FIGURE 1
SOURCE: Mark H. Johnson and Michelle de Haan.
thought that the cortex underlies humans’ complex
intellectual abilities. The cortex is divided into four
lobes: the occipital, parietal, temporal, and frontal
lobes, as illustrated in Figure 1. In all of these lobes
the cortex consists of six layers of cells, and each of
the six layers is made up of particular types of cells
and connections to and from other cells. In adults, the
cortical lobes can be divided even further into areas
that specialize in different functions, such as language
and movement.
Development of the Cerebral Cortex
Neurons (the cells of the cortex that are involved
in processing information) are formed before birth
during the sixth to eighteenth weeks after concep-
tion. In the cerebral cortex, neurons find their way to
the correct position by moving along the long fibers
of radial glia cells, which are like ropes extending
from the inner to the outer surface of the brain. The
length that neurons must travel is especially long for
those that will end up in the frontal lobes, and this
may increase the likelihood that they will end up in
the incorrect position and disrupt information pro-
cessing. Schahram Akbarian and his colleagues sug-
gested in a 1993 paper that such errors might
contribute to schizophrenia.
Once neurons have traveled to their final posi-
tions, they begin to differentiate or take on their ma-
ture characteristics. One aspect of differentiation is
the growth and branching of dendrites. The den-
drites of a neuron are like antennae that pick up sig-
nals from many other neurons and, if the
circumstances are right, pass the signal down the
axon and on to other neurons. The pattern of branch-
ing of dendrites is important because it affects the
amount and type of signals the neuron receives. Dur-
ing development one change that occurs is an in-
crease in size and complexity of neurons’ dendritic
trees. For example, by adulthood the length of the
dendrites of neurons in the frontal cortex can in-
crease to more than thirty times their length at birth.
A second aspect of differentiation that occurs in most
neurons is myelination. Myelin is a fatty sheath that
forms around neurons and helps them transmit sig-
nals more quickly. Myelin begins to form around neu-
rons before birth and continues to do so even into
adulthood in some areas of the cortex.
The points of communication between neurons
are called synapses, and these begin to form in the
brain in the early weeks of gestation. The generation
of synapses occurs at different times in different corti-
cal areas. For example, the maximum density of syn-
apses is reached at about four months in the visual
cortex but not until about twenty-four months after
birth in the prefrontal cortex. This pattern parallels
behavioral development, where functions of the visual
cortex (such as 3-D vision) develop earlier than some
functions of the prefrontal cortex (such as planning
for the future).Regressive Events
At the same time that the brain is growing and in-
creasing in size and complexity, regressive events are
also occurring. One example is the elimination of syn-
apses. During the process of synapse formation, the
number of synapses increases above the level ob-
served in the adult and remains at this level for some
time. Then, synapses are eliminated until the adult
number is reached. For example, in certain parts of
the visual cortex the density of synapses per neuron
reaches a peak of about 150 percent of the adult level
at about age four months then starts to decrease at the
end of the first year of life to reach the adult level by
about age four. The timing of this process is different
for different areas of cortex. In the frontal cortex, the
peak level is reached at about one year of age, and it
then slowly declines to reach adult levels sometime in
adolescence. This loss of synapses does not reduce the
range of behaviors but may be related to the stabiliza-
tion of important networks of neurons in the brain.Neuronal Activity
The adult brain has a very large number of den-
dritic branches and synapses between cells that are or-
ganized in a very specific way. There simply is not
enough space in the human genome to specifically
encode all of this information. Instead of being only
a passive ‘‘readout’’ of genetic information, normal
development of the brain depends in part on the ac-
tivity of the neurons themselves. Even while the baby
is still in the womb, neuronal activity (the electrical
firing of cells) is very important. For example, it has
been discovered that the rhythmical waves of firing of66 BRAIN DEVELOPMENT