Presence hallucinations are those feelings that someone is close by, even though you're actually all alone. Following on from research showing they're common in people with Parkinson's disease, a new study suggests they might predict cognitive decline in the condition too.

One in two people with Parkinson's are thought to regularly experience hallucinations, the researchers report. For a third of patients, they show up before more widely known symptoms, such as trembling.

Data was collected on 75 patients with Parkinson's disease, aged between 60 and 70, using neuropsychological interviews to assess cognitive decline and electroencephalography (EEG) to measure the brain at rest. Patients were also asked about any experiences with presence hallucinations.

The team found that cognitive decline of frontal executive function – which covers attention, problem solving, emotional regulation, and impulse control – was more rapid across five years in those patients who experienced hallucinations early in the disease's progression.

"We now know that early hallucinations are to be taken seriously in Parkinson's disease," says physician and neurologist Olaf Blanke from the Swiss Federal Institute of Technology Lausanne (École Polytechnique Fédérale de Lausanne, or EPFL).

The researchers also spotted a pattern of enhanced frontal theta (4 to 8Hz) oscillatory activity in the brain – but only for those with hallucinations at the onset of Parkinson's. It's another marker that health professionals could look out for in the future.

Blanke and his colleagues are encouraging anybody with Parkinson's who have presence hallucinations to mention their experiences to their doctors. These hallucinations often go unreported, are dismissed, or are considered side effects of treatment.

"Detecting the earliest signs of dementia means early management of the disease, allowing us to develop improved and personalized therapies that try to modify the course of the disease and improve cognitive function," says Blanke.

Hallucinations are defined as false perceptions of sensory experience, typically arising from abnormalities in brain function, so it's not surprising that they become more prevalent with brain diseases such as Parkinson's.

The next step for the researchers is to investigate ways in which this early warning system can be conveniently and reliably used. That could involve spotting brain activity patterns that correlate with hallucinations, perhaps even before they actually appear.

"So far, we only have evidence linking cognitive decline and early hallucinations for Parkinson's disease, but it could also be valid for other neurodegenerative diseases," says Blanke.

The research has been published in Nature Mental Health.