Non-Invasive Electrical Brain Stimulation Montages for Modulation of Human Motor Function

Podsumowanie

Non-invasive electrical brain stimulation can modulate cortical function and behavior, both for research and clinical purposes. This protocol describes different brain stimulation approaches for modulation of the human motor system.

Streszczenie

Non-invasive electrical brain stimulation (NEBS) is used to modulate brain function and behavior, both for research and clinical purposes. In particular, NEBS can be applied transcranially either as direct current stimulation (tDCS) or alternating current stimulation (tACS). These stimulation types exert time-, dose- and in the case of tDCS polarity-specific effects on motor function and skill learning in healthy subjects. Lately, tDCS has been used to augment the therapy of motor disabilities in patients with stroke or movement disorders. This article provides a step-by-step protocol for targeting the primary motor cortex with tDCS and transcranial random noise stimulation (tRNS), a specific form of tACS using an electrical current applied randomly within a pre-defined frequency range. The setup of two different stimulation montages is explained. In both montages the emitting electrode (the anode for tDCS) is placed on the primary motor cortex of interest. For unilateral motor cortex stimulation the receiving electrode is placed on the contralateral forehead while for bilateral motor cortex stimulation the receiving electrode is placed on the opposite primary motor cortex. The advantages and disadvantages of each montage for the modulation of cortical excitability and motor function including learning are discussed, as well as safety, tolerability and blinding aspects.

Wprowadzenie

Non-invasive electrical brain stimulation (NEBS), the administration of electrical currents to the brain through the intact skull, can modify brain function and behavior^1-3. To optimize the therapeutic potential of NEBS strategies understanding the underlying mechanisms leading to neurophysiological and behavioral effects is still needed. Standardization of application across different laboratories and full transparency of stimulation procedures provides the basis for comparability of data which supports reliable interpretation of results and evaluation of the proposed mechanisms of action. Transcranial direct current stimulation (tDCS) or transcranial alternating current stimulation (tACS) differ by parameters of the applied electrical current: tDCS consists of an unidirectional constant current flow between two electrodes (anode and cathode)^2-6 while tACS uses an alternating current applied at a specific frequency⁷. Transcranial random noise stimulation (tRNS) is a special form of tACS that uses an alternating current applied at random frequencies (e.g., 100-640 Hz) resulting in quickly varying stimulation intensities and removing polarity-related effects^4,6,7. Polarity is only of relevance if the stimulation setting includes a stimulation offset, e.g., noise spectrum randomly changing around a +1 mA baseline intensity (usually not used). For the purpose of this article, we will focus on work using tDCS and tRNS effects on the motor system, closely following a recent publication from our lab⁶.

The underlying mechanisms of action of tRNS are even less understood than of tDCS but likely different from the latter. Theoretically, in the conceptual framework of stochastic resonance tRNS introduces stimulation-induced noise to a neuronal system which may provide a signal processing benefit by altering the signal-to-noise ratio^4,8,9. TRNS may predominantly amplify weaker signals and could thus optimize task-specific brain activity (endogenous noise⁹). Anodal tDCS increases cortical excitability indicated by alteration of the spontaneous neuronal firing rate¹⁰ or increased motor evoked potential (MEP) amplitudes² with the effects outlasting the stimulation duration for minutes to hours. Long-lasting increases in synaptic efficacy known as long-term potentiation are thought to contribute to learning and memory. Indeed, anodal tDCS enhances synaptic efficacy of motor cortical synapses repeatedly activated by a weak synaptic input¹¹. In accordance, improved motor function/skill acquisition is often revealed only if stimulation is co-applied with motor training^11-13, also suggesting synaptic co-activation as a prerequisite of this activity-dependent process. Nevertheless, causality between increases in cortical excitability (increase in firing rate or MEP amplitude) on one hand and improved synaptic efficacy (LTP or behavioral function such as motor learning) on the other hand has not been demonstrated.

