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.
Perception
First consider our senses of sight, hearing, smell, taste, and touch, which are major sources of
information about the world. Much is known about the physical basis of how these senses work,
because they can be studied in nonhuman animals whose senses seem to operate much as our own do.
Here is what happens when you see a tree. Light reflects off the tree and into your eyes, where
photons stimulate some of the millions of nerve cells in the retina at the back of your eyeball. These
cells then send signals along your optic nerve to the back of your brain to the occipital lobe, which
begins a complex process of interpreting the retinal input using a series of regions that include parts
of the temporal lobe (see figure 3.2). Eventually, the result is a pattern of activation of neurons in the
several regions that reactivates an approximation to the pattern of neural firing that constitutes your
concept of a tree, allowing you to identify the observed object as a tree.