The Lobes
Dr. C. George Boeree
The Frontal Lobe
Starting from the central sulcus and working forward, we first have the
motor cortex, which sends its
signals down to the body to control the skeletal muscles.
Just in front of the motor cortex there is the premotor cortex, which is where we
compose and rehearse movements before we engage in them. Broca's
area for speech production is part of the premotor cortex.
And then we have the prefrontal cortex,
where
some
of the most interesting things occur. Some say that
will power, our sense of reality, and our sense of our own personality
reside there.
A few areas of the prefrontal cortex are at least partially
understood. The dorsolateral
area (high and to the sides) appears to allow us to hold ideas in
awareness, focus on them, and even manipulate them.
The ventromedial area (low and
close to the midline) seems to be involved in emotional experience and
provides us with the feeling that things make sense and have
meaning. Low levels of activity here are associated with
depression: Nothing makes any sense. High levels, on the
other hand, are associated with mania: Every little thing is full
of importance!
The orbital area of the
prefrontal cortex (just above the eyeballs) tells us when something is
wrong and requires serious attention. It also has the ability to
inhibit behaviors that are inappropriate, such as those that are
harmful to us or are socially unacceptable. This includes the
ability to
counteract the signals for aggression from the amygdala in the limbic
system. It is believed that many violent criminals have had
damage to this area of the brain.
In the most frontal part of the prefrontal lobe is an area devoted to
interpreting people's intentions and motives. Autistic people
seem to have some sort of defect in this location.
The Temporal Lobe
This lobe sits at the two sides of the head, under the temples.
The upper part of the temporal lobe, along the Sylvian fissure that
separates it from the frontal lobe, is the primary auditory cortex, which
receives input from cochlea. The areas around it are devoted to
interpreting sounds, and one of these in particular (Wernicke's area,
toward the boundary with the parietal lobe in the left hemisphere) is
known to be devoted to the understanding of language.
Another area is called the fusiform
gyrus, which sits low in the temporal lobe near the occipital
lobe. In the left hemisphere, it is responsible for word and
number recognition. In the right hemisphere, it is responsible
for the crucial human ability to recognize faces. Problems here
leave one with a disorder called prosopagnosia ("face blindness"),
which makes social life
very difficult.
There are medial (inner) areas of the temporal lobes that are closely
connected to the hippocampus and appear to be devoted to memory for
life events (episodic memory).
One very odd function of the temporal lobe is what some have called the
God spot. Stimulation
here gives people intense feelings of joy and the sense of being close
to some greater power or being "one with the universe." Some
epileptic patients get these intense feelings just before seizures, and
it is believed that some famous saints and other religious figures may
have likewise suffered (if you can call it that!) from such disorders.
The Parietal Lobe
The furthest forward area of the parietal lobe, along the central gyrus
nearest the frontal lobe, is the somatosensory
area, which collects signals coming up from the body. Just
behind that is the somatosensory
association area, which further analyzes these bodily
sensations. Specific areas specialize in locating and orienting
us in three-dimensional space, and focusing on things in the outside
world.
The Occipital Lobe
The occipital lobe is the smallest of the lobes and sits at the very
back of the head. It has no clear borders and is differentiated
primarily by function, i.e. vision. The various parts of the
occipital lobe are labeled with a V (for visual) followed by a number.
The primary visual cortex at
the very back of the occipital lobe is labeled V1, and receives input from the
optic tract. It has a clear map of visual information that
corresponds to the areas of the retina. The center of vision is
greatly magnified. The individual neurons of V1 are extremely
sensitive to very particular changes in input from the eyes.
If there is a lesion somewhere in V1, there will be a "hole" in your
vision called a scotoma.
Oddly, some of the information from that "hole" seems to still be
available, so that some people with scotomas can still react to stimuli
there even though they don't consciously perceive them! This is
called blindsight.
V2 surrounds V1 and has many
reciprocal connections with it. Much of its functioning is a
repeat of V1's, but it detects more complex features, such as contours
and the distinction between figure and ground.
V3, just above V2, gets inputs
from both V1 and V2. It appears to specialize in depth, distance,
and global motion.
V4 lies under V2. V4 is
affected by attentional processes, and specializes in somewhat more
complex perception of specific objects.
V5 (also referred to as MT) is further forward in the
occipital lobe, and processes complex motion.
There seem to be two major pathways for visual information processing
in the occipital lobe. There is a "where?" path, from V1 to V2 to V3
and V5, that interprets location and motion in space. And there
is a "what?" path from V1 to V2
to V4 that determines the identity of an object. There are
additional areas whose functions are not yet known.
The Cingulate Gyrus
The cingulate gyrus sits below the rest of the cerebral cortex, up
against the corpus callosum and partially covering lower areas such as
the basal ganglia, the limbic system, and the thalamus. Many
people see it as the fifth lobe - others see it as part of the lobes
above it, particularly the frontal lobe. It is so intimately
connected with the limbic system that it is sometimes called the limbic gyrus. One of its major
jobs is to keep your attention focused. When it isn't functioning
properly, as seems to happen in schizophrenia, you are unable to
distinguish real voices from imaginary ones.
© Copyright 2006, C. George Boeree
