Transcranial Direct Current Stimulation (tDCS) for Memory Enhancement

The purpose of this video is to demonstrate the transcranial direct current stimulation protocol that is used for memory enhancement. The protocol has been well evaluated in healthy participant studies and is applicable to aging and dementia research as well. Memory plays a vital role in everyday functioning, as it enables us to remember information about people and places, recall past events, learn new facts and skills, as well as to make judgements and decisions.

Here we focus on working and associative memory. Working memory is the ability to temporarily maintain and store information for ongoing cognitive processing, such as problem solving or reasoning. Associative memory on the other hand enables us to remember multiple pieces of experience or information bound together.

These two types of memory underline almost all our daily activities. Unfortunately, memory abilities are among the most vulnerable functions as they decline with normal aging, as well as due to various pathological states and conditions. This is why there is a growing need for novel approaches to prevention and treatment of memory decline.

Transcranial direct current stimulation, tDCS for short is a non-invasive brain stimulation technique that uses weak electrical currents to modulate brain activity. The effects of tDCS are polarity dependent. That is, anodal stimulation increases while cathodal decreases neuronal excitability.

Therefore, anodal tDCS is expected to promote cognitive functions, which rely on targeted brain regions. tDCS is safe, highly tolerable, cost-effective and easy to use. The main challenges for using tDCS are finding the optimal electrode montage and stimulation protocol that will produce reliable effects.

To modulate memory we need to affect the activity within cortical hippocampal network. So the anode is placed either over the dorsolateral prefrontal cortex or posterior parietal cortex as the two most prominent cortical knots of the network. The most usual stimulation protocol is constant analog current between one and two milliamps that lasts between 10 and 30 minutes.

The assumed mechanism behind this protocol is that the electrode with positive charge will increase the excitability of the underlying cortical tissue, which will then result in enhanced subsequent memory performance. Unlike constant anodal tDCS, where current intensity stays the same during the whole stimulation period, the oscillatory tDCS protocol intensity of the current fluctuates at a given frequency around a set value. Therefore this type of protocol modulates not only excitability, but also entrains neuro oscillation of the relevant brain areas.

When planning the tDCS study on memory researchers need to carefully consider the implementation of sham control in either within-or between-subject design, determining adequate sample size to achieve satisfactory power. Furthermore, it is important to develop or select adequate outcome measures that is memory task that are focal, sensitive, reliable, and challenging for participants. For each tDCS session the following materials should be prepared.

In addition, it is advisable to have a protocol sheet that is a fill in form with basic information about the session and a table with a pre-calculated head measures to help with electrode placement. Before tDCS session, check if participant satisfies the inclusion criteria as defined in the ethical approval for the study. Ask participants to sign informed consent.

Use this opportunity to explain the basic aspects of the procedure they will undergo and answer any questions the participants may have. Seat the participant comfortably in the chair and ask to fill out the pre-tDCS sensation checklist. In this session, we will stimulate right posterior parietal cortex.

First measure the distance between nasion and inion going over the top of the head. Mark the halfway distance with a skin marker with a thin line. Then measure the distance between ears going over top of the head and mark the halfway distance with a thin line.

The vertex or Cz is found at the intersection of two midlines. Measure again nasion-inion distance and note it as measure A.Note the distance between the ears as measure B.Write down measures in the protocol sheet for each participant. These can be used to check against when taking measures in subsequent sessions.

Move 20%of distance A backward from Cz, to reach Pz and mark the spot. Move 20%of distance B rightward from Cz to C4 and mark the spot. Starting from C4, go backwards in parallel to the nasion inion line.

Then from Pz go rightwards to reach the area of P4.20%rightwards from Pz and 20%backwards from C4 is where you will find P4.To increase conductance, move away participant's hair from the stimulation site. Use comb and hairbands for participants with longer hair. Inspect for any signs of skin damage at the places of stimulation.

Avoid positioning electrodes over damaged skin. Clean the surface of the skin using alcohol soaked cotton pads and let it dry. Clean the cheek where the return electrode will be placed.

Put a silicon cap on participants head and secure it with a chin strap. Use the silicon strap to adjust the cap to the participant head size and shape, but do not make it too tight. This will be done later.

Soak the sponge pockets with saline solution. The sponges should be moist but not dripping. Usually 10 to 15 milliliters of saline solution per sponge is enough.

Put the electrodes inside the sponge pockets. Set the return electrode on the contro-lateral cheek and adjust the cap. To position the anode, put the sponge electrode under the silicon strap and secure that the center of the electrode is over the marked head location.

The cap should be tight so the electrodes cannot move but still comfortable for the participant. To begin the stimulation, turn on the device, select and run predefined tDCS protocol. To avoid unstructured activities that can interfere with stimulation effects, light cognitive engagement during tDCS is advisable.

For example, participants can perform practice trials of cognitive task or engage in easy memory games during the stimulation. Ask the participants to report how they feel multiple times during the stimulation. When stimulation protocol is finished, turn off the stimulator, remove the sponge electrodes first, then remove the silicone cap.

Ask participant to fill out post-tDCS sensation checklist and to report for any side effects not already listed. Clean the skin on the places where it was marked and inspect the skin for any changes. To standardize memory assessment across participants, it is advisable to use computerized tasks.

If conducting within-subject experiments, strictly parallel forms of the tasks should be used. To assess successfulness of blinding ask participant to try to guess the session in which they have received real and sham stimulation. In line with ethical guidelines, the participants should be debriefed in detail after their involvement is completed.

Wash the sponges with running water and soap so that the saline solution is fully washed away. Use warm water and alcohol to clean all reusable materials, including comb, silicone cap and measuring tape. When it comes to working memory, our results have shown that 20 minutes of right frontal tDCS enhanced verbal memory while the same stimulation protocol applied over the left prefrontal cortex resulted in better spacial performance.

The effects of parietal tDCS on associative memory have been consistent and robust. Namely, in series of within-subject experiments, it was shown that 20 minutes of tDCS over left parietal cortex improves memory of faces and words associations. To summarize, we can say that the application of the described protocol for anodal tDCS, either in the standard or even more in its advanced form, such as oscillatory modulating tDCS, provides a means for reliable enhancement of memory functions.

It allows for investigation of the neurobiology of the functional neuro networks behind memory functions. And it can be used in clinical research as well.