Brain stimulation refers to the application of electric fields to the brain for therapeutic purposes. As a technique, it dates back to ancient times, when early doctors used electric fish to treat – we presume with limited success – a range of malaises. Today, electrical brain stimulation is widely used in neurology and psychiatry to treat ailments such as Parkison’s, major depression, neuropathic pain and for stroke rehabilitation.
We can distinguish two major branches of electrical brain stimulation: invasive and non-invasive. The latter is also referred to as transcranial stimulation (i.e., “through the skull”).
Invasive methods require the surgical implantation of electrodes, either in contact with the cortex of the brain (electrical cortical stimulation or ECS), or in deeply located selected targets (deep brain stimulation or DBS, used successfully to treat pain, Parkinson’s disease and Dystonia).
On the other hand, non-invasive methods such as transcranial direct current stimulation (tDCS) or transcranial magnetic stimulation (TMS) do not involve any surgical procedures, using scalp electrodes (tDCS) or non-contact methods (TMS) for stimulation. There are trade-offs in using one set of methods vs. the other. Invasive methods may involve more accuracy in stimulating appropriate areas in the brain – including subcortical structures –, while non-invasive methods are more patient-friendly and do not present problems associated to surgery and implants.
Non-invasive brain stimulation methods using weak electric currents, namely tDCS and related techniques (transcranial Alternating Current Stimulation, or tACS, and transcranial Random Noise Stimulation, or tRNS) – all of which we refer to generically as tCS – generate electric fields through the transcranial delivery of weak currents at low frequencies (typically < 1 kHz) resulting in small electric fields in the brain (with amplitudes of about 0.2-2 V/m).
On the other hand, TMS is based on the application of magnetically mediated strong, short (pulsed), localized electric fields to the cortex, and, unlike tCS, is capable of inducing action potentials – i.e., making neurons fire. Other supra-threshold non-invasive techniques include Transcranial (current) Electrical Stimulation (TES) and Electro-Convulsive Therapy (ECT), both of which involve much stronger currents and electric fields in the brain than tCS.
From work in the last 50 years, we know that sub-threshold weak electric fields, which can hardly alter the function of a single neuron, can have a substantial impact on a network of neurons. The impact is neuromodulatory: adjusting the direction of the current and hence the resulting electric fields, neuronal populations can be made to fire more or less intensely. And, remarkably, these temporary alterations of excitability have an impact on brain connectivity.
While typical experiments with tDCS are designed to elicit a period of effect on the order of tens of minutes following the end of stimulation, it is known that more frequent applications of brain stimulation induce longer-lasting – and therapeutically relevant – effects in the target area. The brain is plastic, and synaptic connections change with time as encapsulated by Hebb’s rule: “When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased” (Hebb, 1949). Or, stated more simply, “Neurons that fire together, wire together”. When we apply tDCS to some target in the brain, we make a neuronal population more or less excitable. Roughly, the former will favor the establishment of new connections (LTP), the latter their extinction (LTD). Altering brain connectivity is the conduit to treat diseases such as chronic pain and depression or for stroke rehabilitation.
Today, typical tDCS sessions last about 20 minutes, with currents of about 1 mA and voltages applied in the order of 5 V. In order to produce effects, researchers have found out that it is necessary to repeat such sessions during a number of times. tDCS appears to be a safe technique whose only know side effects are skin related temporary alterations (itching, tingling).
While classic tDCS devices rely on the use of two large sponge electrodes to apply controlled currents, modern tDCS devices provide the means for more focal, multisite stimulation with concurrent EEG recording. Clearly, stimulation focality provides for control of the therapeutic scenario, and together with modeling will lead to more successful treatment outcomes. In addition, multisite brain stimulation with controlled frequencies and phases may become an important technique. Support of brain activity involves the orchestrated activity of different and spatially separated brain regions, with specific frequencies and phases. Phase synchronization, entrainment and phase reset are phenomena believed to play a key role in the oscillating brain. Concurrent EEG recording can provide the means for stimulation impact analysis and, ultimately, online tuning of stimulation parameters.
For more info: http://www.ncbi.nlm.nih.gov/pubmed/22949089
Image credit http://cordis.europa.eu/fp7/ict/programme/docs/fp7-fet-nl-11_en.pdf