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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,
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simplified descriptions 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, white light includes
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wavelengths 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 are activated
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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.