NEBS applied to the primary motor cortex (M1) has attracted increasing interest as safe and effective method to modulate human motor function¹. Neurophysiological effects and behavioral outcome may depend on the stimulation strategy (e.g., tDCS polarity or tRNS), electrode size and montage^4-6,14,15. Aside from subject-inherent anatomical and physiological factors the electrode montage significantly influences electric field distribution and may result in different patterns of current spreading within the cortex^16-18. In addition to the intensity of the applied current the size of the electrodes determines the current density delivered³. Common electrode montages in human motor system studies include (Figure 1): 1) anodal tDCS as unilateral M1 stimulation with the anode positioned on the M1 of interest and the cathode positioned on the contralateral forehead; the basic idea of this approach is upregulation of excitability in the M1 of interest^6,13,19-22; 2) anodal tDCS as bilateral M1 stimulation (also referred to as "bihemispheric" or "dual" stimulation) with the anode positioned on the M1 of interest and the cathode positioned on the contralateral M1^5,6,14,23,24; the basic idea of this approach is maximizing stimulation benefits by upregulation of excitability in the M1 of interest while downregulating excitability in the opposite M1 (i.e., modulation of interhemispheric inhibition between the two M1s); 3) For tRNS, only the above mentioned unilateral M1 stimulation montage has been investigated^4,6; with this montage excitability enhancing effects of tRNS have been found for the frequency spectrum of 100-640 Hz⁴. The choice of brain stimulation strategy and electrode montage represents a critical step for an efficient and reliable use of NEBS in clinical or research settings. Here these three NEBS procedures are described in detail as used in human motor system studies and methodological and conceptual aspects are discussed. Materials for unilateral or bilateral tDCS and unilateral tRNS are the same (Figure 2).

Figure 1. Electrode montages and current direction for distinct NEBS strategies. (A) For unilateral anodal transcranial direct current stimulation (tDCS), the anode is centered over the primary motor cortex of interest and the cathode positioned over the contralateral supra-orbital area. (B) For bilateral motor cortex stimulation, anode and cathode are located each over one motor cortex. The position of the anode determines the motor cortex of interest for anodal tDCS. (C) For unilateral transcranial random noise stimulation (tRNS), one electrode is located over the motor cortex and the other electrode over the contralateral supra-orbital area. The current flow between electrodes is indicated by the black arrow. Anode (+, red), cathode (-, blue), Alternating current (+/-, green). Please click here to view a larger version of this figure.

Protokół

Ethics statement: Human studies require written informed consent of participants before study entry. Obtain approval by the relevant ethics committee before recruitment of participants. Make sure studies are in accordance with the Declaration of Helsinki. The representative findings reported here (Figure 4) are based on a study performed in accordance with the Declaration of Helsinki amended by the 59^th WMA General Assembly, Seoul, October 2008 and approved by the local Ethics Committee of the University of Freiburg. All subjects gave written informed consent before study entry⁶.

1. Safety Screening

1. Screen the participant for potential contraindications for noninvasive brain stimulation³, e.g., by using questionnaires²⁵.

