无创式脑调制的神经效应研究进展

Developments of neural effects induced by noninvasive brain modulation

无创式脑调制(noninvasive brain modulation,NBM)技术是基于电磁感应原理,采用电场或磁场以非侵入的方式刺激中枢神经系统(central nervous system,CNS),进而改善脑功能.现在,它不仅是诊断和治疗神经精神疾病(neuropsychiatric disorders)的一个有效手段,同时也是研究脑生理和脑功能的常用工具,此外在探索认知、情感、记忆和语言等方面也有着巨大的应用价值.虽然,NBM技术在认知神经科学、神经生理学及神经精神病学等各个领域被广泛成功的应用,但是它对脑功能和CNS的调节机制目前还不清楚,这严重地限制了该类技术的进一步应用以及研发.NBM技术作用的共同规律是通过在脑组织周围产生感应电场来调节相应脑区的神经活动.因此,明确不同电场作用下神经元放电活动的演化规律以及相应的发生机制是揭示NBM技术神经调节机制的关键.本文首先介绍了NBM在神经科学中的应用现状,然后对近年来有关电场神经调节效应的研究成果进行了综述,包括电生理实验和计算模型仿真,最后对未来的研究方向进行了展望.

Noninvasive brain modulation （NBM） is such a stimulation technique that uses electric or magnetic fields to non-intrusively stimulate the central nervous system （CNS）. Common NBM modalities include transcranial magnetic stimulation （TMS） and transcranial direct current stimulation （tDCS）. TMS usually uses pulsed fields of 1-4 Tesla in intensity with duration less than a millisecond to stimulate brain. Such strong field is sufficient to directly evoke action potentials in relevant neural tissue, which is referred as to suprathreshold field. Unlike TMS, tDCS delivers weak constant current of 0.5-2 mA in intensity to stimulate human cortex by means of a pair of electrode pads placed on the scalp. Such weak stimulus is unable to excite neural tissue to initiate action potentials, which is referred as to subthreshold fields. Nowadays, NBM has become a promising option for the treatment, diagnosis, and rehabilitation of a number of neurologic or psychiatric disorders in clinics, such as, epilepsy, schizophrenia, Alzheimer＇s disease, depression, Parkinson＇s disease, stroke, as well as pain syndromes. What is more, it is also a common tool used to study brain physiology and function, which has great potentials in exploring cognitive, emotion, memory, attention, learning, perception, language, and plasticity. As a noninvasive method of intervening brain activities, the NBM technique has significantly promoted the developments of cognitive science, neurophysiology, neurology and psychiatry in recent years. Although NBM has been widely and successfully applied in clinics and basic researches, it is still largely unknown how it interacts with CNS and modulates brain function. Such insufficient understanding of the potential mechanisms underlying NBM would delay the design of more effective stimulator as well as limit the development and optimization of stimulation protocols. One common principle for NBM techniques is that they modulate neural activity and ultimately behavior through the generation of extracellular electric fields around the interested brain tissue. According to this principle, two key problems should be considered when we are probing how NBM participates in brain activity and function. One is to determine the characteristics of extracellular fields generated in the brain tissue during stimulation. The common method for addressing this problem is finite element analysis. The other one is to investigate how this induced field modulates neural activity to alter brain function and ultimately behavior. The focus of recent researches associated with this problem is to identify the spiking behaviors of neural system stimulated by electric fields as well as to explore the biophysical basis for their generation. In this paper, we first introduce the application status of NBM in neuroscience and clinics. Then, we review the main findings on the neuromodulation effects of electric field, including electrophysiological experiment and computational modeling simulation. Finally, we raise several key issues that need to be addressed in the future.