Variable Current Source for Brain Stimulation

Abstract

Transcranial direct current stimulation (tDCS), is a non-invasive, painless brain stimulation treatment that uses direct electrical currents to stimulate specific parts of the brain. A constant, low intensity current is passed through two electrodes placed over the head which modulates neuronal activity. There are three types of stimulation with tDCS: anodal, cathodal and sham stimulation. Anodal stimulation acts to excite neuronal activity while cathodal stimulation inhibits or reduces neuronal activity. Sham stimulation emits a brief current but then remains off for the remainder of the stimulation time.

Although tDCS is still an experimental form of brain stimulation, it potentially has several advantages over other brain stimulation techniques. It is cheap, non-invasive, painless and safe. It is also easy to administer and the equipment is easily portable. The most common side effect of tDCS is a slight itching or tingling on the scalp.

Several studies suggest it may be a valuable tool for the treatment of neuropsychiatric conditions such as depression, anxiety, Parkinson’s disease, and chronic pain.

Research has also demonstrated cognitive improvement in some patients undergoing tDCS.

 Introduction

What is tDCS?

Transcranial direct current stimulation (tDCS) is a form of neurostimulation which is the modulation of the nervous system`s activity using invasive (microelectrodes) or non-invasive (transcranial magnetic stimulation or transcranial electric stimulation, tES, such as tDCS or transcranianl alternating current stimulation). By using constant, low direct current delivered via electrodes on the head. It can be contrasted with cranial electrotherapy stimulation, which generally uses alternating current the same way to treating variety of conditions such as anxiety, depression and insomnia.

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[1]

It was originally developed to help patients with brain injuries or psychiatric conditions like major depressive disorder (a mental disorder characterized by low mood that is present across most situations). tDCS appears to have some potential for treating depression.[2][3] However, there is mixed evidence about whether tDCS is useful for cognitive enhancement in healthy people. Several reviews have found evidence of small yet significant cognitive improvements.[4][5][6][7]Other reviews found no evidence at all,[8][9] although one of them[9] has been criticized for overlooking within-subject effects[10] and evidence from multiple-session tDCS trials. There is no good evidence that tDCS is useful for memory deficits in Parkinson’s disease and Alzheimer’s disease,[11] schizophrenia,[12] non-neuropathic pain,[13] or improving upper limb function after stroke.[14][15]

Human Brain

To really understand tDCS, the human brain must be understood. The brain is the most complex organ in the human body, comprised of an intricate network of BILLIONS of nerve cells, called neurons. These special cells control and react to everything that happens in our bodies. The neurons in your brain communicate to each other using tiny electrical and chemical impulses called synapses. Electrical synapses, unlike chemical synapses, conduct nerve impulses faster (approximately 10 times faster), causing vital information to pass from neuron to neuron more quickly.

History of tDCS

Transcranial electrical stimulation techniques. While tDCS uses constant current intensity, tRNS and tACS use oscillating current. The vertical axis represents the current intensity in milliamp (mA), while the horizontal axis illustrates the time-course.

In transcranial magnetic stimulation (TMS), an electric coil is held above the region of interest on the scalp that uses rapidly changing magnetic fields to induce small electrical currents in the brain. There are two types of TMS: repetitive TMS and single pulse TMS. Both are used in research therapy but effects lasting longer than the stimulation period are only observed in repetitive TMS. Similar to tDCS, an increase or decrease in neuronal activity can be achieved using this technique, but the method of how this is induced is very different. Transcranial direct current stimulation has the two different directions of current that cause the different effects. Increased neuronal activity is induced in repetitive TMS by using a higher frequency and decreased neuronal activity is induced by using a lower frequency.[26]

Variants related to tDCS include tACS, tPCS and transcranial random noise stimulation (tRNS), a group of technologies commonly referred to as transcranial electrical stimulation, or TES.[34]

Transcranial electrical stimulation techniques. While tDCS uses constant current intensity, tRNS and tACS use oscillating current. The vertical axis represents the current intensity in milliamp (mA), while the horizontal axis illustrates the time-course.

Current applications and outcomes**

The use of transcranial direct current stimulation (tDCS) has been hampered by the lack of knowledge to its long-term adverse effects and by the potentiale misusee of the technologye due to its very ease of its applicatione.

Despitee the restrictionse on the widespreade use of tDCS, it is being researched in severale disorderse discussed in this article.

Neurons, synapses, neural network. Image Credit: Naeblys / Shutterstock

Major Depressive Disorder

Since evidence suggestse that depressione is due to lack of activity in the connectionse between certain specific areas of the cortex and the limbic system and the dorsal and ventrale neural system regulate cognitive control and emotionale evaluation, respectively. Impairment of dorsal and enhancemente of ventral system activity may cause impaired attention, and other negative emotional responses.