2. Motor Cortex Localization

1. Locate the participant's hand motor cortex by one of two distinct approaches, by locating the brain representation of the muscle of interest by transcranial magnetic stimulation (TMS)-induced MEP, or by locating the standard M1 position (C3/C4) based on the EEG 10/20 international system with a measuring tape²⁶.
2. For TMS-induced MEP recording ask the participant to remove any object that may be influenced by TMS magnetic field, including credit cards, mobile phones and metal objects in general.
3. Ask the participant to sit comfortably.
4. Verify connections between EMG amplifier and the computer used for signal configuration and acquisition when using a software interface.
5. Turn on the EMG amplifier and connect EMG electrode cables.
6. Clean participant's skin by softly rubbing with skin preparation paste in the regions of the hand where the electrodes will be placed. Remove excess with clean gauze pad.
7. Attach EMG surface electrodes in a belly-tendon montage on the hand muscle of interest (e.g., M. abductor pollicis brevis of the right hand) and connect a ground electrode (e.g., on forearm). The purpose of the study determines which hand muscle to use.
   Note: For reusable electrodes it is necessary to apply a small amount of conductive paste on the electrode surface before attaching it to the participant's skin.
8. (optional step) Start the recording software for MEP acquisition if MEP data storage is desired.
9. Check the EMG impedance values. Ensure that the impedance is < 20 kOhm.
10. Turn on the magnetic stimulator and charge the capacitor by pressing the corresponding "charge" button.
11. Place a figure-of-eight TMS coil on the participant scalp on the interhemispheric fissure and move it to the motor cortex area (around positions C3/C4 of the EEG 10/20 international system). Hold the TMS coil at a 45^o-50^o angle referenced to the interhemispheric fissure^27,28, with the handle oriented backwards, producing a cortical current flow from posterior to anterior²⁹.
    Note: Two distinct TMS coils are used for motor cortex localization: figure-of-eight or circular coils. If possible, use a figure-of-eight coil as it provides more focal brain stimulation³⁰ and greater reliability of measurements of cortical excitability³¹.
12. When the magnetic stimulator is charged (visible on the display), discharge the stimulator either by pressing the trigger button or by stepping on the foot switch or automatically by a software program. This will subsequently deliver a single TMS pulse through the connected TMS coil placed over the participant's scalp. Default TMS pulse settings (e.g., 100 µs rise time of the induced current and 800 µs decay time for monophasic stimuli; shorter decay times for biphasic stimuli) are specific to the device (firmware).
13. Start with low stimulation intensity (e.g., set the intensity to 45% output using the stimulation intensity controller knob on the stimulator) and watch for MEPs visible on the EMG amplifier.
    1. If no MEP is visible increase the stimulation intensity in 2-5% steps until an MEP is clearly present (e.g., 0.5-1 mV amplitude). Repeat stimulation by pressing the trigger button or activating the foot switch if pulse delivery is not automated. Inform the participant that stimulation will be slightly stronger and that limb movements, facial twitch and eye-blinks are expected.
       Note: Establish a minimum interval of 5 sec between pulses to avoid low-frequency stimulation effects on brain excitability.
14. Move the coil radially in 1 cm steps around the initially stimulated site to find the spot with the largest MEP response following the application of single TMS pulses. From there, start again moving the coil to secure the "hotspot" (cortical area with maximal MEP amplitude).
    Note: The use of a head cap (e.g., used for grid markings) for the localization procedure is not recommended since the cap needs to be removed for NEBS electrode placement and the hotspot position may be lost.
15. Reduce the stimulation intensity in approximately 2%-steps using the stimulation intensity controller knob on the stimulator (MEP must still be present). This will avoid inaccuracy due to supramaximal stimulation. Reconfirm the hotspot by moving the coil radially in 1 cm steps around the hotspot and checking for MEP size. The hotspot should still correspond to the largest and most consistent MEP amplitude.
    Note: Ask the participant to voluntarily contract the muscle of interest if the hotspot is difficult to find (e.g., no MEP present at high stimulation intensities). By doing so, the stimulation intensity needed to elicit MEP is decreased and it may be easier to identify relevant cortical stimulation sites. If this method is used, ask the participant to relax the muscle after finding a relevant stimulation site and adjust stimulation intensity so that reliable MEPs can be found when the muscle is at rest. Proceed to find the hotspot.
16. Mark the hotspot position and coil orientation with non-permanent skin marker.
17. For bilateral M1 stimulation, repeat steps 2.11 to 2.16 for the contralateral limb.

