Investigating a plausible cause for hallucinations in Charles Bonnet Syndrome

Brief lay background

Visual hallucinations – seeing things that are not there – are an alarming symptom often affecting people with central vision loss. Conservative estimates of the prevalence of this condition, known as Charles Bonnet Syndrome (CBS), range between 11% and 15%, with some studies reporting figures of up to 63%.

The condition reduces both functional ability and quality of life. Despite common fears by patients that CBS is a mental health disorder, the condition occurs in healthy individuals with no history of psychiatric problems, i.e., it is a visual disorder. How these hallucinations arise, or develop to clinical intensity, is poorly understood.

One reason for this lack of understanding is the fact that hallucinations are difficult to study because, by definition, they only exist in the mind of the observer. Using a novel method, the research team are able to induce ‘mini-hallucinations’ in the lab, which allows the team  to explore the mechanisms underlying Charles Bonnet Syndrome.

This research could have  implications in other other hallucinatory conditions. Visual attention is sometimes conceived of as a ‘spotlight’, i.e., people are only able to pay attention to a small region of the visual field at any given moment. The team hypothesise that CBS occurs when visual attention is disengaged from the fovea (responsible for sharp central vision), and instead diverted to the visual periphery, where increased ganglion cell pooling results in a poorer visual image. CBS is particularly common secondary to age-related macular degeneration. Except in extremely rare cases, the condition always occurs with a concurrent reduction in visual acuity, and has never been reported without at least some central visual field loss. 

What problem/knowledge gap does it help address

In general, the understanding of CBS is limited. Current models of healthy brain function indicate that the visual system combines sensory inputs with expectations about the environment. Clinical research suggests that an undue reliance on expectation in visual processing is associated with hallucinations.

An excessive influence of expectation sculpts perception to such an extent that, in the extreme, a percept of an expected stimulus is generated even in the absence of sensory input. In short, the percepts we experience do not exclusively rely on sensory input but on a combination of input and pre-existing expectation.

Aim of the research project

This project will investigate a plausible mechanism with the potential to explain how complex hallucinations arise in humans.

Potential impact on people with sight loss

The cause of CBS is still unknown. Despite clinicians knowing that they should discuss CBS with their patients, many do not.Their inability to explain why CBS occurs undoubtedly provides a barrier to educating patients about the condition.

Knowing the cause of CBS could lead to a reduction in suffering by patients who often falsely believe that their experiences are a sign of a mental illness or dementia. Understanding the cause of any condition is the first step toward developing effective treatments.