Applying tDCS with the electrodes over the dorsal area is associated with improved working memory and affective processing, by increasing the activity of the left area, and has been shown to improve response rates in clinical depression.

Bipolar Disorder

The use of tDCS can decrease depressive symptoms in bipolar disorder, with the effects lasting a month after the cessation of therapy sessions.

Schizophrenia

In schizophrenia, positive, negative and cognitive symptoms occur. Positive symptoms include hallucinations and delusions, with aberrant thinking and abnormal movements. Negative symptoms include social withdrawal and blunting of affect. The use of tDCS with the electrodes over the left temporoparietal junction and the left or right prefrontal area has been shown to reduce positive or negative symptoms.

Obsessive-Compulsive Disorder

Both medication and cognitive-behavioral therapy are used in the treatment of this disorder, but tDCS may help to correct the imbalance in the functioning of the cortico-striato-thalamo-cortical neural circuts. Thus, it may be a better alternative for the use of deep brain stimulation, which is currently used in refractory cases, by reducing the pathological hyperexcitability of the premotor and motor system. A trial is currently being carried out using electrodes on the pre-supplementary motor area and deltoid could enable the current to reach the appropriate cortical and subcortical areas.

Childhood Mental Illness

Since the early onset of treatment of mental disorders could lead to a reduced duration of illness and a better outcome, the use of tDCS is being explored to take advantage of greater brain plasticity in this age group, as well as to better understand the relative contribution of each brain area to various pathological mental conditions. For instance, its use has been found to be useful in Broca’s area to facilitate vocabulary acquisition in autistic children with poor use of words. However, the potential risk of abnormal neural development following exposure to extrinsic electrical current should always be kept in mind when dealing with children.

Substance Abuse

The reward circuits in the brain are implicated in the evolution of substance abuse. This shows the important involvement of the prefrontal cortex, for instance, in chemical dependency. The application of tDCS has been explored in users of cocaine, alcohol, nicotine, food, cannabis and other chemical substances, which showed broadly promising results.

Cognitive Disorders

The use of tDCS to improve working memory, executive function, and other aspects of cognition which decline in diseases such as Alzheimer’s disease, has shown improvement in several parameters after one or a few sessions of tDCS.

Electrodes placement*

• How do I know where to position the electrodes?

The great thing about tDCS is that there’s not just one placement! Research has shown that tDCS can produce a variety of affect depending on what areas of the brain are stimulated. To learn more about the different positions, you should familiarize yourself with positioning codes, such as the International 10-20 System which is use to describe the location of the electrodes. As you begin researching different montages (aka electrode placements), it will help to refer to the 10-20 chart to understand what locations the author is discussing. While it is easy to visually estimate electrode placement based on the International 10-20 System, or by using online tDCS montage guides, precise positioning is recommended. Precise positioning involves measuring distances on the head using anatomical landmarks. It’s also good to familiarize ourselves with the names and basic functions of the various cortexes of the brain. The more we know, the more we grow!

Electrode placements are different for various scenarios. using the foc.us headset we will see an examples for electrodes placement as There are alternative placements and new tDCS montage information being published almost every day!  What I hope to convey is the versatility the foc.us headset provides through various electrode placements. So here we go…

DEPRESSION

The most common depression treatment using tDCS places the anode at F3 (high on the left forehead) and the cathode at FP2 (just above the right eye on the forehead).  See below…

(The unusual foc.us “built-on” electrode placement puts the anode at FP1 and F3 and the cathode at FP2 and F4.  So yes, it supplies current in the general area suggested for depression treatment.  Has this unusual arrangement been scientifically studied?  Not to my knowledge. There is anecdotal evidence that indicates that it works.  BTW Notice the air-gap between the sponge and the “head” in the upper right of the photo.  Be careful when you put the headset on your “real” head that there are no air-gaps.)

“SAVANT” LEARNING

This montage is been in the press of late and is easy to do with the foc.us headset with the accessory kit. Normally, the anode is placed at about T4 (the right temple) and the cathode at about T3 (the left temple.)

(The accessory kit includes wire electrodes that connect to the back of the headset. You can then attach the electrodes wherever your montage requires.)

(If you use the wire electrodes, remember that the “built-on” electrodes remain active.  You can use them if appropriate or remove the sponges to not use them.  In this case, the built-on electrodes are not used at all – in fact you can take the headset off and set if on your desk – or do what I do, just let it rest on your neck.)

(If using the wire electrodes, the headset does not need to be on your head!)

MEMORIZATION and LEARNING

An interesting memorization and learning montage involves placing the anode at FP1 (above the left eye on the forehead) and the cathode at FP2 (above the right eye on the forehead).

