Presence hallucinations can affect patient’s social perception: Study

Virtual reality with robotics used to test overcounting of people in a room

Patricia Inácio, PhD avatar

by Patricia Inácio, PhD |

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When asked to quickly assess the number of people in a room, the average person tends to overcount them.

In Parkinson’s disease patients with presence hallucinations — the strong sensation of a presence when no one is there — this overestimation is much more heightened when compared with patients without these hallucinations, a study has found.

No such overestimation was observed when they were asked to estimate the number of boxes in a room, supporting a link between this overestimation and social behavior.

“The fact that patients of Parkinson’s disease have a much higher over-estimation in counting people is mind-blowing because Parkinson’s disease is classically viewed as a movement disorder,” Olaf Blanke, MD, PhD, at the Swiss Federal Institute of Technology (EPFL), in Switzerland, and the study’s lead author, said in a press release. “We show that Parkinson’s may also be a perceptual disorder, especially of social stimuli, and that invisible presences in Parkinson’s disease may impair even more the counting social brain!”

The study, “Numerosity estimation of virtual humans as a digital-robotic marker for hallucinations in Parkinson’s disease,” was published in the journal Nature Communications.

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Minor hallucinations linked to early cognitive decline

Minor hallucinations, like presence hallucinations, can be experienced early in people with Parkinson’s disease, sometimes prior to their diagnosis. They are linked to early cognitive decline, supporting their potential as an early marker for dementia.

However, despite their potential clinical relevance, current assessments of hallucinations rely on verbal self-reports and interviews, which can introduce biases.

In the study, researchers at EPFL merged virtual reality (VR) with robotics — a unique combination they named “technodelics” — to study technology-induced hallucinations.

Specifically, the VR allowed them to show virtual 3D scenes with different numbers of people (five to eight) in an empty room for 200 milliseconds, or a fifth of a second. This is called the “human numerosity experiment” and was combined with a robotic sensorimotor, an established method to induce presence hallucinations artificially.

Basically, participants were asked to perform repetitive movements while operating a robot in front of them, while another robot placed on their back usesd their movements to poke their own backs with a slight delay (500 milliseconds). While doing this, the participants had to estimate the number of people they saw.

“The advantage of our technodelics environment is that it gives us an objective way to measure hallucinations which are highly subjective states,” said Louis Albert, the study’s first author. “We are essentially engineering hallucinations, inducing hallucinations and getting a clear, implicit read-out of hallucination susceptibility at a given time.”

The results confirmed previous findings showing that when healthy people were asked to estimate the number of people in a room without having the time to count them one by one, they overcounted them.

The researchers then created a simplified, online version of the human numerosity experiment that could be performed by people from the comfort of their home and without requiring extra training of medical staff.

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Online test to determine susceptibility to hallucinations in Parkinson’s

“We now have an online test that can determine if someone is prone to hallucinations, a much-needed objective tool for measuring hallucination susceptibility in patients,” Albert said. “The test can be carried out independently by patients, directly from home on their computer or tablet, thus has the potential to reach a large demographic at minimal cost. Without the need for specific equipment or specialized staff for hallucination testing and interviewing, and without the need for patients to travel to the clinic, this test is accessible and can reach people living far away from medical centers and in low-income countries.”

A group of 170 patients with Parkinson’s disease carried out the test remotely on their computer or tablet at their homes from August 2021 to June 2022. The final analysis included 118 patients, of which 63 had presence hallucinations and 55 did not.

The results revealed the overestimation of people in the room was higher in patients with hallucinations compared with those without it. Certain patients saw as many as 11 or more people when only eight were shown. Importantly, they used several controls to show that this heightened overestimation was not confounded by clinical aspects or task demands.

“We have strategies for determining if a patient with Parkinson’s disease experiences presence hallucinations or not, which means that in the future we should be able to identify and monitor those who are more prone to cognitive decline for early treatment,” said Fosco Bernasconi, PhD, a study co-author.