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Here are two images of a house.

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There’s one obvious difference,

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but to this patient, P.S.,

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they looked completely identical.

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P.S. had suffered a stroke that 
damaged the right side of her brain,

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leaving her unaware of everything 
on her left side.

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But though she could discern no difference
between the houses,

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when researchers asked her 
which she would prefer to live in,

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she chose the house that wasn’t burning—

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not once, but again and again.

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P.S.’s brain was still processing 
information

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from her whole field of vision.

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She could see both images

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and tell the difference between them,

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she just didn’t know it.

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If someone threw a ball at her left side,

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she might duck.

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But she wouldn’t have any 
awareness of the ball,

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or any idea why she ducked.

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P.S.’s condition,

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known as hemispatial neglect,

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reveals an important distinction between 
the brain’s processing of information

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and our experience of that processing.

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That experience is what 
we call consciousness.

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We are conscious of both the external 
world and our internal selves—

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we are aware of an image

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in much the same way we are aware of 
ourselves looking at an image,

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or our inner thoughts and emotions.

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But where does consciousness come from?

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Scientists, theologians, and philosophers

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have been trying to get to the bottom of 
this question for centuries—

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without reaching any consensus.

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One recent theory is that

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consciousness is the brain’s imperfect 
picture of its own activity.

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To understand this theory,

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it helps to have a clear idea

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of one important way the brain processes 
information from our senses.

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Based on sensory input,

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it builds models,

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which are continuously updating,
simplified descriptions

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of objects and events in the world.

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Everything we know is based 
on these models.

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They never capture every detail of 
the things they describe,

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just enough for the brain to determine 
appropriate responses.

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For instance, one model built deep 
into the visual system

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codes white light as brightness 
without color.

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In reality,

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white light includes wavelengths

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that correspond to all the 
different colors we can see.

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Our perception of white light is wrong 
and oversimplified,

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but good enough for us to function.

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Likewise, the brain’s model of the 
physical body

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keeps track of the configuration 
of our limbs,

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but not of individual cells 
or even muscles,

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because that level of information 
isn’t needed to plan movement.

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If it didn’t have the model keeping track 
of the body’s size, shape,

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and how it is moving at any moment,

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we would quickly injure ourselves.

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The brain also needs models of itself.

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For example,

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the brain has the ability to pay attention
to specific objects and events.

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It also controls that focus,

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shifting it from one thing to another,

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internal and external,

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according to our needs.

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Without the ability to direct our focus,

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we wouldn’t be able to assess threats, 
finish a meal, or function at all.

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To control focus effectively,

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the brain has to construct a model 
of its own attention.

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With 86 billion neurons constantly 
interacting with each other,

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there’s no way the brain’s model of its 
own information processing

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can be perfectly self-descriptive.

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But like the model of the body,

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or our conception of white light,

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it doesn’t have to be.

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Our certainty that we have a 
metaphysical, subjective experience

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may come from one of the brain’s models,

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a cut-corner description of what it means
to process information

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in a focused and deep manner.

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Scientists have already begun trying 
to figure out

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how the brain creates that self model.

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MRI studies are a promising avenue 
for pinpointing the networks involved.

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These studies compare patterns 
of neural activation

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when someone is and isn’t conscious 
of a sensory stimulus, like an image.

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The results show that the areas needed 
for visual processing

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are activated whether or not the 
participant is aware of the image,

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but a whole additional network lights up

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only when they are conscious 
of seeing the image.

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Patients with hemispatial neglect, 
like P.S.,

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typically have damage to one particular 
part of this network.

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More extensive damage to the network 
can sometimes lead to a vegetative state,

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with no sign of consciousness.

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Evidence like this brings us closer 
to understanding

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how consciousness is built into the brain,

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but there’s still much more to learn.

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For instance,

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the way neurons in the networks 
related to consciousness

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compute specific pieces of information

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is outside the scope of our 
current technology.

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As we approach questions of consciousness 
with science,

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we’ll open new lines of inquiry 
into human identity.