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Today, there are many forms of imaging available to the medical profession. Among the more well-known types include x-rays, ultrasounds, CT scans, PETscans, and the most groundbreaking, the MRI. One lesser known form of imaging is the technique of evoked potentials. The evoked potential uses stimulation of the body to force activity in the brain. Using electrodes, a clinician can take signals directly from the brain without any intrusive methods. The electrodes remain on the surface of the skin and unlike many other imaging techniques, evoked potentials do not involve any type of ionizing radiation that would be dangerous to the body.
Since evoked potentials involve the stimulation of the body, they are directly related to the arousing of the sense. There are three main types of evoked potentials: visual, auditory, and somatosensory. Although there are also experimental studies being conducted with gustatory and olfactory evoked potentials, vision, hearing, and touch have shown the most successful clinical uses. In this paper, visual evoked potentials will be covered in depth and auditory and somatosensory evoked potentials will be reviewed in brief. The most common clinical uses of this technique are to obtain ideas of brain activity by monitoring the size of amplitudes and latencies and subsequently diagnosing diseases and disorders concerning neural activity. The most common potential obtained is the visual evoked potential (VEP).
Before explaining how a VEP is obtained, however, the basic physiology of the visual system must be understood. Each individual eye receives light from both the right and left visual fields. Upon entering the eye, they hit the retina and then they are transferred to the optic nerve. The optic nerve is the eyes connection to the brain. The optic nerve of each eye cross at the optic chips, where the information from both the right and left visual fields are separated. Thereafter, they are directed into the opposite hemisphere of the brain via the optic tract.
The information is then brought to the occipital cortex at the rear of the brain. This is also called the primary visual cortex. Today, a more modern form of the VEP is the multifocal visual evoked potential, which through different testing techniques and forms of analysis can take signals from various locations, forming a more complete image of the brain's activity. When conducting this test, a patient will be seated and will be facing a monitor. On the monitor a checkerboard pattern will be displayed and the squares on the pattern will reverse randomly from black to white. Typically, the stimulus is circular and consists of 60 sectors with each sector containing 16 checks, for a total of 960 checks.
They will alternate very quickly (every few milliseconds) and will serve as the stimulus for the patient. Attached to the patients scalp will be five electrodes. The most important of these electrodes is placed at a landmark bump on the back of the head called the inion. This bump is approximately where a fold in the brain, called the catharine fissure, is located. It is at the catharine fissure where the primary visual cortex is located. In addition to the inion electrode, three other electrodes are placed on the scalp of the back of the head to create three channels with the inion and, altogether, six channels can be derived mathematically.
Also, a ground electrode is placed on the forehead. One eye on the patient is covered and each eye is stimulated for approximately seven minutes. Two runs are typically made causing the entire procedure to last about 45 minutes. Running the test twice is necessary to increase the signal to noise ratio. Once the test is complete, a computer software using complicated algorithms is used to analyze the signals received through the electrodes.
A computer can derive a VEP response in each eye for each of the corresponding 60 sectors found in the stimulus. The VEP does not have a standard response. Every individual will respond to the stimulus with different wave patterns based on sex, age, race, in addition to a number of other factors. Moreover, the same individual may give different responses if tested on two different days. A drawback to this means that one's VEP cannot be compared to a standard response in order to measure normality. Nevertheless, the VEP for each patient's left and right eyes are almost identical.
This allows physicians to compare the two eyes and then evaluate subjects based on abnormalities from eye to eye. There are a few very important applications to the VEP. The most important application deals with the condition of Optic Neuritis. In optic neuritis, the optic nerve becomes inflamed and a patient's vision can become blurry to partially or completely blind.
Optic neuritis is usually a temporary condition and most patients recover their vision after a week. The cause of optic neuritis can be attributed to multiple problems; however, the most prevalent syndrome found in relation to optic neuritis is Multiple Sclerosis (MS). In MS, the myelin, a substance that insulates the nerves in the brain and spinal cord, is destroyed. This is the possible reason for the inflammation that causes optic neuritis.
During an episode of optic neuritis, the VEP is very obviously abnormal. After it has subsided, however, a patient with MS would still show some abnormalities in the latency of the Vep's. The eye that previously had optic neuritis would show signals that are slower than the other eye. This difference in time indicates demyelination and therefore multiple sclerosis. There are many other diseases and conditions for which the VEP can be used for. Besides comparing time difference between the left and right eyes, one can also measure the difference in amplitude.
This can lead to diagnoses of glaucoma or tumors that are pressing on the optic nerve. In addition, the VEP has been able to diagnose different kinds of migraines as well as optic neuropathy. Although the MRI can be used to view most abnormalities in the brain, the VEP has its advantages. The VEP is more sensitive than the MRI, especially concerning disorders of the optic nerve. Moreover, the VEP costs substantially less, both in equipment and running of the test.
Overall, the visual evoked potential is a leading imaging technique used in diagnosing disorders concerning the entire visual system as well as diagnosing diseases such as multiple sclerosis. Another form of evoked potentials is the auditory evoked potential, otherwise known as the brainstem auditory evoked potential (BAEP). This is mainly used in the clinical setting. To understand how the BAEP works, a basic understanding of the auditory system must be established. Sound enters through the outer ear, which consists of the lobe and the ear canal. The main purpose of the outer ear is to collect sound waves and send them to the middle ear.
The most important part of the middle ear is the ear drum and three little bones called the ossicles. The ear drum receives the vibrations and sends the vibrations through the ossicles which bring the sound to the inner ear. The middle ear is also responsible for controlling sound pressure. In the inner ear, the vibrations lead to the cochlea where tiny hair cells cause signals that go to the vestibulocochlear nerve in the brain. To obtain the BAEP, about three of four electrodes are used. They are placed on each earlobe and two are placed on the scalp of the patient.
This setup creates two primary channels. The patient then wears headphones which provide stimuli which consist of clicks in each ear. These clicks produce five different signals for each eye. The first wave represents the signal from the vestibulocochlear nerve. The second and third are from the cochlear nuclei and the superior oliver nucleus, respectively. These are both part of the brainstem section of the auditory pathway.
The fourth and fifth waves are received from the lateral lemniscus and inferior colliculcus. These are found further up in the brain, closer to the auditory cortex. The first, third, and fifth waves are the most important when it comes to clinical use. When the BAEP is taken, it is similar to the VEP in that there is not a set normal standard as the response depends on the attributes of the patient.
There are important clinical applications for the BAEP as well. The most obvious use is to measure the exte...
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