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An elderly woman named Rosalie
was sitting in her nursing home
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when her room suddenly burst
to life with twirling fabrics.
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Through the elaborate drapings,
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she could make out animals,
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children,
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and costumed characters.
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Rosalie was alarmed,
not by the intrusion,
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but because she knew this entourage
was an extremely detailed hallucination.
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Her cognitive function was excellent,
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and she had not taken any medications
that might cause hallucinations.
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Strangest of all, had a real-life crowd
of circus performers burst into her room,
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she wouldn’t have been able to see them:
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she was completely blind.
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Rosalie had developed a condition known
as Charles Bonnet Syndrome,
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in which patients with either impaired
vision or total blindness
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suddenly hallucinate whole scenes
in vivid color.
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These hallucinations appear suddenly,
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and can last for mere minutes
or recur for years.
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We still don’t fully understand
what causes them to come and go,
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or why certain patients develop them
when others don’t.
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We do know from fMRI studies
that these hallucinations
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activate the same brain areas as sight,
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areas that are not activated
by imagination.
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Many other hallucinations,
including smells,
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sights,
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and sounds,
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also involve the same brain areas
as real sensory experiences.
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Because of this, the cerebral cortex is
thought to play a part in hallucinations.
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This thin layer of grey matter
covers the entire cerebrum,
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with different areas processing
information from each of our senses.
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But even in people
with completely unimpaired senses,
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the brain constructs the world we perceive
from incomplete information.
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For example, our eyes have blind spots
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where the optic nerve
blocks part of the retina.
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When the visual cortex processes light
into coherent images,
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it fills in these blind spots with
information from the surrounding area.
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Occasionally, we might notice a glitch,
but most of the time we’re none the wiser.
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When the visual cortex is deprived
of input from the eyes, even temporarily,
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the brain still tries
to create a coherent picture,
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but the limits of its abilities
become a lot more obvious.
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The full-blown hallucinations of
Charles Bonnet Syndrome are one example.
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Because Charles Bonnet Syndrome
only occurs in people
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who had normal vision
and then lost their sight,
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not those who were born blind,
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scientists think the brain uses
remembered images
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to compensate for
the lack of new visual input.
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And the same is true for other senses.
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People with hearing loss
often hallucinate music or voices,
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sometimes as elaborate as the cacophony
of an entire marching band.
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In addition to sensory deprivation,
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recreational and therapeutic drugs,
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conditions like epilepsy and narcolepsy,
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and psychiatric disorders
like schizophrenia,
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are a few of the many known causes
of hallucinations,
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and we’re still finding new ones.
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Some of the most notorious hallucinations
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are associated with drugs
like LSD and psilocybin.
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Their hallmark effects include
the sensation that dry objects are wet
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and that surfaces are breathing.
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At higher doses, the visual world
can appear to melt,
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dissolve into swirls,
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or burst into fractal-like patterns.
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Evidence suggests these drugs also
act on the cerebral cortex.
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But while visual impairment typically
only causes visual hallucinations,
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and hearing loss auditory ones,
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substances like LSD cause perceptual
disturbances across all the senses.
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That’s likely because they activate
receptors in a broad range of brain areas,
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including the cortical regions
for all the senses.
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LSD and psilocybin both function
like serotonin in the brain,
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binding directly to one type of
serotonin receptor in particular.
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While serotonin’s role in the brain
is complex and poorly understood,
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it likely plays an important part
in integrating information
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from the eyes,
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nose,
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ears,
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and other sensory organs.
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So one theory is that LSD and psilocybin
cause hallucinations
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by disrupting the signaling
involved in sensory integration.
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Hallucinations associated
with schizophrenia
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may share a similar mechanism with those
caused by LSD and psilocybin.
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Patients with schizophrenia
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often have elevated levels
of serotonin in the brain.
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And antipsychotic drugs relieve
symptoms of schizophrenia
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by blocking the same serotonin
receptors LSD and psilocybin bind to.
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And, in some cases,
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these drugs can even relieve
the hallucinations
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of patients with Charles Bonnet Syndrome.
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We’re still a long way from understanding
all the different causes
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and interconnected mechanisms
of hallucinations.
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But it’s clear that
hallucinatory experiences
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are much more closely tied to ordinary
perception than we once thought.
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And by studying hallucinations,
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we stand to learn a great deal
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about how our brains construct
the world we see,
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hear,
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smell,
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and touch.
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As we learn more,
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we’ll likely come to appreciate
just how subjective and individual
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each person’s island universe
of perception really is.