A mechanism for retina-linked hallucinations

Hallucinations linked to vision loss from macular degeneration are caused by abnormally heightened activity in the visual cortex of the brain, according to new Australian research.

That means they are nothing to particularly worry about and do not need special treatment.

Macular degeneration causes progressive deterioration of the central region of the retina, affecting the centre of the field of vision but not peripheral vision. In Australia, it is the leading cause of legal blindness in people over the age of 40.

Many people with this condition also develop Charles Bonnet Syndrome (CBS), in which they experience hallucinations as the brain adjusts to the significant vision loss. These range from flashes of light or simple shapes to complex visions of people, animals or entire scenes.

Although CBS was first described more than 250 years ago, the neural basis for the hallucinations, and the reason why only some people get them, remains unclear.

To test a theory that this is linked to “excitability” in the brain, a team from the Queensland Brain Institute at the University of Queensland stimulated the peripheral visual fields of study participants and found that those who suffered hallucinations showed significantly heightened activity in parts of their visual system.

They don’t yet know how or why, but they know it occurs, and straight away that could help reduce misdiagnosis.

The findings are reported in a paper published in the journal Current Biology.

“When people get older and they start having these unusual experiences, they are often worried that something is wrong with them, such as dementia or something similar, so they tend to not report the hallucinations for fear they may be treated differently,” says lead author David Painter.

“Doctors sometimes don’t recognise the disease either, and therefore can give people inappropriate medication; but our method potentially allows us to detect people who might have Charles Bonnet Syndrome by looking at their brain excitability in response to flickering stimuli.

“Once people realise it’s not a brain disorder as such, they tend to have a neutral or even positive experience of their hallucinations. Unlike the hallucinations in people with schizophrenia, for example, individuals with Charles Bonnet Syndrome are aware their hallucinations aren’t real.”