0:00:06.670,0:00:10.020
An elderly woman named Rosalie [br]was sitting in her nursing home

0:00:10.020,0:00:14.291
when her room suddenly burst [br]to life with twirling fabrics.

0:00:14.291,0:00:16.022
Through the elaborate drapings,

0:00:16.022,0:00:17.457
she could make out animals,

0:00:17.457,0:00:18.213
children,

0:00:18.213,0:00:20.035
and costumed characters.

0:00:20.035,0:00:22.709
Rosalie was alarmed,[br]not by the intrusion,

0:00:22.709,0:00:27.800
but because she knew this entourage [br]was an extremely detailed hallucination.

0:00:27.800,0:00:30.163
Her cognitive function was excellent,

0:00:30.163,0:00:34.642
and she had not taken any medications [br]that might cause hallucinations.

0:00:34.642,0:00:39.662
Strangest of all, had a real-life crowd [br]of circus performers burst into her room,

0:00:39.662,0:00:41.741
she wouldn’t have been able to see them:

0:00:41.741,0:00:44.249
she was completely blind.

0:00:44.249,0:00:48.439
Rosalie had developed a condition known [br]as Charles Bonnet Syndrome,

0:00:48.439,0:00:52.127
in which patients with either impaired [br]vision or total blindness

0:00:52.127,0:00:55.545
suddenly hallucinate whole scenes [br]in vivid color.

0:00:55.545,0:00:57.987
These hallucinations appear suddenly,

0:00:57.987,0:01:02.107
and can last for mere minutes [br]or recur for years.

0:01:02.107,0:01:05.621
We still don’t fully understand [br]what causes them to come and go,

0:01:05.621,0:01:08.778
or why certain patients develop them [br]when others don’t.

0:01:08.778,0:01:13.011
We do know from fMRI studies [br]that these hallucinations

0:01:13.011,0:01:15.519
activate the same brain areas as sight,

0:01:15.519,0:01:20.023
areas that are not activated [br]by imagination.

0:01:20.023,0:01:22.905
Many other hallucinations, [br]including smells,

0:01:22.905,0:01:23.785
sights,

0:01:23.785,0:01:24.758
and sounds,

0:01:24.758,0:01:29.428
also involve the same brain areas [br]as real sensory experiences.

0:01:29.428,0:01:35.626
Because of this, the cerebral cortex is[br]thought to play a part in hallucinations.

0:01:35.626,0:01:39.533
This thin layer of grey matter [br]covers the entire cerebrum,

0:01:39.533,0:01:44.169
with different areas processing [br]information from each of our senses.

0:01:44.169,0:01:47.110
But even in people[br]with completely unimpaired senses,

0:01:47.110,0:01:51.840
the brain constructs the world we perceive[br]from incomplete information.

0:01:51.840,0:01:54.130
For example, our eyes have blind spots

0:01:54.130,0:01:57.091
where the optic nerve [br]blocks part of the retina.

0:01:57.091,0:02:00.893
When the visual cortex processes light [br]into coherent images,

0:02:00.893,0:02:05.529
it fills in these blind spots with [br]information from the surrounding area.

0:02:05.529,0:02:10.462
Occasionally, we might notice a glitch,[br]but most of the time we’re none the wiser.

0:02:10.462,0:02:15.678
When the visual cortex is deprived [br]of input from the eyes, even temporarily,

0:02:15.678,0:02:18.810
the brain still tries [br]to create a coherent picture,

0:02:18.810,0:02:22.953
but the limits of its abilities [br]become a lot more obvious.

0:02:22.953,0:02:27.971
The full-blown hallucinations of[br]Charles Bonnet Syndrome are one example.

0:02:27.971,0:02:30.595
Because Charles Bonnet Syndrome [br]only occurs in people

0:02:30.595,0:02:33.572
who had normal vision [br]and then lost their sight,

0:02:33.572,0:02:35.311
not those who were born blind,

0:02:35.311,0:02:38.299
scientists think the brain uses[br]remembered images

0:02:38.299,0:02:41.922
to compensate for [br]the lack of new visual input.

0:02:41.922,0:02:44.067
And the same is true for other senses.

0:02:44.067,0:02:47.978
People with hearing loss [br]often hallucinate music or voices,

0:02:47.978,0:02:52.929
sometimes as elaborate as the cacophony [br]of an entire marching band.

