The mirror mechanism: recent findings and perspectives

Mirror neurons are a specific type of visuomotor neuron that discharge both when a monkey executes a motor act and when it observes a similar motor act performed by another individual. In this article, we review first the basic properties of these neurons. We then describe visual features recently investigated which indicate that, besides encoding the goal of motor acts, mirror neurons are modulated by location in space of the observed motor acts, by the perspective from which the others’ motor acts are seen, and by the value associated with the object on which others’ motor acts are performed. In the last part of this article, we discuss the role of the mirror mechanism in planning actions and in understanding the intention underlying the others’ motor acts. We also review some human studies suggesting that motor intention in humans may rely, as in the monkey, on the mirror mechanism.     

Selective increase in corticospinal excitability in the context of tactile exploration

In this study, we compared changes in corticomotor excitability under various task conditions engaging the index finger of each hand. Functional demands were varied, from simple execution to demanding sensory exploration. In a first experiment, we contrasted facilitation in the first dorsal interosseus (FDI) by monitoring changes in motor evoked potentials (MEPs) when participants (young adults, n = 18) performed either a simple button pressing (BP) task or a more demanding tactile exploration (TE) task (i.e., discrimination of raised letters). This experiment showed a large effect of task conditions (p < 0.01) on MEP amplitude but no effect of “Hand”, while latency measurements were unchanged. In fact, MEPs were on average 40% larger during TE (2410 ± 1358 µV) than during BP (1670 ± 1477 µV). The two tasks produced, however, different patterns of electromyographic (EMG) activity, which could have accounted for some of the differences observed. A second experimental session involved a subset of participants (10/18) tested in third task condition: finger movement (FM). The latter task consisted of scanning a smooth surface with the tip of the index finger to reproduce the movements seen with the TE task. The addition of this third condition task confirmed that MEP facilitation seen during TE reflected task-specific influences and not differences in background EMG activity. These results, altogether, provide further insights into the effect of task conditions on corticomotor excitability. Our findings, in particular, stress the importance of behavioural context and tactile exploration in leading to selective increase in corticomotor excitability during finger movements.     

综述: 无创穿颅电刺激对人脑神经活动和认知功能的调控

穿颅电刺激被认为可以无创调节大脑的神经活动,为研究特定脑区与某一认知功能间的因果关系提供了可能.近些年,对穿颅电刺激作用机制和其对认知、运动功能调控的研究方面取得了很多重要进展.在这篇综述中,我们总结了以往关于穿颅直流电刺激、穿颅交流电刺激和穿颅随机噪声电刺激三种刺激方式的发展历史及其作用机制,同时总结了其对感知觉(主要是视觉知觉)、注意和记忆等认知功能的调控,并对未来的研究方向进行了展望.     

Global perception depends on coherent work of bilateral visual cortices: Transcranial magnetic stimulation (TMS) studies

Previous research suggests that the right and left hemispheres dominate global and local perception of hierarchical patterns, respectively. The current work examined whether global perception of hierarchical stimuli requires coherent work of bilateral visual cortices using transcranial magnetic stimulation (TMS). Subjects discriminated global or local properties of compound letters in Experiment 1. Reaction times were recorded when single-pulse real TMS or sham TMS was delivered over the left or right visual cortex. While a global precedence effect (i.e., faster responses to global than local targets and stronger global-to-local interference than the reverse) was observed, TMS decreased global-to-local interference whereas increased local-to-global interference. Experiment 2 ruled out the possibility that the effects observed in Experiment 1 resulted from perceptual learning. Experiment 3 used compound shapes and observed TMS effect similar to that in Experiment 1. Moreover, TMS also slowed global RTs whereas speeded up local RTs in Experiment 3. Finally, the TMS effects observed in Experiments 1 and 3 did not differ between the conditions when TMS was applied over the left and right hemispheres. The results support a coherence hypothesis that global perception of compound stimuli depends upon the coherent work of bilateral visual cortices.     

综述与专论: 经颅磁刺激和经颅直流电刺激在调控大脑共情功能中的应用

共情可以帮助人们建立和谐的人际关系,更好地适应现实社会,是一种重要的社会认知功能。已有研究表明,诸多神经和精神类疾病的发生发展和复发与共情缺陷有关。非侵入性脑刺激技术（经颅磁刺激和经颅直流电刺激）可以通过调节大脑皮层兴奋性来调控个体的共情水平,缓解共情缺陷症状。针对健康群体使用该技术的现有证据显示：内侧前额叶、初级运动皮层、额下回、背外侧前额叶和颞顶交界处的活动有助于提升个体的认知共情水平,其中双侧背外侧前额叶的活动还有助于下调个体的情感共情水平,而右侧颞顶交界处的活动则可以增强自我表征从而支持个体在共情时进行自我和他人的区分。少数针对共情缺陷群体使用该技术的临床证据提示,增强左侧背外侧前额叶和内侧前额叶的活动可以分别提升精神疾病和神经退行性疾病患者的认知共情水平。未来的研究应探讨在统一的行为测量范式下针对不同刺激参数和刺激位点进行共情干预研究,通过融合其他神经生理技术进一步考察非侵入性脑刺激技术改善共情功能的作用机制,并考虑个体差异性对大脑共情功能干预效果的影响。     

