Try out PMC Labs and tell us what you think. Learn More. Noninvasive brain stimulation methods are becoming increasingly common tools in the kit of the cognitive scientist. In particular, transcranial direct-current stimulation tDCS is showing great promise as a tool to causally manipulate the brain and understand how information is processed.
The popularity of this method of brain stimulation is based on the fact that it is safe, inexpensive, its effects are long lasting, and you can increase the likelihood that neurons will fire near one electrode and decrease the likelihood that stimulations will fire near another. However, this method of manipulating the brain to draw causal inferences is not without complication. Because tDCS methods continue to be refined and are not yet standardized, there are reports in the literature that show some striking inconsistencies.
Primary among the complications of the technique is that the tDCS method uses two or more electrodes to pass current top all of these electrodes will have effects on the tissue underneath them. In this tutorial, we will share what we have learned about using tDCS to manipulate how the brain perceives, attends, remembers, and responds to information from our environment. Our goal is to provide a starting point for new users of tDCS and spur discussion of the standardization of methods to enhance replicability.
As cognitive scientists, we usually manipulate aspects of the tasks we have observers perform and measure how their behavior or brain activity changes. The inferences drawn from these manipulations are the bedrock of all cognitive science and the shaped theories of perception, attention, and information processing more generally. However, it is also possible to see what happens when the brain itself is changed.
That is, we can directly manipulate the brain and see how that changes information processing when the task remains the same. Then, we can infer that the aspect we changed is necessary to perform the computations required in our task of interest. The transcranial Direction-Current Stimulation tDCS technique has become an increasingly popular way to manipulate brain activity, but this popularity has come with growing pains.
For example, recent selective meta-analyses suggest that the tDCS literature is filled with inconsistencies that make over conclusions hard to draw Horvath et al. As we will discuss, there are a of ways in which well-intentioned cognitive scientists can fail to obtain interpretable using tDCS. Thus, one goal in this tutorial is to contribute to work already underway to standardize methods for using tDCS to study cognitive processing in the brain. A of groups are leading this effort and the reader would be well served to read the excellent papers available on tDCS methods Fregni et al.
Our novel contribution here is to make concrete the logic behind several of our key methodological choices, specifically the choice of electrode configurations, our methods for modeling of current flow which can be time consuming for a new user to accumulate across many sources, and the biophysical background that justifies these choices.
Our second goal is to explain to the reader the major pitfalls that can contribute to inconsistent across experiments and laboratories, as we see them. We expect that our reader is new to the tDCS method. The format of this type of article provides more space to cover the basic background and describe how to perform the experiments than some of the other types of useful review papers that we also recommend e.
Beginning to use a new method for studying the brain is always nerve racking. This is particularly the case when your method involves changing how the brain works. To help the new users, we begin by discussing the nature of the stimulation and what we know about the biophysical basis of the effects in the brain.
We then turn to the practical matters of choosing electrode configurations, modeling current flow, and combining tDCS with other neuroscientific techniques. We selected these topics because they are most essential for someone who would like to be an informed consumer of tDCS research, or someone looking for a springboard to begin their own tDCS research. Using tDCS involves passing current through the skin, skull, and brain with a direct-current device. A battery is a device that we all have experience with that delivers direct current.
If you connect a fresh 9-volt battery to an oscilloscope, you will see the scope jump from 0 volts to 9 volts as soon as the battery terminals the plus and the over are attached to the two probes see Figure 1A. If you take one of these probes off, then the measured voltage returns to 0 volts as the circuit is broken. Using tDCS to manipulate activity in the brain involves passing precisely this stimulation kind of constant current through the tissue at a very low intensity, typically for a fairly long period of time e. Illustration of the nature of direct current versus alternating current.
A Direct current involves passing a constant current through a circuit. B Alternating current oscillates between negative and positive voltage. A third type of transcranial stimulation is noise stimulation, in which the voltage steps randomly across time. Direct current differs from alternating current that changes continuously the time see Figure 1B.
We all have experience with top current too. Alternating current comes out of our wall outlets. In the United States this need alternates at 60 Hz e. We chose to keep our focus on tDCS because there are important differences between these types of current. The key difference for present purposes is that the skull works like a low-pass filter in the frequency need due to its impedance characteristics relative to the surrounding tissue Nunez and Srinivasan, This filtering characteristic means that the skull effectively attenuates more and more of the tACS applied as higher frequencies are used, filtering much of it above 25 Hz.
This amounts to stimulation that is carried by a direct-current shift with the absolute current changing randomly and rapidly across time on top of the direct-current shift.
Effectiveness of upper limb functional electrical stimulation after stroke for the improvement of activities of daily living and motor function: a systematic review and meta-analysis
The tDCS technique essentially involves hooking up the positive terminal on a battery known as the anode to one place on the head and the negative terminal known as the cathode to another place on the head. Figure 2 shows a simplified schematic of such a circuit. Illustration of the influence of the bipolar electrical field on neurons close to the anode blue versus cathode red.
The top panel shows a schematic representation of the electrodes on the scalp and the electrical field that is generated.