0:02:52.929,0:02:55.662
In addition to sensory deprivation,

0:02:55.662,0:02:57.941
recreational and therapeutic drugs,

0:02:57.941,0:03:00.859
conditions like epilepsy and narcolepsy,

0:03:00.859,0:03:03.635
and psychiatric disorders [br]like schizophrenia,

0:03:03.635,0:03:06.996
are a few of the many known causes [br]of hallucinations,

0:03:06.996,0:03:09.229
and we’re still finding new ones.

0:03:09.229,0:03:11.619
Some of the most notorious hallucinations

0:03:11.619,0:03:15.472
are associated with drugs [br]like LSD and psilocybin.

0:03:15.472,0:03:20.399
Their hallmark effects include [br]the sensation that dry objects are wet

0:03:20.399,0:03:23.068
and that surfaces are breathing.

0:03:23.068,0:03:26.978
At higher doses, the visual world [br]can appear to melt,

0:03:26.978,0:03:28.318
dissolve into swirls,

0:03:28.318,0:03:31.340
or burst into fractal-like patterns.

0:03:31.340,0:03:35.521
Evidence suggests these drugs also [br]act on the cerebral cortex.

0:03:35.521,0:03:39.900
But while visual impairment typically [br]only causes visual hallucinations,

0:03:39.900,0:03:41.794
and hearing loss auditory ones,

0:03:41.794,0:03:48.390
substances like LSD cause perceptual[br]disturbances across all the senses.

0:03:48.390,0:03:53.115
That’s likely because they activate [br]receptors in a broad range of brain areas,

0:03:53.115,0:03:56.653
including the cortical regions [br]for all the senses.

0:03:56.653,0:04:00.679
LSD and psilocybin both function [br]like serotonin in the brain,

0:04:00.679,0:04:05.063
binding directly to one type of[br]serotonin receptor in particular.

0:04:05.063,0:04:08.990
While serotonin’s role in the brain [br]is complex and poorly understood,

0:04:08.990,0:04:12.596
it likely plays an important part [br]in integrating information

0:04:12.596,0:04:13.378
from the eyes,

0:04:13.378,0:04:14.111
nose,

0:04:14.111,0:04:14.881
ears,

0:04:14.881,0:04:16.743
and other sensory organs.

0:04:16.743,0:04:21.022
So one theory is that LSD and psilocybin[br]cause hallucinations

0:04:21.022,0:04:25.243
by disrupting the signaling [br]involved in sensory integration.

0:04:25.243,0:04:27.943
Hallucinations associated [br]with schizophrenia

0:04:27.943,0:04:33.374
may share a similar mechanism with those[br]caused by LSD and psilocybin.

0:04:33.374,0:04:35.031
Patients with schizophrenia

0:04:35.031,0:04:38.008
often have elevated levels [br]of serotonin in the brain.

0:04:38.008,0:04:42.209
And antipsychotic drugs relieve [br]symptoms of schizophrenia

0:04:42.209,0:04:47.612
by blocking the same serotonin [br]receptors LSD and psilocybin bind to.

0:04:47.612,0:04:49.117
And, in some cases,

0:04:49.117,0:04:51.684
these drugs can even relieve [br]the hallucinations

0:04:51.684,0:04:54.282
of patients with Charles Bonnet Syndrome.

0:04:54.282,0:04:57.848
We’re still a long way from understanding [br]all the different causes

0:04:57.848,0:05:01.234
and interconnected mechanisms [br]of hallucinations.

0:05:01.234,0:05:03.698
But it’s clear that [br]hallucinatory experiences

0:05:03.698,0:05:08.361
are much more closely tied to ordinary[br]perception than we once thought.

0:05:08.361,0:05:09.871
And by studying hallucinations,

0:05:09.871,0:05:11.453
we stand to learn a great deal

0:05:11.453,0:05:14.377
about how our brains construct [br]the world we see,

0:05:14.377,0:05:15.086
hear,

0:05:15.086,0:05:15.918
smell,

0:05:15.918,0:05:17.220
and touch.

0:05:17.220,0:05:18.147
As we learn more,

0:05:18.147,0:05:22.248
we’ll likely come to appreciate[br]just how subjective and individual

0:05:22.248,0:05:26.435
each person’s island universe [br]of perception really is.