5 Steps to a 5 AP Psychology, 2010-2011 Edition (19 page)

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.

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.

Your patterns of behavior generally involve masses of neural tissue rather than a few neurons. All of the neurons in your body are organized into your nervous system. Your nervous system has subdivisions based on location and function. The two major subdivisions are your central nervous system and your peripheral nervous system. Your
central nervous system
consists of your brain and your spinal cord. Your
peripheral nervous system
includes two major subdivisions: your somatic nervous system and your autonomic nervous system. Your peripheral nervous system lies outside the midline portion of your nervous system carrying sensory information to and motor information away from your central nervous system via spinal and cranial nerves. Your
somatic nervous system
has motor neurons that stimulate skeletal (voluntary) muscle. Your autonomic nervous system has motor neurons that stimulate smooth (involuntary) and heart muscle. Your autonomic nervous system is subdivided into the antagonistic
sympathetic nervous system
and
parasympathetic nervous system
. Sympathetic stimulation results in responses that help your body deal with stressful events including dilation of your pupils, release of glucose from your liver, dilation of bronchi, inhibition of digestive functions, acceleration of heart rate, secretion of adrenalin from your adrenal glands, acceleration of breathing rate, and inhibition of secretion of your tear glands. Parasympathetic stimulation calms your body following sympathetic stimulation by restoring digestive processes (salivation, peristalsis, enzyme secretion), returning pupils to normal pupil size, stimulating tear glands, and restoring normal bladder contractions. Your
spinal cord
, protected by membranes called meninges and your spinal column of bony vertebrae, starts at the base of your back and extends upward to the base of your skull where it joins your brain. The cord is composed mainly of interneurons and glial cells, which are all bathed by cerebrospinal fluid produced by your glial cells.

Your
brain
, which has the consistency of soft-serve yogurt, is covered by protective membranes called meninges, and is housed in your skull. The evolutionary approach describes the brain’s evolution from more primitive organisms, reasoning that new types of behavior developed as each new layer of the brain evolved. According to one evolutionary model (triune brain), the human brain has three major divisions, overlapping layers with the most recent neural systems nearest the front and top. The reptilian brain, which maintains homeostasis and instinctive behaviors, roughly corresponds to the brainstem, which includes the medulla, pons, and cerebellum. Developmental psychologists call the
brainstem the hindbrain. The old mammalian brain roughly corresponds to the limbic system that includes the septum, hippocampus, amygdala, cingulate cortex, hypothalamus; the thalamus, which are all important in controlling emotional behavior, some aspects of memory, and vision. The new mammalian brain or neocortex, synonymous with the cerebral cortex, accounts for about 80% of brain volume and is associated with the higher functions of judgment, decision making, abstract thought, foresight, hindsight and insight, language and computing, as well as sensation and perception. Developmental psychologists call the structures of the “mammalian brains” the forebrain. The surface of your cortex has peaks called
gyri
and valleys called
sulci
, which form
convolutions
that increase the surface area of your cortex. Deeper valleys are called fissures. The last evolutionary development of the brain is the localization of functions on different sides of your brain.

Although multiple representations of information can be located within different areas of your brain, specific regions of your brain seem most critical in handling particular functions. This localization of structure and function has been identified for numerous regions (see
Figure 7.1
).
Association areas
are regions of the cerebral cortex that do not have specific sensory or motor functions, but are involved in higher mental functions, such as thinking, planning, remembering, and communicating. In general, crossing over of nerves sending information from one side of your body to the other side of your brain results in
contralaterality
, control of one side of your body by the other side of your brain.

Figure 7.1 Major structures of the brain in medial view
.

Just as a map or globe can be divided into hemispheres and continents, your cerebral cortex can be divided into eight lobes, four in the left cerebral hemisphere and four in the right cerebral hemisphere (see
Figure 7.2
). Go to
www.g2conline.org
and click on 3-D Brain for a more detailed view of a three-dimensional brain model.

Figure 7.2 Regions of the left cerebral cortex in lateral view
.

Although specific regions of the brain are associated with specific functions, if one region is damaged, the brain can reorganize to take over its function, which is called
plasticity
. In phantom limb syndrome, a somewhat unfortunate example of plasticity, reorganization of the somatosensory cortex leads to experiencing sensations where a missing limb used to be.