Techniques to Learn About Structure and Function for AP Psychology (page 2)
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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 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 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 cuts 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 hemisphere 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 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 magnetic 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 tomography (PET) produces color computer graphics that depend on the amount of metabolic activity in the imaged brain region. When neurons are active, an automatic increase in blood flow to the active region of the brain brings more oxygen and glucose necessary for respiration. Blood flow changes are used to create brain images when tracers (such as radioactively tagged glucose) injected into the blood of the subject emit particles called positrons, which are converted into signals detected by the PET scanner. Functional MRI (fMRI) shows the brain at work at higher resolution than the PET scanner. Changes in oxygen in the blood of an active brain area alters its magnetic qualities, which is recorded by the fMRI scanner. After further computer processing, a detailed picture of that local brain activity emerges. With new brain imaging technology, psychologists can explore far more about our abilities than ever before, from well-known systems like perception to less understood systems like motivation and emotion.
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