Background

The expanding array of neurostimulation techniques in recent years has led to a greater understanding of functional-anatomic relationships of the human brain. An improved understanding of the underlying mechanisms of action from these emerging technologies has in turn spurred the development of novel therapeutic interventions. Transcranial direct current stimulation (tDCS) is one such technique that has been the focus of significant recent attention. This has partly been due to its non-invasive nature and also a result of the utility of the technique. Reports indicate that tDCS has been used in the neuropsychiatric domain for the treatment of affective disorders such as depression and anxiety Reference Murphy, Boggio and Fregni(1) and also within the neurological domain assisting in the motor rehabilitation of stroke patients Reference Hummel and Cohen(2) making it a viable non-surgical treatment option.

Subject comfort and safety

• 1. Most tDCS machines use a battery-derived power source. Ensure that the DC stimulator is fully charged before commencing any study. An interruption of power supply, such as battery failure, without ramping down of the current during the time of stimulation could potentially cause the subject pain and also compromise clinical treatment or testing.

• 2. To increase stimulation efficiency and limit subject anxiety, stimulus parameters such as current intensity, duration of stimulation, ramp up/down duration, stimulation mode (e.g. single or pulse mode) and impedance limit should be pre-programmed before the subject arrives. All electrodes and leads should be plugged in and electrode sponges pre- moistened with saline solution.

• 3. Ideally, the tDCS machine should be placed out of view of the subject and all cables and wires should not obstruct the face during stimulation. This is important as it creates a non-threatening environment for the subject.

• 4. Optimum hygiene must be observed at all times in the lab. The tDCS technician is exposed to the subjects' skin, scalp and hair, and therefore gloves should be worn. After every stimulation session, electrode sponges and straps should be cleaned and disinfected. Common surfaces such as testing chairs and computer equipment should also be wiped down with disinfectant.

• 5. When equipment faults arise during stimulation, the delivery of current should immediately be stopped and the fault attended to. Troubleshooting the DC stimulator or electrodes while the current is being administered should be avoided.

With such a rapidly emerging literature, it is likely that the popularity of the tDCS technique will continue to increase and result in further research with novel therapeutic applications. As such, special care must be taken to ensure that researchers and clinicians in this field use the appropriate practices when conducting tDCS investigations and treatments. The purpose of this issue's Brain Bytes is to discuss a set of standard guidelines and parameters for the use of tDCS.

Before testing begins

• 1. When scheduling a tDCS session, understandably, subjects may feel anxious about the procedure. It is important that the subject be informed about the experimental protocols and all concerns and questions should be addressed prior to the commencement of the tDCS.

• 2. It is equally important to emphasise that tDCS is non-invasive.

tDCS involves the application of a mild current to the scalp through cathodal and anodal electrodes which results in the modulation of cortical excitability. These focal reversible shifts in cortical excitability are not totally homogeneous but are dependant on polarity of stimulation, cortical layer and neuronal spatial orientation and vary with strength and duration of stimulation Reference Lang, Nitsche and Paulus(3). tDCS is capable of inducing duration-dependant, long-lasting cerebral excitability that outlasts the duration of the stimulation period. This method of stimulation has been shown to affect a range of motor, somatosensory, visual, affective and cognitive functions. Therapeutic effects have been shown in clinical trials of tDCS for a variety of conditions including tinnitus, post-stroke motor deficits, fibromyalgia, depression, epilepsy and Parkinson's disease (see Reference Fregni, Marcondes and Boggio(4) for review). Its effects can be modulated by combining with pharmacological treatment, and tDCS may influence the efficacy of other neurostimulatory techniques such as transcranial magnetic stimulation Reference Lang Siebner and Ernst(5).

In most studies, a battery-driven constant current stimulator provides a steady flow of direct current to a pair of 5 cm × 7 cm conductive rubber electrodes that are surrounded by a sponge soaked in isotonic sodium chloride solution and applied to the scalp. One electrode is placed on the area of interest and a reference electrode is placed on the contralateral supraorbital region Reference Paulus(6). Current is then applied for approximately 10 min with an intensity of 1–2 mA.

Procedure

• 1. For the subject's comfort, it is ideal to provide a waterproof, protective smock to prevent water that is used to moisten the electrodes during stimulation from saturating the subject.

• 2. Importantly, electrode montages that could result in brainstem or cardiac stimulation can be dangerous and should never be employed. Lippold and Redfearn Reference Lippold and Redfearn(7) have described one case of disturbed breathing, speech arrest and psychosis that may have been a result of the current travelling through the brainstem.

• 3. The sites of stimulation need to be determined and marked. Elastic or velcro straps are then placed across the forehead and under the chin perpendicular to each other. It is important that the straps remain firm in order to hold the electrodes in place but do not cause the subject discomfort.

• 4. The moistened electrodes are then secured under the straps and adjusted so that they lie over the scalp region of interest and reference area. The long edge of the electrode is placed parallel to the direction of the velcro strap to increase contact pressure.

• 5. Maximum surface area contact between the electrode and the scalp ensures that low electrode impedances are maintained which reduces the risk of burns and electrical injury to the patient.

• 6. Before stimulation commences additional pressure, further moistening of the electrode sponges with saline solution and further adjustments to electrodes' position may be required at this point.

• 7. The technician must remain vigilant throughout the stimulation session and regularly check impedance levels as well as constantly monitor all of the stimulation parameters.

Conclusion

Previous studies have shown that non-invasive brain stimulation techniques such as tDCS provide a viable option for the treatment of a range of neuropsychiatric conditions. However, as data emerge from clinical trials about the efficacy of tDCS, it is important that laboratory procedures be standardised in order to be able to objectively evaluate the potential of tDCS as a therapeutic application.