Seeing what isn't there: how Parkinson's affects social perception

Visual hallucinations are a common symptom in Parkinson’s disease (PD), and they often involve seeing animate figures such as people or animals. But current scientific theories do not explain why hallucinations in PD, a movement disease - have such a strong social element.

Our study investigated this question by exploring how people with PD perceive social information—like how many people are in a group—and whether this perception is different in those who experience visual hallucinations.

We tested 28 people with PD who experience hallucinations (PD-VH), 55 with PD who do not (PD-nH), and 45 healthy people of similar age. Participants were asked to estimate how many humans were in a picture, along with a control task involving non-human shapes. They also answered questionnaires about two social traits: feelings of loneliness and the tendency to see human-like qualities in non-human things (a trait called "anthropomorphism").

We found that people with PD and hallucinations strongly and consistently overestimated the number of people they saw in the images—more than those without hallucinations and healthy participants.

Critically, this bias did not appear when the images showed non-human objects, suggesting the effect is specific to social perception.

The same individuals also reported feeling more socially isolated and showed a greater tendency to attribute human-like qualities to non-human things.

These findings suggest that hallucinations in PD are linked to changes in specific brain mechanisms that process social information, such as brain areas responsible for recognizing and interpreting social visual cues and that this neurodegeneration leads to visual hallucinations, involving people or animals. Latter mechanisms are closely related to movement-related fronto-parietal brain mechanisms, thereby explaining why social visual hallucinations are typical for a movement disorder like PD.

Of note, the present study was administered in an online web-based fashion that each patient comfortably carried out at their home. In combination with home-based clinical-neuropsychological testing, the present online web-based method will become an important tool in the future, because it enables regular and frequent clinical, affective and cognitive assessments that can be performed at the patient’s convenience, with reduced costs (clinician time, patient travel time and associated expenses), and without specific equipment or trained staff. Such online approaches further facilitate reaching patients who have important physical disabilities that limit their mobility (as in PD) or those living far away from medical centers or in low-income countries.