Yet, through his research into Parkinson's disease, he began to realise that – thanks to these monkeys – he might have a unique window into the brain pathways of addiction and ways of reshaping them for the better.

"I strongly believe that we have to view all brain disorders, such as addictions, through the lens of different wiring," he says. "The brain is incredible at organising connections, but certain individuals are more predisposed to being wired in certain ways."

Both Parkinson's and addictions are intrinsically linked to a substance called dopamine, often known as the pleasure chemical. Activities such as sex, drinking, taking illicit drugs, winning £10 on a slot machine or watching pornography all flood the brain with surges of dopamine, creating a desire to repeat the experience.

Some people are particularly susceptible to swiftly becoming dependent on these dopamine surges, which can subsequently develop into an addiction. When these behaviours turn into chronic habits, the brain starts to produce less and less dopamine each time, which is why addicts often explain that their drinking or drug use does not actually provide them with any pleasure.

Parkinson's, a disease which gradually kills off dopamine-producing cells in the brain, is very different, but many of the latest therapeutic approaches utilise ingenious ideas for replacing the lost dopamine. One particular therapy being studied involves delivering the gene for a protein called glial-derived neurotrophic factor (GDNF) into the brain, which then encourages dopamine production.