Stimulating the Lip Motor Cortex with Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) has proven to be a useful tool in investigating the role of the articulatory motor cortex in speech perception. Researchers have used single-pulse and repetitive TMS to stimulate the lip representation in the motor cortex. The excitability of the lip motor representation can be investigated by applying single TMS pulses over this cortical area and recording TMS-induced motor evoked potentials (MEPs) via electrodes attached to the lip muscles (electromyography; EMG). Larger MEPs reflect increased cortical excitability. Studies have shown that excitability increases during listening to speech as well as during viewing speech-related movements. TMS can be used also to disrupt the lip motor representation. A 15-min train of low-frequency sub-threshold repetitive stimulation has been shown to suppress motor excitability for a further 15-20 min. This TMS-induced disruption of the motor lip representation impairs subsequent performance in demanding speech perception tasks and modulates auditory-cortex responses to speech sounds. These findings are consistent with the suggestion that the motor cortex contributes to speech perception. This article describes how to localize the lip representation in the motor cortex and how to define the appropriate stimulation intensity for carrying out both single-pulse and repetitive TMS experiments.     

Disrupting the experience of control in the human brain: pre-supplementary motor area contributes to the sense of agency

The feeling of controlling events through one''s actions is fundamental to human experience, but its neural basis remains unclear. This ‘sense of agency’ (SoA) can be measured quantitatively as a temporal linkage between voluntary actions and their external effects. We investigated the brain areas underlying this aspect of action awareness by using theta-burst stimulation to locally and reversibly disrupt human brain function. Disruption of the pre-supplementary motor area (pre-SMA), a key structure for preparation and initiation of a voluntary action, was shown to reduce the temporal linkage between a voluntary key-press action and a subsequent electrocutaneous stimulus. In contrast, disruption of the sensorimotor cortex, which processes signals more directly related to action execution and sensory feedback, had no significant effect. Our results provide the first direct evidence of a pre-SMA contribution to SoA.     

Motor imagery—Anatomical representation and electrophysiological characteristics

Experience and results of neuropsychological studies have shown that motor imagery can improve motor performance and enhance motor learning. In recent years several electro-physiological and functional imaging studies have investigated the physiological basis for this observation. In the present essay we review two of our recent studies, in which we compared motor imagery with motor preparation and motor execution. In the first we used positron emission tomography to describe their functional anatomy and in the second we employed electromyography, H-reflexes and transcranial magnetic stimulation to delineate their electrophysiological characteristics. Both studies demonstrated that motor imagery shares some characteristics with motor preparation and other, additional ones with motor execution. Thus it can be seen as a special form of motor behaviour, similar but distinct from both motor preparation and execution. This combination of mutual and distinct characteristics may be the key to its successful role in motor learning. Special issue dedicated to Dr. Herman Bachelard.     

Do changes in spinal reflex excitability elicited by transcranial magnetic stimulation differ based on the site of cerebellar stimulation?

Abstract

Purpose: The present study aimed to investigate whether spinal reflex excitability is influenced by the site of cerebellar transcranial magnetic stimulation (C-TMS).

Materials and methods: Fourteen healthy volunteers (mean age: 24.6?±?6.6?years [11 men]) participated. Participants lay on a bed in the prone position, with both ankle joints fixed to prevent unwanted movement. Right tibial nerve stimulation was provided to elicit the H-reflex in the right soleus muscle. Conditioning transcranial magnetic stimulation (TMS) was delivered at one of the following sites 110?ms prior to tibial stimulation: right, central, or left cerebellum; midline parietal (Pz) region; or sham stimulation. A total of 10 test trials were included for each condition, in random order. The unconditioned and conditioned H-reflexes were measured during random inter-test trials, and the cerebellar spinal facilitation (CSpF) ratios for each site were calculated (the ratio of conditioned to unconditioned H-reflexes). CSpF ratios were compared among TMS sites.

Results: CSpF ratios were significantly higher at cerebellar sites than at the Pz site or during sham stimulation. However, there was no significant difference in CSpF ratio among cerebellar sites.

Conclusions: TMS conditioning over any part of the cerebellum facilitated the excitability of the spinal motoneuron pool. Facilitation of the H-reflex due to C-TMS may involve the effects of the bilateral descending tract of the spinal cord on the spinal motoneuron pool. Alternatively, direct brainstem stimulation may have activated portions of the bilateral descending tract of the spinal cord.     

Relationship between blood lactate and cortical excitability between taekwondo athletes and non-athletes after hand-grip exercise

Objectives: In taekwondo competitions, fatigue has a large influence on performance. Recent studies have reported that the excitability in the primary hand motor cortex, investigated with transcranial magnetic stimulation (TMS), is enhanced at the end of a maximal exercise and that this improvement correlates with blood lactate. The aim of the present study was to investigate the relationship between blood lactate and cortical excitability in taekwondo athletes and non-athletes.