3. NEBS Electrode Preparation

1. Connect cables to rubber electrodes, and place the electrodes inside the sponge bags. Make sure electrode size and sponge bag size do match. Materials are commercially available in standard sizes (e.g., 5x5 cm², 5x7 cm²).
2. Soak sponge bags on both sides with isotonic NaCl solution, but avoid excessive soaking to prevent salt bridges or dripping onto the volunteer.
   1. This step is optional: To prevent leakage of NaCl solution when using bandages instead of rubber bands, place the electrodes and sponge bags inside non-conductive rubber sponge covers.
      Note: Alternatively, cover the rubber electrode with conductive paste and place them directly on the participant's head, i.e., not using sponge bags or rubber sponge covers.

4. NEBS Electrode Placement (Figure 1)

1. Find the head marking(s) indicating the motor cortical hotspot and separate the hair around the area.
2. To improve conductance clean the skin before electrode placement by gently rubbing the skin area around the head markings with a swab soaked with 40-50% alcohol or skin preparation paste. Do not scratch the skin! Remove excess with a swab and clean area again with isotonic NaCl solution. Dry the area afterwards.
   Note: Make sure the head marking(s) remain visible; remark if needed.
3. Place one electrode following the head marking for the M1 of interest (contralateral to the hand of interest). Bring the sponge as much as possible in direct contact with the skin. Place the electrode cable towards the participant's back to avoid disturbance during stimulation and/or task execution and to ease connection to the NEBS device.
   Note: The hair below the electrode should get damp. In case of excessive hair moistening, use paper or hand towels to absorb the excess.
   Note: For anodal tDCS, the electrode placed on the motor cortical hotspot of interest (increase of excitability is desired) corresponds to the anode, usually connected to the red cable. The cathode (usually connected to a black or blue cable) is placed on the opposite supraorbital area or M1 (see below). Conventionally, electrode placement is the same for tRNS, although in the classical protocol there is no polarity specificity due to the alternating current flow. Specific placement may be important if the stimulation settings include a stimulation offset.
4. For unilateral M1 stimulation place the second electrode (for anodal tDCS: the cathode) over the contralateral supra-orbital area (corresponding to electrode Fp2 in the EEG 10/20 international system). Make sure the cable is oriented towards the back of the participant.
5. For bilateral M1 stimulation skip step 4.4. Place the second electrode (for anodal tDCS: the cathode) on the opposite M1 following the head marking ipsilateral to the limb used in the study. Make sure the cable is oriented towards the back of the participant.
6. Cover the head twice with an elastic bandage circularly in the medio-lateral direction to stabilize the M1 electrode, then use the remaining bandage to cover the head circularly in the anterior-posterior direction to stabilize both electrodes.
7. Use an adhesive tape to fix the end of the bandage.
8. Secure the cables with an adhesive tape on the participant's neck or shirt.
9. Connect electrode cables to the NEBS device.

Figure 2. Materials used for NEBS protocols. Conventional materials used in non-invasive electrical brain stimulation protocols include an NEBS device, electrode cables, conductive rubber electrodes, perforated sponge bags, rubber sponge cover (optional), isotonic NaCl solution and bandages. Please click here to view a larger version of this figure.