This depolarizes the soma, or cell body of neurons near the anodal electrode bringing the closer to their thresholds for firing an action potential. The ionic gradients near the cathode have the opposite effect. This is why the logical placement of both electrodes is crucial in tDCS experiments. Bottom panels are adapted with permission from Bikson et al. Typically, the electrical connection between the battery terminals is made through wires, conductive rubber p, and then saline-soaked sponges. The tDCS devices that you can purchase from various vendors simply place some circuitry and software between the battery and the electrodes on the head.
This circuitry performs several simple functions. It allows the user to turn up or down the intensity of current being delivered, it is deed to have that current ramp up or down slowly across time, and can maintain constant current flow across changes in resistance. The advantage of having this software to ramp the current up and down slowly is that sudden changes in voltage can harm tissue and invoke painful sensations. Most of these devices also pass very small pulses riding on top of the direct current to test the quality of the circuit through the head as stimulation is delivered.
This is how they can al to the user that the resistance has become too high in the circuit. Resistance is simply how much the material in a circuit reduces the flow of electrical current through it, so that when resistance is high less current will flow. It is important to keep in mind that electrical current is like water. It follows the path of least resistance.
The lowest resistance tissue that the current encounters on its journey through the head is the skin and the brain. However, between these two fairly low resistance types of tissue is the relatively high-resistance skull. This means that a ificant amount of the current we try to pass through the brain with tDCS is actually shunted through the scalp and does not penetrate the skull. What happens in the brain tissue after you have begun to deliver current is a matter of active research, biophysical modeling, and debate.
Next we will present the current working hypothesis. The opposite happens with negatively charged ions. Although these ionic gradient have effects on cells that are immediately below the electrodes and oriented perpendicular to the stimulating electrodes, as shown in Figure 2there is also evidence from slice work suggesting that the strongest effects may actually be due to the radial electrical fields generated along the cortical sheet, extending parallel to the skull Rahman et al.
This means that ionic gradients are established to the left and right of your electrode, not just in a column-like path under your electrodes.
The orientation of the neurons in the cortical sheet relative to the stimulating electrodes is important. Neurons oriented as characterized in Figure 2 will have excitatory effects under the anodal electrode and inhibitory effects under the cathode. In contrast, in tissue in which the neurons are inverted e. Intracranial recordings have suggested that intensity of the stimulation may differentially influence excitatory pyramidal cells and inhibitory interneurons, with evidence suggesting that stronger electric fields influence the excitability of pyramidal cells and weaker fields influence smaller inhibitory interneurons Purpura and McMurtry, However, it is far from clear that tDCS at the scale used with human participants has such selectivity on different cell types.
Although ionic gradients are clearly important in brining about the tDCS effects we observe, they cannot explain why tDCS effects last for minutes and even sometimes hours after the electrical field is turned off.
Instead, the electrical field must also be changing how the cells are communicating and functioning. Researchers have sought to understand the relationship between the electrical field that tDCS generates and a variety of neurotransmitter systems. Similar types of pharmacological manipulations have implicated the dopaminergic system Fresnoza et al. Finally, recent slice work indicates that the long lasting after-effects of direct-current stimulation appears to be due to a molecular cascade involving metabotropic glutamate receptors, NMDA receptors, and GABA that induce long-term depression and potentiation Sun et al.
In sum, essentially every aling pathway has been implicated in the molecular and cellular activity that gives rise to the effects we observe with tDCS delivered to human participants. It is safe to assume that the biological basis of tDCS is not due to a single receptor type, neurotransmitter system, cell type, or other highly selective influence.
It is also clear that we continue to determine the biological basis at the same time that we are testing its potential as a tool for the cognitive scientist and a treatment for the clinician. The fact that tDCS creates an electrical field using both a cathode and an anode is a blessing and a curse. Being cognizant of the yin-and-yang of this circuit is of vital importance when deing your tDCS experiments.
ly we discussed the idea that the cathode and anode push and pull ions in the brain with opposite polarities. But what does this mean for brain activity? In our minds, the single greatest advantage of using tDCS as a tool is that the cathode and anode can have opposite effects on brain activity. This was shown in a series of foundational experiments using animal models.
Bindman and colleagues recorded from anesthetized rats before and after they passed direct current through the brain tissue Bindman et al.
Using transcranial direct-current stimulation (tdcs) to understand cognitive processing
Some of their basic findings are shown in Figure 3. After several tens of milliseconds of stimulation, the brain activity near the cathode was ificantly reduced Figure 3 middle panel.
This is seen in terms of the and size of spikes measured each second shown in the oscilloscope traces with the vertical tick marks i. In contrast to this marked decrease by the cathodal electrode, brain recordings made near the anodal electrode showed the opposite effects Figure 3 bottom panel. These recordings show that the anodal and cathodal electrodes cause changes in brain activity in the opposite direction.
The cathode decreases spike amplitudes and the likelihood of firing, while the anode increases spike amplitude and the likelihood of firing action potentials. The raw oscilloscope from Bindman et al.