The procedure does not involve any surgery or hair removal. We simply put a small amount of electrode gel on your head and then hold two electrodes on the top of your head for a short period of time (15-20 minutes) while they generate a small electrical current. It is quite common for participants to feel a tingling or an itch near where the electrode is applied, but if so, this usually wears off within a few minutes. Previous research has shown this method can occasionally lead to mild side-effects (e.g. headache, nausea, fatigue, exacerbation of scalp skin conditions). However, there are no known harmful long term effects. The positive benefits of brain stimulation can include improving the functioning of the brain for a short while after the stimulation session, in a way that may help with things like memory problems. Some people may not notice any change, but an improvement may still be detected using tests of complex mental skills including memory tasks. It is also possible that mood symptoms (e.g. depressive thoughts) could improve. The study will explore these possibilities. Looking at whether brain stimulation can help with health symptoms can help us develop new treatments.

This trial will include patients with two different movement disorders: Tourette syndrome and Huntington's disease. In both of these conditions, changes deep within the brain can have a knock on effect on higher brain areas important for complex mental abilities, as well as brain areas involved in movement control. Participants will be male and female adults (aged 18-65). Patients will have either Tourette syndrome (as diagnosed according to current medical criteria) or Huntington's disease (diagnosed by a genetic test). Patients will have few (if any) signs of additional medical conditions. Healthy controls (who will be compared to patients) will have no current significant medical or psychiatric diagnoses, history of drug use or brain injury.

There will be least two separate appointments for you to attend the study center. We will first assess your mental abilities (e.g. memory, attention) and interview you about current mood or medical symptoms. Brain stimulation will be given to slightly different areas of the outer part of the brain, near the top and front of the head, for each patient group. You may be asked to take part in the trial where there is a no-treatment placebo condition, where the procedure will feel similar to real brain stimulation, but real brain stimulation will not take place. If you receive the placebo condition you will also receive real stimulation, but you will not know which condition is which. This means the effects of expectations are less likely to influence the results. After brain stimulation we will take measures of mental abilities, and compare them to measures taken before brain stimulation, to show whether brain stimulation has had a greater effect on mental abilities than expected by chance.

You may be asked to complete a simple task (e.g. remembering numbers) while having the brain stimulation, as it has been suggested that this could make the effects of stimulation more effective in improving mental skills. As the trial continues, some participants will be invited to have repeated sessions of stimulation over a few days (e.g. three appointments over three days in a row) to look at whether this could make the effects last longer. We will record the effects of brain stimulation on participant's mental skills, movement symptoms and mood, as well as any side effects. We will collect data about your symptoms and health conditions in order to look for relationships between these factors and the effects of brain stimulation. Finally, you may be asked to have a brain scan straight after receiving brain stimulation, so we can look for any changes in brain activity.