How Brains Explain
It was not always obvious that brains have much to do with thinking. Aristotle believed that the
primary organ supporting thought was the heart rather than the brain, whose main function was to cool
the blood. By the sixteenth century, however, rough connections between the brain and thinking were
generally recognized—for example, that vision and hearing depend on brain anatomy. Understanding
of how brains work began only at the end of the nineteenth century when the development of new
chemical techniques for staining cells made possible identification of the cells that constitute brains.
It was decades before the electrical nature of brain cells—neurons—was appreciated. Only with the
development of computers did it become possible to formulate and test detailed hypotheses about
how the interactions among large numbers of neurons might be able to support different kinds of
thinking.
When the psychologist Stephen Pinker was on the TV show The Colbert Report in 2007, Stephen
Colbert insisted that he explain, in five words or fewer, how the brain works. Pinker's brilliantly
concise response was “Brain cells fire in patterns.” Neurons are different from the cells that make up
other bodily organs in that they build up electrical charges; they can pass these on to other neurons
that are connected to them. Firing is a kind of electrical discharge. The flow of electrical charge in
neurons is only a few millivolts, compared to as much as a billion in lightning flashes, and also
differs in being directed along pathways formed by the thousand or so connections that a neuron has
with other neurons. These connections are called synapses. Figure 3.1 depicts how one neuron can
send messages in the form of electric signals to another neuron by means of synaptic connections.
3.1 A functional model of two connected neurons.
A firing neuron does not simply send a spark across to another neuron but rather sends a chemical
signal in the form of neurotransmitters that flow from the firing neuron across the synaptic gap to the
neurons to which it is connected. Using these signals, one neuron can either excite the neurons to
which it is connected, increasing their electrical activity, or inhibit the activity of the connected
neurons. Whereas a lightning flash is like a single trumpeter producing a loud sound with no intended
direction, the synaptic connections between neurons enable them to perform like a trained orchestra
with many coordinated musicians. Just as a band performance is a complex pattern of activity in a
group of musicians, a brain function is accomplished by patterns of coordinated firing activity in
interconnected neurons. The brain is not like a symphony orchestra that has a conductor to keep
everyone synchronized, but more like a bunch of jamming jazz musicians whose coordinated playing
emerges from their dynamic interactions.
At first it seems incredible that patterns of electrochemical activity in a bunch of cells could
generate thought. Then again, it is also not obvious that a hundred musicians playing together could
produce a beautiful symphony, or that billions of tiny water molecules in a cloud could accumulate a
huge electrical charge that generates bright flashes of lightning and loud rolls of thunder. But much is
coming to be known about how patterns of neural firing can produce complex kinds of perception,
memory, learning, inference, language, and other mental functions. In what follows I will be
extremely introductory. I don't need to convince neuroscientists or cognitive psychologists that minds
are brains, so the explanations that follow are aimed at people new to the idea that thinking might be
explained neurologically.