialization is related to perceptual processing and experience and not merely to the acoustic properties of stimuli. Additionally, no sex differences were observed.