4.5: Techniques for Modulating Brain Activity
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TMS
History: Transcranial magnetic stimulation (TMS) is an important technique for modulating brain activity. The first modern TMS device was developed by Antony Baker in the year 1985 in Sheffield after 8 years of research. The field has developed rapidly since then with many researchers using TMS in order to study a variety of brain functions. Today, researchers also try to develop clinical applications of TMS, because there are long lasting effects on the brain activity it has been considered as a possible alternative to antidepressant medication.
Method: UMTS utilizes the principle of electromagnetic induction to an isolated brain region. A wire-coil electromagnet is held upon the fixed head of the subject. When inducing small, localized, and reversible changes in the living brain tissue, especially the directly under laying parts of the motor cortex can be effected. By altering the firing-patterns of the neurons, the influenced brain area is disabled. The repetitive TMS (rTMS) describes, as the name reveals, the application of many short electrical stimulations with a high frequency and is more common than TMS. The effects of this procedure last up to weeks and the method is in most cases used in combination with measuring methods, for example: to study the effects in detail.
Application: The TMS-method gives more evidence about the functionality of certain brain areas than measuring methods on their own. It was a very helpful method in mapping the motor cortex. For example: While rTMS is applied to the prefrontal cortex, the patient is not able to build up short term memory. That determines the prefrontal cortex, to be directly involved in the process of short term memory. By contrast measuring methods on their own, can only investigate a correlation between the processes. Since even earlier researches were aware that TMS could cause suppression of visual perception, speech arrest, and paraesthesias, TMS has been used to map specific brain functions in areas other than motor cortex. Several groups have applied TMS to the study of visual information processing, language production, memory, attention, reaction time and even more subtle brain functions such as mood and emotion. Yet long time effects of TMS on the brain have not been investigated properly, Therefore experiments are not yet made in deeper brain regions like the hypothalamus or the hippocampus on humans. Although the potential utility of TMS as a treatment tool in various neuropsychiatric disorders is rapidly increasing, its use in depression is the most extensively studied clinical applications to date. For instance in the year 1994, George and Wassermann hypothesized that intermittent stimulation of important prefrontal cortical brain regions might also cause downstream changes in neuronal function that would result in an antidepressant response. Here again, the methods effects are not clear enough to use it in clinical treatments today. Although it is too early at this point to tell whether TMS has long lasting therapeutic effects, this tool has clearly opened up new hopes for clinical exploration and treatment of various psychiatric conditions. Further work in understanding normal mental phenomena and how TMS affects these areas appears to be crucial for advancement. A critically important area that will ultimately guide clinical parameters is to combine TMS with functional imaging to directly monitor TMS effects on the brain. Since it appears that TMS at different frequencies has divergent effects on brain activity, TMS with functional brain imaging will be helpful to better delineate not only the behavioral neuropsychology of various psychiatric syndromes, but also some of the pathophysiologic circuits in the brain.
tDCS
transcranial Direct Current Stimulation: The principle of tDCS is similar to the technique of TMS. Like TMS this is a non-invasive and painless method of stimulation. The excitability of brain regions is modulated by the application of a weak electrical current.
History and development: It was first observed that electrical current applied to the skull lead to an alleviation of pain. Scribonius Largus, the court physician to the Roman emperor Claudius, found that the current released by the electric ray has positive effects on headaches. In the Middle Ages the same property of another fish, the electrical catfish, was used to treat epilepsy. Around 1800, the so-called galvanism (it was concerned with effects of today’s electrophysiology) came up. Scientists like Giovanni Aldini experimented with electrical effects on the brain. A medical application of his findings was the treatment of melancholy. During the twentieth century among neurologists and psychiatrists electrical stimulation was a controversial but nevertheless wide spread method for the treatment of several kinds of mental disorders (e.g. Electroconvulsive therapy by Ugo Cerletti).
Mechanism: The tDCS works by fixation of two electrodes on the skull. About 50 percent of the direct current applied to the skull reaches the brain. The current applied by a direct current battery usually is around 1 to 2 mA. The modulation of activity of the brain regions is dependent on the value of current, on the duration of stimulation and on the direction of current flow. While the former two mainly have an effect on the strength of modulation and its permanence beyond the actual stimulation, the latter differentiates the modulation itself. The direction of the current (anodic or cathodic) is defined by the polarity and position of the electrodes. Within tDCS two distinct ways of stimulation exist. With the anodal stimulation the anode is put near the brain region to be stimulated and analogue for the cathodal stimulation the cathode is placed near the target region. The effect of the anodal stimulation is that the positive charge leads to depolarization in the membrane potential of the applied brain regions, whereas hyperpolarisation occurs in the case of cathodal stimulation due to the negative charge applied. The brain activity thereby is modulated. Anodal stimulation leads to a generally higher activity in the stimulated brain region. This result can also be verified with MRI scans, where an increased blood flow in the target region indicates a successful anodal stimulation.
Applications: From the description of the TMS method it is should be obvious that there are various fields of appliances. They reach from identifying and pulling together brain regions with cognitive functions to the treatment of mental disorders. Compared to TMS it is an advantage of tDCS to not only is able to modulate brain activity by decreasing it but also to have the possibility to increase the activity of a target brain region. Therefore the method could provide an even better suitable treatment of mental disorders such as depression. The tDCS method has also already proven helpful for apoplectic stroke patients by advancing the motor skills.