Blind World

Ghosts and witches on the brain -- Charles Bonnet hallucinations.

November 6, 2002.


The brain is a knowledge-gathering machine that carries out endless abstractions to form concepts, whether of a car, a line or falling in love. However, witches and fairies also lurk within it - along with more ghosts than can be seen at a Deathday party. These have been revealed by studies of hallucinations suffered by people who are losing their sight.

They are called Charles Bonnet hallucinations, after the Swiss naturalist who reported his grandfather's strange experiences and later went on to suffer the hallucinations himself. Dominic ffytche, of the Institute of Psychiatry in London, has studied many of these patients. "The hallucinations tend to be brief - lasting a few seconds or minutes - re-appearing after hours, days, months or even years. The experiences may be frightening, but with time most sufferers recognise them as hallucinations and learn to ignore them." Strikingly, ffytche found patterns in these visions. Rather than witnessing anything and everything, the patients report apparitions that usually fall into a handful of categories, including distorted faces and costumed figures. "I'm sure ghosts, fairies and witches all relate in some respect to these disembodied hallucinations," he says.

Ghosts are among the categories. One patient described how a friend working in front of a tall privet hedge suddenly disappeared, as if he had suddenly put on a cloak of invisibility. "There was an orange peaked cap bobbing around in front of the hedge and floating in space by its own devices."

There are also ghouls. The disembodied or distorted face of a stranger with staring eyes and prominent teeth is seen by about half of all patients, sometimes only in an outline, cartoon-type form. The faces "are often described as being grotesque, or like gargoyles".

In the hallucinations, objects or people often appeared much smaller than usual, but sometimes were much larger (micropsia and macropsia). Could these distortions be linked to sightings of the little people and giants?

One patient described "two half heads joined like Janus", which sounds rather like what Harry Potter encountered, in his first adventure, at the back of Quirrell's head, where he gazed at a second face, the most terrible he had ever seen.

Another patient seemed to catch a glimpse of the dreaded Dementors: some of the faces conjured up by his failing eyesight have blank eye sockets with "expressions [that] are all evil and malevolent".

As well as random patterns and geometrical forms, ffytche's patients also saw serene landscapes and vortices. Near-death experiences could be activating the same brain areas that produce these hallucinations, he speculates.

Reassuringly for the patients, these figments are not a sign of madness. These phantoms appear when vision deteriorates to a certain point, usually as a result of eye diseases such as age-related macular degeneration, glaucoma and retinitis pigmentosa.

As the brain is starved of sufficient information from the eyes, it compensates with abnormally increased activity and conjures up hallucinations from the random firing of nerve cells. "You get the same phenomenon in patients who have had both eyes removed," he says. "When there is no information coming in, the brain is idle, cells are firing away and producing these stereotyped categories of hallucination."

Using a scanner, ffytche has observed that the thin rind on the back of the brain, where vision is processed, is active during these hallucinations, just as it is when a sighted person has his eyes open.

But where did the gargoyles come from? Ffytche points out that one part of the brain - the lateral occipital region - alerts us to the possibility that what we are looking at might be a face. This region detects a face's component features - the eyes, nose lips and chin, for example - but does not register where these features are. "Our results showed that it was this face-feature detector that caused the gargoyle-like hallucinations - its indifference to the position of each feature leads to the characteristic distortions of the gargoyle and the overemphasis of face features, the prominent staring eyes."

The recurring association of figure and garden hallucinations seems to relate to how the area of the brain that processes landscapes, gardens and vistas lies next to the one responsible for processing figures and objects. Activity in one is likely to spill over to the other, leading to a spurious association of the two hallucination categories.

The abundance of hats could reflect how cells in the temporal lobe in the brain, which encode elongated shapes, are firing. But ffytche admits that it remains a mystery why the figures should so often be costumed and sporting flamboyant hats, looking like they just walked out of Hogwarts.

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