Techniques to Learn About Structure and Function
As technology has improved, scientists have used a wide range of techniques to learn about
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 and vocal cords weren’t dam-
aged 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 areato loss of
the ability to speak, known as expressive aphasia. Carl Wernicke similarly found another
brain area involved in understanding 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
process, 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 that cut neural connections (the corpus callosum) between the
cerebral hemispheres revealed that the left cerebral hemisphere is specialized for verbal,
mathematical, and analytic 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
enabled 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 potentialswhen 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 visual66 ❯ STEP 4. Review the Knowledge You Need to Score High“Structure is
always related to
function in living
things.”—
Adrianne,
AP teacher