Biological Bases of Behavior ❮ 79Techniques to Learn about structure and Function
As technology has improved, scientists have used a wide range of techniques to learn about the
brain and neural function. Over 150 years ago, studying patients with brain damage linked
loss of structure with loss of function. Phineas Gage was the level-headed, calm foreman of a
railroad crew (1848) until an explosion hurled an iron rod through his head. After the injury
severed the connections between his limbic system and frontal cortex, Gage became hostile,
impulsive, and unable to control his emotions or his obscene language. Observed at autopsy,
his loss of tissue (where the limbic system is connected to the frontal lobes) revealed the
relationship between frontal lobes and control of emotional behavior. In another case, Paul
Broca (1861) performed an autopsy on the brain of a patient, nicknamed Tan, who had lost
the capacity to speak although his mouth, his vocal cords weren’t damaged, and he could still
understand language. Tan’s brain showed deterioration of part of the frontal lobe of the left
cerebral hemisphere, as did the brains of several similar cases. This connected destruction of
the part of the left frontal lobe known as Broca’s area to loss of the ability to speak, known as
expressive aphasia. Carl Wernicke similarly found another brain area involved in understand-
ing language in the left temporal lobe. Destruction of Wernicke’s area results in loss of the
ability to comprehend written and spoken language, known as receptive aphasia.
Gunshot wounds, tumors, strokes, and other diseases that destroy brain tissue enabled
further mapping of the brain. Because the study of the brain through injury was a slow pro-
cess, quicker methods were pursued. Lesions, precise destruction of brain tissue, enabled
more systematic study of the loss of function resulting from surgical removal (also called
ablation), cutting of neural connections, or destruction by chemical applications. Surgery
to relieve epilepsy severs neural connections at the corpus callosum, between the cerebral
hemispheres. Studies by Roger Sperry and Michael Gazzaniga of patients with these “split
brains” have revealed that the left and right hemispheres do not perform exactly the same
functions (brain lateralization) that the hemispheres specialize in. The left cerebral hemi-
sphere is specialized for verbal, mathematical, and analytical functions. The nonverbal right
hemisphere is specialized for spatial, musical, and holistic functions such as identifying
faces and recognizing emotional facial expressions.
Direct electrical stimulation of different cortical areas of the brain during surgery ena-
bled scientists to observe the results. Stimulating the back of the frontal cortex at particular
sites caused body movement for different body parts enabling mapping of the motor cortex.
In recent years, neuroscientists have been able to look inside the brain without surgery.
Computerized axial tomography (CAT or CT) creates a computerized image using x-rays
passed through various angles of the brain showing two-dimensional “slices” that can be
arranged to show the extent of a lesion. In magnetic resonance imaging (MRI), a mag-
netic field and pulses of radio waves cause emission of faint radio frequency signals that
depend upon the density of the tissue. The computer constructs images based on varying
signals that are more detailed than CT scans. Both CT scans and MRIs show the structure
of the brain, but don’t show the brain functioning.Measuring Brain Function
Scientists have developed a number of tools to measure the brain functions of people. An
EEG (electroencephalogram) is an amplified tracing of brain activity produced when
electrodes positioned over the scalp transmit signals about the brain’s electrical activity
(“brain waves”) to an electroencephalograph machine. The amplified tracings are called
evoked potentials when the recorded change in voltage results from a response to a specific
stimulus presented to the subject. EEGs have been used to study the brain during states of
arousal such as sleeping and dreaming, to detect abnormalities (such as deafness and visual
disorders in infants), and to study cognition. Another technology, positron emission“Structure is
always related
to function in
living things.”
—Adrianne,
AP teacher