A complete study of human cognition would involve all three Flanagan/Marr
levels; in this segment of the course, we have studied the lowest of these
levels: Implementation. More specifically, we have concentrated on genes
and the "wetware" of the brain.
What is a Neuron?
A neuron is fundamental structural unit of the nervous system. There are
over one hundred billion of these cells in the human brain - twenty times
the current population of the entire world or five-hundred thousand time
the number of pixels on a television screen. And they all fit into the
space of three pints. There are over one hundred trillion connections
between these neurons.
A neuron can fire up to one thousand times per second. Although this is a
thousand times slower than a microchip, the processing capabilities of the
human brain still exceed that of any computer - apparently these systems
have extremely different modes of operation.
A neuron is a cell consisting of a nucleus, an axon, and dendrites; the
primary function of these cells is to transmit information from one area
of the body to another. The largest part of the neuron, the axon,
transmits electrical impulses across the cell to dendrites. These regions
of the cell receive electrical stimuli and, in response, produce
neurotransmitters, chemicals that transfer information between neurons.
Chemical Pathways of the Brain
Although the brain transmits the majority of the information it
processes through electrical intraneural impulses, the chemical transfer
of information between neurons is also important. Vacuoles filled with
neurotransmitters make their way to the dendritic membrane and are
released and travel to other neurons - stimulating the production of an
electrical signal. Two examples of neurotransmitters are serotonin and
dopamine; they are involved with normal functions of the brain, such as
sleep.
The Structure of the Human Brain
An analysis of the lateral and vertical differentiation of the
brain indicates that functional relationships between areas of dedicated
processing parallel their spatial orientation. (See the Neuroscience for Kids
for a diagram of the structure of the brain.) The presence of all these
mediating layers between input and output adds to the complexity and
flexibility of the brain.
What is a Gene?
A gene is the unit of inheritance. It's a piece of the genetic material
that determines the inheritance of a particular characteristic, or group
of characteristics. Genes are carried by chromosomes in the cell nucleus
and are arranged in a line along each chromosome. Genetic material takes
the form of deoxyribonucleic acid (DNA), a long ladder-like molecule in
the shape of a double helix. The "rungs" of the ladder encode genetic
information as a sequence of nucleotides (adenine, guanine, cytosine, and
thymine). Cells utilize this genetic information by producing various
proteins, the structure of which are stored in the arrangement of
nucleotides on the DNA molecule.
How do Genes Influence Behavior?
Contrary to popular belief, genetic make-up is not the only
determinant of brain structure and function; however, it does play a
significant role. Genes set the norms of behavior, not its limits.
Interactions with the environment are an additional contributor to
behavior. The extent of influence each has on the brain is hard to
determine.
In an effort to quantify the influence genes - as opposed to
environment - have on behavior, cognitive scientists have conducted
various studies of identical twins. The validity of these experiments is
extremely difficult to ensure because, even when the twins are raised
apart, their genetic similarities lead them to encounter similar
environments.
There is also significant research aimed at tracing the link from behavior
back to its genetic roots. Comparing William's Syndrome, Turner's
Syndrome, and Simultagnosia, we see that the same behavior at the
ecological level can be attributed to different bases at the
implementation level. This analysis also indicated that it is not always
possible to trace an ecological disorder down through every other
Flanagan/Marr level.
Evolutionary Theory
Our brain is the result of millions of years of evolution. The
earliest named hominoid species lived between three and four million years
ago, and evidence suggests that meat eating began two to three million
years ago. One million years later, man began to develop stone tools and
harness fire. Thirty-two thousand years ago Neanderthal man became
extinct, and Cro-Magnon man, our earliest ancestor with a brain nearly
identical to ours, lived fifty to seventy-five thousand years ago.