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.
3.2 Rough sketch of some major regions of the brain. For a more detailed diagram, see figure 5.1.
Smell similarly involves the stimulation of receptor cells and subsequent processing to enable
recognition and storage. When you sniff a banana, for example, molecules of it are drawn into your
nose where they stimulate cells called olfactory receptors. When these cells fire, they send the signals
to areas of the brain specifically dedicated to smell, such as the olfactory bulb, but also to related
areas such as the amygdala, which is important for emotional experience. The resulting pattern of
activation may produce recognition of a banana if your previous experiences with the substance have
produced synaptic connections that generate a similar pattern of activation. Or, if you have never
smelled a banana before, the resulting pattern of activation produces synaptic changes that can lead to
recognition of bananas in the future.
The senses of taste, hearing, and touch also involve complex processes of stimulation of receptor
cells along with neural encodings and transformations. Similarly, the perception of pain, temperature,
balance, and internal states such as gastrointestinal fullness are increasingly well understood in terms
of how dedicated brain areas interpret signals from various kinds of receptors. Some animals have
sense receptors not found in humans—for example, those that enable some fish to detect electric
fields. Although there is still much that is unknown about how brains perceive sights, smells, and
other sensations, neural explanations of perception are sufficiently rich to justify thinking of
perception as a collection of different kinds of brain processes. Chapter 4 discusses further how
brains perceive the world