Methods: The excitability of the primary motor cortex was measured before and after fatiguing hand-grip exercise by TMS. Capillary blood lactate was measured at rest (pre-test), at the end (0?min), and at 3 and 10?min after the exercise by using a “Lactate Pro” portable lactate analyzer.

Results: Significant differences in cortical excitability between the two groups were found after the exercise (p?p?Conclusion: The present findings showed changes in the excitability in the athletes group and also in the non-athletes group. However, blood lactate seems to have the greater effect in trained subjects compared to untrained subjects. In fact, it appears that, during extremely intensive exercise in taekwondo athletes, lactate may delay the onset of fatigue not only by maintaining the excitability of muscle, but also by increasing the excitability of the primary motor cortex more than in non-athletes.     

重复经颅磁刺激联合生物反馈疗法对帕金森病患者血清IL-6, CRP及TNF-α水平的影响

目的:探讨重复经颅磁刺激联合生物反馈用于帕金森病非运动症状的临床疗效及对血清IL-6,CRP及TNF-α水平的影响。方法:收集我院就诊的84例帕金森病患者,随机分为实验组和对照组,每组42例。对照组患者给予肌电、脑电生物反馈治疗,实验组患者在对照组基础上给予重复经颅磁刺激治疗。观察并比较两组患者治疗前后血清C反应蛋白(CRP)、白介素-6(IL-6)及肿瘤坏死因子-α(TNF-α)水平以及非运动症状筛查问卷(NMSQuest)、帕金森病睡眠量表(PDSS)、汉密尔顿抑郁量表(HAMD)评分的变化情况。结果:与治疗前相比,治疗后患者CRP,IL-6及TNF-α水平以及NMSQuest、HAMD评分均下降,而PDSS评分均升高,差异具有统计学意义(P0.05);与对照组相比,实验组患者血清CRP,IL-6及TNF-α水平以及NMSQuest、HAMD评分较低,而PDSS评分较高,差异具有统计学意义(P0.05)。结论:重复经颅磁刺激联合生物反馈能够降低帕金森病患者血清IL-6、CRP、TNF-α水平且对于非运动症状的临床疗效较好。     

Is high-frequency repetitive transcranial magnetic stimulation of the left primary motor cortex superior to the stimulation of the left dorsolateral prefrontal cortex in fibromyalgia syndrome?

Objective: To investigate effectiveness of two different high-frequency repetitive transcranial magnetic stimulation (rTMS) protocols on pain, fatigue, quality of life (QoL) and depression in female patients with fibromyalgia.

Methods: Thirty patients were randomized into three groups. Fifteen sessions of 10?Hz (90% resting motor threshold-RMT, 1200 pulses) rTMS were applied to left primary motor cortex and left dorsolateral prefrontal cortex (DLPFC) in Group M1 (n:10) and Group DLPFC (n:10), respectively. Group sham (n?=?10) received 15 sessions of sham rTMS over 3?weeks. Visual Analogue Scale, Fibromyalgia Impact Questionnaire, Fatigue Severity Scale, Short-form 36, and Beck Depression Inventory were assessed at baseline and at the end of the treatments by a blinded-experienced assessor.

Results: Significant improvements in pain, QoL, and depression scores were observed in three groups. However, improvements in depression, physical functioning, physical role functioning, and general health perceptions were greater in active rTMS groups than in sham group. Emotional role functioning was only improved in Group M1. The decrease in VAS scores was significantly greater in Group M1 when compared to sham group. Change in physical role functioning was significantly greater in Group DLPFC than in Group M1.

Conclusions: Significant improvements in physical role functioning, physical functioning, depression, and general health perceptions were achieved in active rTMS groups. Further clinical studies on larger samples involving both sexes with longer follow-up durations are needed.     

Changes in cortical excitability and blood lactate after a fatiguing hand-grip exercise

Excitability of primary hand motor cortex (PHMC), evaluated with transcranial magnetic stimulation by using the “1?mV resting motor threshold” method, and capillary blood lactate were measured at the end, as well as 5 and 10?min after a fatiguing hand-grip exercise. The relation between blood lactate and the amplitudes of motor-evoked potentials showed a significant direct proportionality. Blood lactate seems to exert a protective role on PHMC against fatigue reduction during extremely intensive isometric exercises.     

The transfer of motor functional strategies via action observation

When someone is choosing one piece from a bowl full of fruit, many pieces are within reach and visible. Although the desired piece seems to govern the particular pattern and direction of that person's reaching movement, the selection process is not impervious to the presence of task-irrelevant information (i.e. the other fruits). Evidence suggests that the kinematics of reach-to-grasp actions for a desired object integrates the motor features of all the objects which might become potential targets. Transcranial magnetic stimulation (TMS) and motor-evoked potentials (MEPs) were used by us to establish if that motor integration process can be transferred to an onlooker. Our results indicate that observation of hybrid reach-to-grasp movement kinematics is reflected in the observer's pattern of MEP amplitudes. This effect can be defined as a form of motor resonance which operates by 'reading' the kinematics of an observed action. The brain's ability to mirror motor integration processes while observing someone else's action helps an onlooker to understand what the other person is doing and to predict his/her motor alternatives.