Developmental psychology.pdf

Physiology and Behavior 99
Motor Projection Areas We now know that the primary area for controlling body
movements is a narrow strip of cortical tissue at the rear of the frontal lobe directly
in front of the central fissure, known as the motor projection areas. Here the body parts
are represented in an inverted sequence with reference to an upright human being.
Control of the toes, feet, and legs appears near the top; the trunk is toward the middle;
and the arms, hands, neck, and head are near the bottom of each hemisphere.
When you begin to skip rope, impulses occur at the top of the cerebral cortex.
When you are reciting a poem, impulses occur lower down the side. In both cases, the
locations are just forward of the central fissure (Figure 4.10).
Sensory Projection Areas Research on the sensory projection areas, which receive
incoming sensory information, proceeded more slowly, but the primary visual areas are
now known to be in the occipital lobes. Visual experience depends on the dispersion of
impulses in this region, and surgeons have noted that flashing lights and whirling colors
are seen during direct electrical stimulation of the occipital cortex.
Investigators now study highly specific portions of the visual cortex, examining
the response of just a single nerve cell. These studies are called visual coding
because they are concerned with the way in which visual symbols are translated or
coded in the brain. They have shown that even the single cell is not generally responsive
to any visual stimulus but rather to certain visual features. One nerve cell, for example,
is activated by a straight line in a vertical position but not by a straight line in another
position. Another cell responds to a different visual feature, such as a straight line in
a nonvertical position. Integrated activities in billions of such cells, together with responses
from other brain areas, result in visual perceptions (Hubel & Wiesel, 1962).
Auditory experiences involve activities in the temporal lobes. Direct stimulation
of these areas produces humming, buzzing, and similar experiences, but hearing
is also dependent on the integration of activities in other brain areas. In auditory coding,
microelectrodes record responses from single nerve cells in the temporal region,
and the researcher attempts to follow the transmission of information from one temporal
site to another by observing the brain activity associated with specific auditory
stimuli.
The primary area for body feeling, called somesthesis, is the front part of the
parietal lobes, immediately adjacent to and behind the central fissure. Somesthesis has
two divisions, cutaneous, which is sensitivity of the skin, and kinesthetic, which involves
feelings in the muscles, tendons, and joints. Impulses of both types are received from
Figure 4.10
Motor Control Areas. The relative
sizes of the body parts reflect the
relative amounts of neural tissue
devoted to motor control in that area.
Note that large amounts of tissue
serve the hands, face, and speech
organs. The drawing shows only one
side of this system; there is a
counterpart on the other side of the
brain, serving motor functions on the
opposite side of the body.