1-1 Neuroscience in the Twenty-First Century

Studying the brain and behavior leads us to better understand our origins, our human nature, the causes of many behavioral disorders, and the rationale behind treatment for disorders.

The human nervous system is composed of the CNS, which includes the brain and the spinal cord, and the PNS, through which the brain and spinal cord communicate with sensory receptors, with muscles and other tissues, and with the internal organs. The cerebrum and the cerebellum have undergone the most growth in large-brained animal species.

We define behavior as any kind of movement, including mental processes such as thinking and imagining. In animals, behavior is caused by nervous system activity. Behavioral flexibility and complexity vary greatly across species, as does the nervous system.

For some species, including humans, the brain is the organ that exerts control over behavior. The brain seems to need ongoing sensory and motor stimulation to maintain its intelligent activity.

1-2 Perspectives on Brain and Behavior

Mentalism views behavior as a product of an intangible entity called the mind (psyche); the brain has little importance. Dualism is the notion that the immaterial mind acts through the material brain to produce language and rational behavior, whereas the brain alone is responsible for the “lower” actions that we have in common with other animal species.

Materialism, the view that brain function fully accounts for all behavior, language and reasoning included, guides contemporary research on the brain and behavior. Support for the materialistic view comes from the study of natural selection—the evolutionary theory that behaviors such as human language evolved from the simpler language abilities of human ancestors—and from discoveries about how genes function. Experiments follow the process of science: first, formulate a question (hypothesis), then design a procedure to test it, evaluate the results, and confirm or modify the hypothesis.

After severe TBI, the brain demonstrates a remarkable ability to recover, but after either mild or severe injury, a person can be left with a permanent disability that prevents full recovery to former levels of function. Brain imaging techniques can confirm severe disabilities such as the MCS, locked-in syndrome, and PVS.

1-3 Evolution of Brains and of Behavior

Behavioral neuroscientists subscribe to the evolutionary principle that all living organisms are descended from a common ancestor. Brain cells and muscles are quite recent developments in the evolution of life on Earth. Because they evolved only once, a similar basic pattern exists in the nervous systems of all animals.

The nervous systems of some animal lineages have become more complex, with evolution featuring first a nerve net, followed by a bilaterally symmetrical and segmented nervous system, a nervous system controlled by ganglia, and eventually, in chordates, a nervous system featuring a brain and spinal cord.

Mammals are a class of chordates characterized by a large brain relative to body size. Modern humans belong to the primate order, which is distinguished by especially large brains, and to the family of great apes, whose members’ limber shoulders allow them to brachiate (hang and swing by the arms).

1-4 Evolution of the Human Brain and Behavior

One of our early hominid ancestors was probably an Australopithecus, who lived in Africa several million years ago. It is from an australopith species that Homo evolved through species such as Homo habilis and Homo erectus. Modern humans, Homo sapiens sapiens, appeared about 200,000 years ago.

Since Australopithecus, the hominid brain has increased in size almost threefold, as has its number of brain cells. The EQ describes brain size relative to body size, but a complete comparison of different species’ brains requires brain cell counts. Among the factors hypothesized to have stimulated brain evolution in human species are environmental challenges and opportunities, such as climate changes that favored the natural selection of adaptability and more complex behavior patterns. Also proposed are lifestyle changes such as social cooperation and cooking food, changes in physiology, and changed maturation rate.

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1-5 Modern Human Brain Size and Intelligence

Evolutionary principles learned from studying the brain and behavior across species do not easily apply to the brain and behavior within a single species, such as Homo sapiens. As animals evolved, larger brains were associated with more complex behavior. Yet within our species, the complexity of different brain regions is related to behavioral abilities. People vary widely in body size and in brain size as well as in varying kinds of intelligence, making a simple comparison of brain size and general intelligence unwise.

Recognizing the great extent to which modern human behavior, rather than being inherent in our nervous systems, results from cultural learning and transmission is paramount to understanding how our brains function. Memes may spread from person to person and culture to culture.