78 CHAPTER 2
covering on all the other brain structures. This will be the cortex, which will get more and
more wrinkled as the brain increases in size and complexity. This increase in wrinkling is
called “corticalization.”
CEREBRAL HEMISPHERES The cortex is divided into two sections called the cerebral
hemispheres, which are connected by a thick, tough band of neural fibers (axons)
called the corpus callosum (literally meaning “hard body,” as calluses on the feet are
hard). (Refer to Figure 2.12.) The corpus callosum allows the left and right hemispheres
to communicate with each other. Each hemisphere can be roughly divided into four
sections or lobes by looking at the deeper wrinkles, or fissures, in its surface. The lobes
are named for the skull bones that cover them (see Figure 2.14).
Another organizational feature of the cortex is that for specific regions, each hemi-
sphere is responsible for the opposite side of the body, either for control or for receiving
information. For example, the motor cortex controls the muscles on the opposite side of
the body. If we are writing with our right hand, the motor cortex in the left hemisphere
is responsible for controlling those movements. This feature, referred to as contralateral
organization, plays a role in information coming from many of the sense organs to the
brain and in the motor commands originating in the brain going to the rest of the body.
Information from our body can also be transmitted to both sides of the brain, or
bilaterally (as in hearing and vision), or to only one side of the brain, or ipsilaterally (as in
taste and olfaction). These aspects are also important in the study of brain lateralization,
which we will come back to later in the chapter. Why do we have this arrangement for
some functions and not for others? No one really knows, but at least for some informa-
tion, it assists with identifying where information from the environment is coming from.
For auditory information from the ears, having sensory information projected to both
hemispheres allows us to localize sounds by comparing the slightly different information
coming from each ear.
OCCIPITAL LOBES At the base of the cortex, toward the back of the brain, is an area
called the occipital lobe. This area processes visual information from the eyes in the
primary visual cortex. The visual association cortex, also in this lobe and in parts of thecorpus callosum
thick band of neurons that con-
nects the right and left cerebral
hemispheres.
cerebral hemispheres
the two sections of the cortex on the
left and right sides of the brain.
From top to bottom, a rat brain, sheep brain,
and human brain (not to scale!). Note the
differences in the amount of corticalization,
or wrinkling, of the cortex between
these three brains. Greater amounts of
corticalization are associated with increases
in size and complexity.
occipital lobe
section of the brain located at the
rear and Dottom of each cereDral
hemisphere containing the primary
visual centers of the brain. Figure 2.14 Lobes and Cortical Areas of the Brain