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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.