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| George Wallis |
This
is a guest post by George Wallis, one of my PhD students. We recently attended a seminar in which
Oliver Sacks discussed his recent book ‘Hallucinations’. In this post George discusses the ways in which
hallucinations provide neuroscientists with clues about the hidden workings of
the brain. This article is also cross-posted at Brain Metrics, a Scitable Blog hosted by Nature Education.
Oliver Sacks is a
neurologist and a writer, and close to a household name. For many readers, he will be a familiar
figure. Since 1970 he has been writing
humane accounts of the ways in which different forms of neurological illness or
damage affect the lives of his patients – or occasionally Sacks himself. Amongst his book-length works are The
Man Who Mistook His Wife For a Hat, and Awakenings,
an account of the almost miraculous effect of the drug l-DOPA on sleeping sickness
patients at the Beth Abraham hospital, that has been adapted into a feature
film starring Robin Williams. Mark and I
were lucky to be invited to a small discussion session with Dr Sacks at Warwick
University where he is a visiting professor.
The topic of discussion was his most recent book, Hallucinations.
Hallucinations
is known for its detailed account of Sack’s own
hallucinatory experiences during his remarkably excessive drug-taking phase in
the 1960s. Before tight drug laws, and
with access to the most potent compounds to be found in a doctor’s medicine
cabinet, Sacks experimented with a wide range of compounds – often in huge
doses. He describes the mind-altering
experiences he had with classic psychedelics, the disturbingly real-seeming
hallucinations experienced whilst on Artane, frightening episodes of psychotic
delirium following withdrawal from some lesser known toxic agents, and the
time-eating stupor of opiates. Most
fascinating for Sacks fans is his description of the amphetamine fuelled
epiphany that crystallized his desire to write about the neurology and the
experiences of his patients.
Beyond the spectacle of these
autobiographical chapters, Sacks’ book is a catalogue of the many varieties of
hallucination. For students of
neuroscience, this makes for engrossing reading. Hallucinations can tell us a lot about the
brain.
What are hallucinations? Sack’s defines them as ‘percepts arising in
the absence of any external reality – seeing things or hearing things that are
not there’. A few hundred years ago
hallucinations might have been ascribed to the influence of Gods or ghosts. Nowadays, neuroscientists and psychologists
see hallucinations as the result of abnormal activity in the brain. Crucially, neuroscientists consider all of the things we experience to
result from models the brain builds.
When you look at something in the outside world, your brain doesn’t
magically ‘reach out and touch’ the object so you can perceive it (though, some
philosophers might disagree with neuroscientists on this point!). Instead, the brain builds a model of what is
probably out there in the world, doing its best to match the model to the
sensory input we receive at our sense organs (for example, in the retina of the
eye). The things you perceive reflect
the model the brain builds – a model built out of the buzzing activity of
billions of neurons in your brain. It’s
basically intelligent guesswork, but mostly our brains do pretty well, and we
have the impression of a stable world.
Importantly, we tend to agree with other people about what’s out there -
which gives an indication that our brains are getting things right! However, if the activity of the brain is in
some way altered by a neurological disturbance of one form or another (illness,
drugs, damage from a stroke or injury), the model can diverge from its normal
faithful representation of the outside world, and we can have hallucinatory
perceptions.
Depending on the type of neural
disturbance, these hallucinations can take many different forms. These are all interesting to neuroscientists,
as they all have the potential to tell us something about the workings of the
brain.
For example, there is Charles Bonnet
Syndrome, which Sacks describes in his opening chapter. The brain’s intelligent guesswork about the
outside world is normally informed by a stream of activity from the sense
organs. What happens if you cut off that stream of incoming information? In some cases, the brain keeps on ‘making up
a story’ – except now, it has no information to go on, so the percepts that are
produced bear no relation to reality.
For example, diseases of the eye can deprive someone of the visual input
their brain has been used to receiving.
If part of the retina is damaged, this can leave a blind patch called a ‘scotoma’, and people with a
scotoma can sometimes have vivid hallucinations in just their blind patch.
Charles Bonnet type hallucinations can also
occur if someone goes completely blind. These
hallucinations can be highly ornate – for example little ‘lilliputian’ people
are sometimes seen, often in very colorful and ornate clothing. Some people describe these hallucinations as
being like a movie. For most people,
however, Charles Bonnet syndrome involves simpler hallucinations – shapes,
colours and patterns. The patterns in
the scotoma can ‘scintillate’, giving the impression of constant movement.
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| Scintillating scotoma patterns |
Just because the retina is damaged doesn’t
imply that the visual parts of the brain are damaged too – this isn’t necessary
for hallucination. Charles Bonnet
syndrome reflects the normal activity of a brain forced to guess in the absence
of information – and people with Charles Bonnet are often well aware that their
hallucinations aren’t real, even if they seem very solid and detailed. Interestingly, some people with disrupted
sensory input experience hallucinations and some do not – it isn’t clear why.
Does this mean that you could hallucinate
too if you were deprived of sensory input?
Yes – though as with Charles Bonnet syndrome, it seems to vary from
person to person. There have been
various experiments with sensory deprivation.
