Many methods exist that allow doctors, biopsychologists, and researchers to study the brain and its functions. They are: stereotaxic surgery, conventional lesion, stimulation, recording methods, pharmacological methods, selective chemical lesions, contrast x-rays, x-ray computed tomography, magnetic resonance imaging, position emission tomography, functional magnetic resonance imaging, gene knockout techniques, and gene replacement techniques (Pinel, 2007). Each of these methods provides valuable information, but they each have limitations to what information they can provide.
Stereotaxic Surgery
Stereotaxic surgery is typically the first method used in many experiments. Stereotaxic surgery is surgery that uses devices that must be placed with precision in the brain. Two things are required to perform stereotaxic surgery, and they are a stereotaxic atlas and stereotaxic instruments. A stereotaxic atlas is much like an atlas that locates roads, rivers, and other landmarks in a geographic atlas; the only difference between the two are that the earth's surface is only two dimensional, while the brain is three dimensional. Stereotaxic instruments typically include a head holder and an electrode holder, both of which are designed to increase the precision at which the instruments are inserted in the brain. This precision is an absolute necessity since measurements in the brain atlas are in millimeters. (Pinel, 2007)
Conventional Lesion, Stimulation, & Recording Methods
Conventional lesion, stimulation and recording methods are the three most important methods used in surgical and electrophysiological studies of brain functioning. There are three conventional lesion methods: the knife cut, the electrolytic lesion, and the aspiration lesion. The knife cut involves the insertion of a hollow tube into the brain, that once in place, a knife pivots out of the tube, slicing the intended section of the brain. The electrolytic lesion destroys a specific section of the brain through the use of an electrode tip that carries enough current to destroy the brain tissue. An aspiration lesion is the removal of a portion of the brain's surface by suctioning or vacuuming it out. Stimulation methods use small currents applied to electrodes that have been permanently placed in the brain to stimulate neurons located near the electrodes' tip; the effect of the currents on behavior on then measured. These methods typically cannot be used on humans due to ethical concerns. (Pinel, 2007)
There are two primary methods for recording brain activity: unit recording and electroencephalographic recording (EEG). Unit recording enables researchers to record the activities of neurons on an individual basis; there is also a method for recording multiple neurons that measures their overall firing rate, called multiple-unit recording. The EEG is a non-invasive method of unit recording that records the variations of electrical activity between two large electrodes that are placed on the scalp of the subject. The EEG method, while it can be used on humans, is limited in the detail that it can provide because the electrodes are so large they record multiple electrical signals. (Pinel, 2007)
Drugs
Pharmacological methods use drugs to study their effect on human and nonhuman behavior and the brain. The most frequently used are agonists and antagonists. Agonist drugs increase the effects of neurotransmitters while antagonist drugs decrease the effects of neurotransmitters. Cocaine is an example of an agonist that increases the effect of dopamine and norepinephrine on neurotransmitters. Botox is an example of an antagonist drug that blocks the effect of acetylcholine on neuromuscular junctions. (Pinel, 2007)
Selective chemical lesions are the use of drugs to make brain lesions. Chemical lesions differ from surgical and electrolytic lesion because they allow for a more specific targeting of an area. For example, injection of the neurotoxin 6-hydroxydopamine (6-OHDA) target only the neurons that release nerepinephrine or dopamine while leaving other neurons largely unscathed. (Pinel, 2007)
Brain Imaging:
Before 1970, the only way for the human brain to be viewed was by removing it from a cadaver; now, there are several technological advances that allow for the viewing of the brain on living subjects. The primary methods are: contrast x-rays, x-ray computed tomography (CT scan), magnetic resonance imaging (MRI), position emission tomography (PET scan), and a functional MRI. For the subject, these methods are relatively pain-free; although the subject may experience some discomfort from lying for extended periods of time on a cold hard table, these methods are non-invasive. (Pinel, 2007)
Contrast x-rays involve the injection of a substance that either increases or decreases the absorption of radiation in surrounding tissues which creates a contrast between surrounding tissue and the compartment. The CT scan is an x-ray that, with the assistance of a computer, enables the brain and other organs to be visualized; the multiple x-rays enable the computer to generate a three-dimensional view of the brain. The MRI uses a magnetic field to activate radio-frequency waves that measure the waves produced by hydrogen atoms, the result of which is a clear, two- or three-dimensional image that has a high degree of spatial resolution. The PET scan measures brain activity while the subject is performing an activity, such as reading, through the injection of a radioactive substance into a carotid artery (artery in the neck that supplies blood to the brain). The functional MRI is an MRI that measures the flow of oxygenated blood to the active areas of the subject's brain. (Pinel, 2007)
Genetic Engineering:
The science of genetic engineering is exploding on the scene in recent years; there are two primary methods of genetic engineering: gene knockout techniques and gene replacement techniques. Genetic engineering is not conducted on humans, with mice the more frequent subject of choice. Gene knockout techniques is the creation of a subject that lacks a particular gene, enabling scientists to study the effects of observable differences in neural processes and behavioral differences between a normal subject versus the subject with the gene missing. The data collected by gene knockout may be unreliable, since behaviors are the result of genetic interaction, genetic compensation may disguise the effects of the knocked-out gene, and genetic expression can also be influence by experiences. (Pinel, 2007)
Genetic replacement techniques are the replacement of one gene with another. One example would be the replacement of a gene from another species into that of a mouse, creating transgenic mice. Genes can also be replaced with genes that act as a light switch that have the ability to turn a gene on or off when exposed to certain chemicals that are able to activate or suppress gene during certain stages of development. (Pinel, 2007)
Conclusion
Many of these methods for studying the brain are controversial and riddled with questions about ethics; yet their usage continues to add to the development of new techniques. Though several of these methods are not approved for use on humans due to the invasiveness of the procedure, their use on other mammals allow scientists to have a greater understanding of the workings of the human brain through their functional similarities.
Reference
Pinel, J. P. J. (2007). Basics of biopsychology. Boston, MA: Allyn and Bacon.
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