What unemployed neurons get up to, and why Charles Bonnet syndrome needs wider recognition

I’ve just a watched marvellous TED talk from Oliver Sacks.

It’s about Charles Bonnet syndrome, CBS, which I mentioned in previous posts. Many people who are blind or visually impaired see hallucinations. These are not psychotic symptoms because the people concerned are completely aware that what they are seeing is an illusion and nothing more.

The hallucinations may be ‘unformed’, such as abstract shapes like triangles or hexagons that do not represent real experiences. They may also be ‘formed’, which means they correspond to things the patient has actually seen in their visual field, for example their arm.

In his talk Sacks gives examples of extraordinary range of visual hallucinations CBS patients experience.

I described my CBS in earlier postings on this blog. I had CBS for about a month and saw amongst other hallucinations triangles, squares, hexagons, cube frameworks, flags, herringbone patterns, Buckyballs, chequerboards, packs of running dogs, groups of sprinters, a Cheshire cat, Homer Simpson, my own face but strangely plasticised, a miniature version of myself diving into a vortex, mathematical tables, the structural of molecules, and a white horse crossing the road.

These are but a shadow of what some patients experience however. Some examples are buildings, landscapes, carriages, groups of people trooping up and down stairs, and many other scenes far more complex than anything I witnessed.

The hallucinations come and go suddenly, and they are silent and do not interact with the viewer in any way: they go about their own business, whatever that may be, with complete indifference to the patient. Moreover patients are always aware they are seeing hallucinations and never imagine the phantoms have any reality.

What is going on and why are similar themes (such as cartoon characters or hexagons) seen by a large proportion of CBS sufferers? Oliver Sacks puts forward the following theory, which I have summarised as well as I am able.

Studies using functional magnetic resonance imagine, which can reveal patterns of blood flow in the brain in real time, indicate that there is a level of the visual cortex in which very specific patterns are recognised. Many particular groups of neurons exist whose job it is to react to shapes or lines or faces or cartoon characters — and not just cartoon characters in general but Marge Simpson say. Oliver Sacks uses the example of recognising not just a car, but an Aston Martin. So I guess there are square neurons and hexagon neurons and so on, which is very intriguing.

Without the sight loss these neurons would respond to real-world stimuli and the responses would be integrated by the higher visual cortex into a normal view of the world. But presumably when they receive no incoming signals they may begin to fire at random and the phantoms produced — Sacks calls them figments — swirling around are what are experienced as hallucinations. The peculiar nature of what CBS sufferers see is to do with the fact that the neurons are so specific.

CBS used to be considered rare until as recently as 25 years ago but in fact it is very common. Sacks estimates that 10% of those with vision loss encounter CBS and 10% of those report their symptoms. Other studies that the prevalence of CBS amongst elderly people with sight loss may be up to 40%.

The consensus is that the figure is under-reported because sufferers are afraid to speak what they are seeing out of fear that others with think they are losing their minds and that they will be stigmatised.

Near the end of his talk Oliver Sacks remarks that there must be thousands of CBS suffers. From WHO statistics I estimate the number must be around 30 million world wide. Imagine all those people too scared and anxious to mention their symptoms, or perhaps mistaking them for something else.

Sack ends by making a plea: for CBS to be brought into notice ‘for patients, for doctors, for the public’.

I couldn’t agree more!

You can find the TED talk here.

Professor Sachs just talks, there are no visuals, and there is a transcript, so it is very accessible.


About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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