About half of all Parkinson's patients experience hallucinations. These include so-called presence hallucinations. In these, people experience the sensation that someone is standing behind or beside them. Such events tend to get worse and more frequent as Parkinson's disease progresses. They can also lead to a higher risk of psychosis and cognitive impairment. Nevertheless, the underlying neurobiological processes remain unclear.

The "Robot Ghost Test"

To specifically induce hallucinations in the laboratory, researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) used a robotic procedure they call the "Robot Ghost Test." In it, Parkinson's patients were asked to operate a robot placed on a table in front of them. A robotic arm behind the subjects imitated the corresponding gestures with nudges on their backs. When both actions were synchronized, the study participants reported nothing out of the ordinary. However, if the robotic arm reacted with a tiny delay, this caused hallucinations.

Predicting symptom severity

Using the test, the researchers identified the neuronal networks in the brain that are responsible for the presence hallucinations. In a subsequent experiment, they were thus able to predict the severity of symptoms based on brain scans.

Ultimately, the scientists hope to provide physicians with an objective method to improve diagnosis for hallucinations. However, they say more research is needed to examine the affected brain networks in more detail.