Transcranial Direct Current Stimulation of the Cerebellum to Modulate Cognition in Humans

Begin with a participant wearing a headset with a microphone, seated before a computer screen.

First, announce the numbers through the headset, enabling the participant to perform an addition task by vocalizing each answer, followed by a subtraction task.

Next, display words on the screen so the participant can perform three distinct tasks: reading nouns, generating verbs associated with the nouns, and reading verbs.

Position a saline-soaked cathodal electrode over the right cerebellar cortex and an anodal electrode on the right shoulder muscle.

Apply cathodal tDCS.

The current hyperpolarizes cerebellar Purkinje cells, reducing inhibitory neurotransmitter release.

This disinhibits the deep cerebellar nuclei, which send excitatory signals to the thalamus.

The thalamus increases excitatory input to the prefrontal cortex, enhancing cognitive function.

After stimulation, the participant repeats the tasks.

Compare this performance with the pre-stimulation results to assess the neurotherapeutic potential of cathodal tDCS.

To begin the task again, ensure that the participant is seated comfortably and that the stimulus and responses are audible. Begin the task and present the 60 items at the appropriate rate.

During the task, write down each answer on the printed score sheet for subsequent verification. Give no score if the participant provides an incorrect answer or fails to respond and mark each correct answer in turn. Tell participants that the instructions for the subtraction task are the same as for the addition task, except this time they're required to subtract the number they just heard from the number immediately before it vocalize the answer, and then continue to subtract the number they hear from the one immediately before it.

Again, make sure that the microphone has not moved away from the participant's mouth and have them read the instructions presented to them on the computer screen. After confirming that the participant understands the instructions and reminding them not to perform calculations orally or with the aid of the fingers, perform the practice task to determine the presentation rate, as before.

Begin the paced auditory, serial subtraction task and record answers on the printed score sheet as before. For this part of the experiment, first, explain to the participant that they have to say an appropriate verb in response to the presented noun. Clarify this noun verb relationship to participants at the beginning of the task.

Present the words centrally on the computer screen in a different random order in blocks, one to five consisting of repeated words, and in block six, consisting of novel words. The word lists in session one and two should be different and counterbalanced between participants. Start the task and ask the participant to read the standard instructions that are presented on the computer screen, which formally explains how to perform the verb generation task.

Once the participant has fully understood the task, position the microphone in front of the mouth and instruct them to produce words as soon as they appear on the computer screen. Make sure each word is replaced by the next word. When the microphone detects a response, write down or record each answer spoken aloud by the participant for subsequent verification.

Make a note of any errors or missed responses. Present words in the same manner as in the verb generation task. Participants read nouns in the noun reading task and verbs in the verb reading task.

For both reading tasks, ensure the position of the microphone has not moved from the participant's mouth. And instruct the participant to read each word aloud as soon as it appears on the computer screen. Verify that the participant has read each word correctly during both reading tasks by looking at the screen as words are being read aloud.

TDCS is considered safe to use in humans, however, it is advisable that a first aid is at hand when performing TDCS to ensure that the participant's safety is not compromised, especially if they feel unwell or faint during the procedure. Never leave a participant unattended when administering TDCS. Presoak two sponge electrodes with a surface area of 25 centimeter square in a standard 0.9%sodium chloride solution until they are saturated. To administer inhibitory cathodal stimulation over the right cerebellar cortex, place the blue electrode, one centimeter under and four centimeters to the right of the most prominent projection of the occipital bone, approximately the location of cerebellar lobule seven.

To complete the electrode montage, place the red reference or anodal electrode on the right shoulder over the deltoid muscle. Secure the wet electrodes firmly to the head and upper arm with rubber straps or self-adherent wrap to ensure an optimal electrode skin interface. Make sure the electrodes are placed flat on the scalp and not over the hair.

Check that the intended position of the electrodes has not moved after they have been secured. Place a paper towel around the back of the participant's neck to mop up dripping saline solution. Set the intensity of stimulation at two milliamps and deliver for 20 minutes using a reliable current regulated DC stimulator to onset and offset each stimulation intervention increase and decrease respectively the DC current in a ramp like manner over 10 seconds.

To administer sham TDCS set the parameters to deliver sham stimulation for 20 minutes. Instead of the stimulation current, Apply anodal, cathodal or sham stimulation to three separate groups of participants in pseudo random order. Ensure that the overall number, gender and average age of participants is comparable between groups.

Following brain stimulation repeat the P-A-S-A-T and P-A-S-S-T as before in counterbalanced order. Then repeat the noun and verb reading tasks and the verb generation task in this order. Perform calculation task experiments and language task experiments in pseudorandom order.