A recent example
was published in the Journal of
Neuro-opthalmolagy in 2004, by Lofti Merabet, Alvaro Pascual-Leone, and
their collaborators (Merabet et al., 2004). They simply blindfolded thirteen healthy
volunteers for four days – otherwise, their volunteers were able to walk inside
and outside, talk to others, and listen to the TV. 10 out of 13 people reported
hallucinations. Just like in Charles
Bonnet syndrome, these were sometimes simple (flashing lights, geometric
patterns) and sometimes complex (landscapes, people, buildings, sunsets – often
seeming extremely vivid; more vivid than normal visual perceptions).
Hallucinations resulting from sensory
deprivation are evidence for the neuroscientists’ view of perception – that the
brain generates a model and fits it to the world. Sometimes the brain tissue responsible for
generating that model is disturbed in a way that alters the things people
perceive. For example, in epilepsy, the normally
controlled activity of the brain briefly goes haywire. Out of control neuronal firing emerges, and can
spread over the brain surface. Another
form of disturbed brain activity is experienced by many people in the form of migraine. Migraines are sometimes accompanied by a
visual hallucination superimposed on the real visual scene – often termed a
‘migraine aura’.
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| A migraine sufferer’s recreation of a ‘migraine aura’ |
In migraine or epilepsy, people sometimes
perceive geometric patterns – for example chequer-boards, zig-zag lines, or
concentric rings. These geometric
hallucinations are so consistent across people, they were catalogued in the
1920s by the psychologist
Heinrich
Klüver. He divided them into four
types: tunnels and funnels, spirals, lattices, and cobwebs.
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Kluver’s four categories of hallucination pattern.
Bressloff et al., 2002; used with permission. |
These patterned hallucinations are
interesting because they seem to reflect the structure of the parts of the
brain responsible for early visual processing - parts of the brain that are
quite organized in their layout. In the
1970s, mathematicians Jack Cowan and G Ermentrout built models of aberrant
activity patterns, given what they knew about the structure of the visual
cortex. These models have been extended
by the Oxford mathematician Paul Bressloff (Bressloff, Cowan, Golubitsky, Thomas, & Wiener, 2002). By modeling unusual
activity in the visual cortex, and then also taking account of the way the
neurons in our visual cortex map onto visual space, these researchers are able
to predict the kind of hallucinatory patterns catalogued by Klüver.
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| A mathematical simulation of a hallucination pattern |
Whilst migraines and epilepsy are certainly
not pleasant, the actual hallucinations experienced are rarely
frightening. The same is true for
Charles Bonnet Syndrome. People
experiencing these hallucinations are usually able to tell them apart from
reality, though sometimes only once they have become used to them and know what
to expect! Of course, this isn’t true of
all hallucinations. Sacks also discusses
the more terrifying types of hallucinations, for example, those of psychosis,
or of the ‘night terror’ associated with sleep paralysis – in which people
awake unable to move, with the feeling that they are trapped beneath a horrible
intruder who is trying to suffocate them (the ‘night mare’ or ‘night hag’).
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| Nicolai Abildgaard’s ‘Nightmare’ |
What do these more frightening
hallucinations - in particular, the hallucinations associated with psychosis
(in which people often also experience delusions) - say about the brain? This is a fascinating but difficult area, as
yet poorly understood. Here it becomes
more difficult to draw the line between perceptions and beliefs, and emotional
and motivational factors seem to be more involved. Researchers are currently trying to
understand how hallucinations in diseases like schizophrenia are related to the
other symptoms of the disorder, and how they may be similar or different to the
kind of hallucinations produced by sensory deprivation or epileptic activity
patterns in the brain.
Finally, an interesting speculation that
may haunt you as you read Sacks’ book is that hallucinatory experiences –
which, as Sacks points out, are much more common than one might think – could
be responsible for the religious, mystical, and paranormal parts of our
culture. For example, Sacks points out
that Joan of Arc’s
visions are classic manifestations of epileptic activity in the temporal
lobes. He speculates that these seizure
related visions were the reason an uneducated farmer’s daughter became a
religious leader who rallied thousands of followers.
Sacks’ book is a engrossing survey of
hallucinatory experiences of all types.
In their variety (far more extensive than described in this blog post)
hallucinations provide many insights into the way our ordinary perception works. Reading Sacks’ book is also a good preparation
for the possibility – not too slim, as Sacks points out – that you will one day
have a hallucinatory experience of one form or another (if you haven’t
already!).
References
Bressloff, P. C., Cowan, J. D., Golubitsky, M.,
Thomas, P. J., & Wiener, M. C. (2002). What Geometric Visual Hallucinations
Tell Us about the Visual Cortex. Neural Computation, 14(3),
473–491. doi:10.1007/BF00288786
Merabet, L. B., Maguire, D., Warde, A., Alterescu, K., Stickgold, R.,
& Pascual-Leone, A. (2004). Visual Hallucinations During Prolonged
Blindfolding in Sighted Subjects. Journal of Neuro-Ophthalmology, 24(2),
109.
All images Creative Commons except Kluver patterns, from Bresloff et
al., used with permission.
