Joseph Rey has Parkinson’s disease.
As part of his condition, he experiences what’s called ‘presence hallucinations’ – a feeling that somebody is behind when there’s actually no one there.
"They feel like angels protecting me. They do me no harm. They follow me around. It's reassuring in a way, because I am not alone."
Around half of the people with Parkinson's disease experience hallucinations of some sort.
The spontaneous nature of the event has made the phenomena hard to study.
Now, scientists in Switzerland have devised a way of awakening the 'ghosts' hidden in the brains of Parkinson's disease sufferers to help evaluate their mental health.
The process involves using robots to provoke the onset of 'presence hallucinations.'
Professor Olaf Blanke of the Swiss Federal Institute of Technology.
"The system is actually quite simple. We have two robots, not one. One robot is in front of the subject and will measure the movement and the second robot will feedback signals to the individual that we're testing, Parkinson's patients or healthy subject, and then when we induce a mis-match, so if the front robot is doing something else from the back robot, this is the condition when the ‘presence hallucination’ occurs."
The study involved 56 Parkinson's sufferers in Switzerland and Spain.
While the disease has been traditionally defined as a movement disorder, some patients also suffer from mental symptoms like psychosis, depression, cognitive decline and even dementia.
Researchers say the growing evidence suggests that hallucinations might be precursors to these more severe mental health symptoms.
But they often remain under diagnosed.
"Hallucinations occur spontaneously. The patient, the doctor, nobody knows when they will occur and we have no control over it so far. So this is an important achievement, that this method and achievement does. We know when it will happen, we know the conditions when it will happen and we can control it across several conditions...What we want to do is try to see whether even 10 years before you develop Parkinson's disease, and there are certain ways of detecting those individuals who may be at risk of developing Parkinson's disease, whether among these individuals we can also have a similar discrimination. So this could lead to findings that, before you have any motor symptoms of Parkinson's disease we can already tell you, based on the test, whether you will have the more severe forms of Parkinson's or not."