Here are two images of
a house. There’s one obvious difference, but to this patient, P.S., they looked
completely identical. P.S. had suffered a stroke that damaged the right side of
her brain, leaving her unaware of everything on her left side.
But though she could
discern no difference between the houses, when researchers asked her which she
would prefer to live in, she chose the house that wasn’t burning-not once, but
again and again. P.S.’s brain was still processing information from her whole field
She could see both
images and tell the difference between them, she just didn’t know it. If
someone threw a ball at her left side, she might duck. But she wouldn’t have
any awareness of the ball, or any idea why she ducked. P.S.’s condition, known
as hemispatial neglect, reveals an important distinction between the brain’s
processing of information and our experience of that processing. That
experience is what we call consciousness.
We are conscious of
both the external world and our internal selves-we are aware of an image in
much the same way we are aware of ourselves looking at an image, or our inner
thoughts and emotions. But where does consciousness come from?
theologians, and philosophers have been trying to get to the bottom of this
question for centuries-without reaching any consensus. One recent theory is
that consciousness is the brain’s imperfect picture of its own activity. To
understand this theory, it helps to have a clear idea of one important way the
brain processes information from our senses.
Based on sensory input,
it builds models, which are continuously updating, simplified descriptions of
objects and events in the world. Everything we know is based on these models.
They never capture every detail of the things they describe, just enough for
the brain to determine appropriate responses. For instance, one model built
deep into the visual system codes white light as brightness without color.
In reality, white light
includes wavelengths that correspond to all the different colors we can see.
Our perception of white light is wrong and oversimplified, but good enough for
us to function. Likewise, the brain’s model of the physical body keeps track of
the configuration of our limbs, but not of individual cells or even muscles,
because that level of information isn’t needed to plan movement.
If it didn’t have the
model keeping track of the body’s size, shape, and how it is moving at any
moment, we would quickly injure ourselves. The brain also needs models of
itself. For example, the brain has the ability to pay attention to specific
objects and events. It also controls that focus, shifting it from one thing to
another, internal and external, according to our needs.
Without the ability to
direct our focus, we wouldn’t be able to assess threats, finish a meal, or
function at all. To control focus effectively, the brain has to construct a
model of its own attention. With 86 billion neurons constantly interacting with
each other, there’s no way the brain’s model of its own information processing
can be perfectly self-descriptive.
But like the model of
the body, or our conception of white light, it doesn’t have to be. Our
certainty that we have a metaphysical, subjective experience may come from one
of the brain’s models, a cut-corner description of what it means to process
information in a focused and deep manner.
Scientists have already
begun trying to figure out how the brain creates that self model. MRI studies
are a promising avenue for pinpointing the networks involved. These studies
compare patterns of neural activation when someone is and isn’t conscious of a
sensory stimulus, like an image. The results show that the areas needed for
visual processing are activated whether or not the participant is aware of the
image, but a whole additional network lights up only when they are conscious of
seeing the image.
hemispatial neglect, like P.S., typically have damage to one particular part of
this network. More extensive damage to the network can sometimes lead to a vegetative
state, with no sign of consciousness. Evidence like this brings us closer to
understanding how consciousness is built into the brain, but there’s still much
more to learn.
For instance, the way
neurons in the networks related to consciousness compute specific pieces of
information is outside the scope of our current technology. As we approach
questions of consciousness with science, we’ll open new lines of inquiry into