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