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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 [br]damaged the right side of her brain,

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leaving her unaware of everything [br]on her left side.

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But though she could discern no difference[br]between the houses,

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when researchers asked her [br]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 [br]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 [br]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 [br]the brain’s processing of information

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and our experience of that processing.

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That experience is what [br]we call consciousness.

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We are conscious of both the external [br]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 [br]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 [br]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 [br]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 [br]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,[br]simplified descriptions

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of objects and events in the world.

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Everything we know is based [br]on these models.

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They never capture every detail of [br]the things they describe,

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just enough for the brain to determine [br]appropriate responses.

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For instance, one model built deep [br]into the visual system

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codes white light as brightness [br]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 [br]different colors we can see.

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Our perception of white light is wrong [br]and oversimplified,

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but good enough for us to function.

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Likewise, the brain’s model of the [br]physical body

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keeps track of the configuration [br]of our limbs,

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but not of individual cells [br]or even muscles,

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

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If it didn’t have the model keeping track [br]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[br]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, [br]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 [br]of its own attention.

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With 86 billion neurons constantly [br]interacting with each other,

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there’s no way the brain’s model of its [br]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 [br]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[br]to process information

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in a focused and deep manner.

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Scientists have already begun trying [br]to figure out

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how the brain creates that self model.

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

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These studies compare patterns [br]of neural activation

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

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The results show that the areas needed [br]for visual processing

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are activated whether or not the [br]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 [br]of seeing the image.

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Patients with hemispatial neglect, [br]like P.S.,

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typically have damage to one particular [br]part of this network.

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More extensive damage to the network [br]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 [br]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 [br]related to consciousness

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compute specific pieces of information

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is outside the scope of our [br]current technology.

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As we approach questions of consciousness [br]with science,

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