(As with the depression montage shown earlier, the unusual placement of the built-on electrodes is “more” than required for this montage. Possibly the upper sponges could be left out to more precisely match the montage requirements. However, current density could be too high or irritation might result as the sponges are small. Research is needed! An alternative is to use the wire electrodes and not put the headset on the head at all.)

CHRONIC PAIN

Chronic pain is addressed in at least a couple of different montages.  One example is to place the anode at C4 (above the right ear, halfway to the center of the head) and the cathode at FP1 (above the left eye on the forehead).  There are other montages for chronic pain – so look those up on the web if the one I show is not what you are looking for.) Another montage places the anode at either C3 or C4 and the corresponding cathode at FP1 or FP2 on the same side of the head (left or right) to treat chronic pain on the opposite side of the body (left or right).

(This is another example of how versatile the foc,us headset can be. Use the wire electrodes for this chronic pain montage and set the headset on your desk or leave it resting on your neck – with no sponges.)

* Dr. Brent William’s tDCS Blog – Another good blog

What are montages and protocols?

Montages are the many different possible tDCS positions of both the anode and cathode electrodes on the body. The Protocol is actually a combination of the tDCS montage (electrode position) being used, the current level being used, and the total duration of the session. The most officially researched protocol is anodal (positiv me: red) stimulation of the left dorsolateral prefrontal cortex (L-DLPFC – F3), and cathode (negative: black) positioned on right Frontopolor (Fp2), located on the forehead above the right eyebrow, using 1mA of current for 20 minutes. This montage has been shown to improve many cognitive functions such as working memory, impulse control, reasoning, and learning.

What are some montages and their uses?

There are many websites which provide information on tDCS montages, electrode placement instructions, and types of effects to be expected. Below are a few of the most reliable websites currently providing tDCS electrode montage placement information. In particular, Total tDCS has 3D graphics which makes estimating electrode placement much easier.

Adverse effects and Contraindication

People susceptible to seizures, such as people with epilepsy should not receive tDCS.[18]As of 2017, at stimulation up to 60 min and up to 4 mA over two weeks, adverse effects include skin irritation, a phosphene (A phosphene is a phenomenon characterized by the experience of seeing light without light actually entering the eye.) at the start of stimulation, nausea (is an unpleasant, diffuse sensation of unease and discomfort, often perceived as an urge to vomit), headache, dizziness, and itching under the electrode.

Adverse effects of long term treatment were not known as of 2017.[19] Nausea most commonly occurs when the electrodes are placed above the mastoid (The mastoid part of the temporal bone is the back part of the temporal bone. Its rough surface gives attachment to various muscles and it has openings for the transmission of blood vessels. From its borders the mastoid part articulates with two other bones) for stimulation of the vestibular system(The vestibular system, in most mammals, is the sensory system that provides the leading contribution to the sense of balance and spatial orientation for the purpose of coordinating movement with balance.). A phosphene is a brief flash of light that can occur if an electrode is placed near the eye.[18][20]

Studies have been completed to determine the current density at which overt brain damage occurs in rats. It was found that in cathodal stimulation, a current density of 142.9 A/m2delivering a charge density of 52400 C/m2 or higher caused a brain lesion (A lesion is any abnormal damage or change in the tissue of an organism, usually caused by disease or trauma) in the rat. This is over two orders of magnitude higher than protocols that were in use as of 2009.[21][22][23]

Mechanisms of tDCS

One of the aspects of tDCS is its ability to achieve cortical changes even after the stimulation is ended. The duration of this change depends on the length of stimulation as well as the intensity of stimulation. The effects of stimulation increase as the duration of stimulation increases or the strength of the current increases.[24] The way that the stimulation changes brain function is either by causing the neuron’s resting membrane potential (Membrane potential also transmembrane potential or membrane voltage is the difference in electric potential between the interior and the exterior of a biological cell.

With respect to the exterior of the cell, typical values of membrane potential, normally given in millivolts, range from –40 mV to –80 mV) to depolarize  or hyperpolarize which is essential to the function of many cells, communication between cells, and overall physiology of an organism. When positive stimulation (anodal tDCS) is delivered, the current causes a depolarization of the resting membrane potential, which increases neuronal excitability and allows for more spontaneous cell firing. When negative stimulation (cathodal tDCS) is delivered, the current causes a hyperpolarization of the resting membrane potential. This decreases neuron excitability due to the decreased spontaneous cell firing.[18][25]

tDCS has been proposed to promote both the persistent strengthening of synapses based on recent patterns of activity(long term potentiation) and long term depression which produces a long-lasting decrease in synaptic strength.[18][25]

Differences in the concentrations of ions on opposite sides of a cellular membrane lead to a voltage called the membrane potential.