5. Stimulation

1. Switch on the NEBS device.
2. Adjust NEBS device settings regarding stimulation type (tDCS or tRNS), intensity (e.g., 1 mA, 1.5 mA or 2 mA), duration (e.g., 10-40 min), ramping up and down (time between beginning of stimulation and maximum intensity, typically 8-15 sec), and additional factors related to stimulation type (e.g., frequency spectrum for tRNS).
   Note: Conventionally, sham stimulation includes ramping up immediately followed by ramping down. Accordingly, the participant has the sensation of the stimulation but the duration of the stimulation is not sufficient to exert lasting effects on brain function. Some NEBS devices include a study mode which allows blinding of participant and investigator by entering a study specific subject code. The code determines stimulation settings automatically. Alternatively, a second experimenter may set the stimulation settings in each session and cover the display from the experimenter conducting the stimulation.
3. Inform the participant about potential side effects associated with NEBS. Common adverse effects include skin itching/tingling or burning sensation underneath the electrodes, headache, and discomfort³². Burning sensation may be a sign of poor electrode contact with the skin.
4. Start the stimulation.
   Note: Common stimulation duration lasts approximately 10-20 min based on reports investigating changes on cortical excitability (see representative results section). Empirically, the maximum stimulation duration was set to 40 min³.
5. Check for continuity of stimulation during ramping up and stimulation. If impedance is too high or electrodes are in bad contact with the skin, the stimulation may terminate automatically.
   Note: In case the impedance is too high or the participant reports increasing discomfort during the stimulation try to decrease impedance by, e.g., better fixating the electrodes at the stimulation sites or adding conductive medium. NaCl solution may be added by using a syringe directly in the sponges after their placement on the head.
   Note: For safety reasons some devices report the impedance throughout the stimulation. The NEBS device may shut off if impedance reaches a specific threshold (e.g., 55 kOhms).
6. If NEBS is co-applied with execution of a motor task, start the testing/training after stimulation is ramped up and the participant is feeling comfortable with stimulation. In case the study does not include a motor task during stimulation, make sure the participant remains seated and awake during the stimulation period, and wait until stimulation is over.
7. Check with the participant for side effects of the stimulation, e.g., by handing out a standardized questionnaire³² or directly asking the participant. In case of studies including multiple days of stimulation, take note of any possible side effects between days.
   Note: For assessment of blinding efficacy, ask the participant after each stimulation session to guess which stimulation type (sham/condition) the participant underwent. If the experimenter is also blinded, the experimenter could also note his guess regarding the participant's stimulation type. Compare answers with the actual stimulation type to verify rate of correct guesses³³.
8. Disinfect electrodes and sponges with non-hazardous substances such as 40-50% alcohol. Thoroughly rinse in water afterwards. Let materials dry before storing.

Wyniki

To investigate the effects of NEBS on the human motor system it is important to consider appropriate outcome measures. One advantage of the motor system is the accessibility of the cortical representations by electrophysiological tools. Motor evoked potentials are frequently used as an indicator of motor cortical excitability. After application of 9 or more minutes of anodal tDCS at a current density of 29 µA/cm², motor cortical excitability is increased for at least 30 mi...

Dyskusje

This protocol describes typical materials and procedural steps for modulation of hand motor function and skill learning using NEBS, specifically unilateral and bilateral M1 stimulation for anodal tDCS, and unilateral tRNS. Before choosing a particular NEBS protocol for a human motor system study, e.g., in the context of motor learning, methodological aspects (safety, tolerability, blinding) as well as conceptual aspects (montage or current type specific effects on a particular brain region) need to be taken into...

Materiały

Name	Company	Catalog Number	Comments
NEBS device (DC Stimulator plus)	Neuroconn
Electrode cables	Neuroconn
Conductive-rubber electrodes	Neuroconn		5x5 cm
Perforated sponge bags	Neuroconn		5x5 cm
Non-conductive rubber sponge cover	Amrex-Zetron	FG-02-A103	Rubber pad 3"*3"
NaCl isotonic solution	B. Braun Melsungen AG	A1151	Ecoflac, 0,9%
Cotton crepe bandage	Paul Hartmann AG	931004	8x5m, textile elasticity
Adhesive tape (Leukofix)	BSN medical	02122-00	2,5cm*5m
Skin preparation paste	Weaver	10-30
Magnetic stimulator	Magstim	3010-00	Magstim 200
EMG conductive paste	GE Medical Systems	217083
EMG bipolar electrodes	e.g., Natus Medical Inc. Viking 4
EMG amplifier	e.g., Natus Medical Inc. Viking 4
Cable for EMG signal transmission	e.g., Natus Medical Inc. Viking 4
Data acquisition unit	Cambridge Electronic Design (CED)	MK1401-3	AD converter
Computer for signal recording and offline analysis
Signal 4.0.9	Cambridge Electronic Design (CED)		Software
non-permanent skin marker	Edding	8020	1 mm, blue