Operation

Transcranial direct current stimulation works by sending constant, low direct current through the electrodes. When these electrodes are placed in the region of interest, the current induces intracerebral current flow. This current flow then either increases or decreases the neuronal excitability in the specific area being stimulated based on which type of stimulation is being used. This change of neuronal excitability leads to alteration of brain function, which can be used in various therapies as well as to provide more information about the functioning of the human brain.[18]

Parts

Transcranial direct current stimulation is a relatively simple technique requiring only a few parts. These include two electrodes and a battery-powered device that delivers constant current. Control software can also be used in experiments that require multiple sessions with differing stimulation types so that neither the person receiving the stimulation nor the experimenter knows which type is being administered. Each device has an anodal, positively charged electrode and a cathodal, negative electrode. Current is ‘conventionally’ described as flowing from the positive anode, through the intervening conducting tissue, to the cathode, creating a circuit. Note that in traditional electric circuits constructed from metal wires, current flow is created by the motion of negatively charged electrons, which actually flow from cathode to anode. However, in biological systems, such as the head, current is usually created by the flow of ions, which may be positively or negatively charged—positive ions will flow towards the cathode; negative ions will flow toward the anode. The device may control the current as well as the duration of stimulation.[26]

Set Up

To set up the tDCS device, the electrodes and the skin need to be prepared. This ensures a low resistance connection between the skin and the electrode. The careful placement of the electrodes is crucial to successful tDCS technique. The electrode pads come in various sizes with benefits to each size. A smaller sized electrode achieves a more focused stimulation of a site while a larger electrode ensures that the entirety of the region of interest is being stimulated.[27] If the electrode is placed incorrectly, a different site or more sites than intended may be stimulated resulting in faulty results.[18] One of the electrodes is placed over the region of interest and the other electrode, the reference electrode, is placed in another location in order to complete the circuit.

This reference electrode is usually placed on the neck or shoulder of the opposite side of the body than the region of interest. Since the region of interest may be small, it is often useful to locate this region before placing the electrode by using a brain imaging technique such as fMRI or PET.[18] Once the electrodes are placed correctly, the stimulation can be started. Many devices have a built-in capability that allows the current to be ‘ramped up’ or increased gradually until the necessary current is reached. This decreases the amount of stimulation effects felt by the person receiving the tDCS.[28] After the stimulation has been started, the current will continue for the amount of time set on the device and then will automatically be shut off. Recently a new approach has been introduced where instead of using two large pads, multiple (more than two) smaller sized gel electrodes are used to target specific cortical structures. This new approach is called High Definition tDCS (HD-tDCS).[27][29] In a pilot study, HD-tDCS was found to have greater and longer lasting motor cortex excitability changes than sponge tDCS.[30]

Types of stimulation

There are three different types of stimulation: anodal, cathodal, and sham. The anodal stimulation is positive (V+) stimulation that increases the neuronal excitability of the area being stimulated. Cathodal (V-) stimulation decreases the neuronal excitability of the area being stimulated. Cathodal stimulation can treat psychological disorders that are caused by the hyper-activity of an area of the brain.[31] Sham stimulation is used as a control in experiments. Sham stimulation emits a brief current but then remains off for the remainder of the stimulation time. With sham stimulation, the person receiving the tDCS does not know that they are not receiving prolonged stimulation. By comparing the results in subjects exposed to sham stimulation with the results of subjects exposed to anodal or cathodal stimulation, researchers can see how much of an effect is caused by the current stimulation, rather than by the placebo effect (is a substance or treatment of no intended therapeutic value)..

Results:

In this part a suggested circuits will be examined to find out whether they can deliver and maintain four different constant current values with the variation of the load which is in this case is the body skin impedance.

Circuit (1)

This is a simple basic circuit in which the current controlled by changing the voltage across the transistor by changing the value of R5. By using Multisim program the results of the simulation shows that this circuit couldn’t maintain constant current with the variation of load (skin impedance)

Variation of the current with load, circuit (1)

Circuit (2):

In this circuit the use of the OP-AMP help to maintain constant voltage and then constant current through the load making use of the negative feedback which return back portion of the output voltage to the inverting input of the Op-Amp. The simulation results shows a good maintain to current value with the change of the load.

Variation of the current with load, circuit (2)

Conclusions

While tDCS has shown a lot of promise in early trials focused on its uses in various mental and cognitive disorders, much more work needs to be done to derive data as to the optimum dosage, pattern of electrode placement, and number of sessions used. The adverse effects appear to be few but at the same time the clinical utility has also not been proven.

As of now, guidelines (according to the European Chapter of the International Federation of Clinical Neurophysiology) endorse clinical applications only in the following fields, as being of probable efficacy:

•  Fibromyalgia
•  Major depressive disorder without drug resistance
•  Drug addiction or cravings

Possible efficacy is seen with chronic neuropathic pain of the lower limb, and it is thought to be ineffective in treating tinnitus and drug-resistant major depression.