When light enters the eye it falls on the retina, which stimulates receptors. This results in depolarization and the generation of action potentials in the optic nerves. These action potentials pass through the anterior visual pathways, optic chiasm, and posterior visual pathways, to reach the occipital lobe in the cerebral cortex. This is where the visual information is processed.
A visual evoked potential (VEP) is an electrical potential elicited by a sensory stimulus in the volunteer’s visual field. VEPs are recorded with electrodes from the scalp above the visual cortex. Information can be determined from VEPs, such as the conduction velocity from the optic nerve to the occipital cortex and the functioning of the visual pathway.
","boxStyle":"defaultStyle","scrollable":false,"height":0}},"dataVersion":1,"uuid":"5b5af487-e30f-4d58-bbbe-0cc3ea266146","revisionUuid":"4be4c863-3772-4d74-b00a-fc33e14c4bfc"}],"10217":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10217","dataValue":{"default":{"theText":"This pre-lab prep is designed to test your understanding and knowledge before you begin the lab. You will identify key concepts that you will need to understand before you proceed with this module.
","boxStyle":"tipStyle","scrollable":false,"height":0}},"dataVersion":1,"uuid":"497df36f-2cd1-4474-b254-b2a93959be4a","revisionUuid":"2d1600e1-6e76-4336-b03a-625a65bd7e94"}],"10617":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10617","dataValue":{"default":{"theText":"The visual system is the part of the nervous system that allows us to see. When light enters the eye it falls on the retina, which stimulates the receptors (rods and cones) (see the two images below). This results in depolarization and the generation of action potentials in the optic nerve.
","boxStyle":"defaultStyle","scrollable":false,"height":0}},"dataVersion":1,"uuid":"da0a1a65-cb30-4334-93ff-32ab5e979b8c","revisionUuid":"ed2eceb5-e091-488d-93db-1de427fd1036"}],"10215":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10215","dataValue":{"default":{"theText":"The VEP measures the conduction velocity from the optic nerve to the occipital cortex, where it is recorded with electrodes. VEP amplitude is decreased and latency is increased in demyelinating diseases (since myelin acts as an insulator of electrical activity and increases conduction speed).
","checked":true},{"answerDescription":"VEPs provide a measure of the conduction velocity from the optic nerve to the occipital cortex (where it is recorded with electrodes), therefore it is not expected that diseases of the cornea and lens of the eye affect this.
","checked":false},{"answerDescription":"VEPs provide a measure the conduction velocity from the optic nerve to the occipital cortex (where it is recorded with electrodes), therefore it is not expected that diseases of the eye muscles would affect this.
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"},{"answer":"Diseases that affect the cornea and lens.
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"}],"singleAttempt":false,"assessmentType":"instantFeedback","maximumGrade":1.0}},"dataVersion":2,"uuid":"8d2f58a1-0c79-4f95-8392-9a11b826da4f","revisionUuid":"d02458c6-06ed-4712-9deb-63190932abed"}],"1030":[{"pluginKey":"select","typeKey":"modelAnswer","dataKey":"1030","dataValue":{"default":{"answers":[{"answerDescription":"The N135 occurs approximately 135 ms poststimulus and has a negative polarity (a trough).
","checked":false},{"answerDescription":"The P100 occurs approximately 100 ms poststimulus and has a positive polarity (a peak).
","checked":false},{"answerDescription":"Great job! The N75 occurs approximately 75 ms poststimulus and has a negative polarity (a trough).
","checked":true}]}},"dataVersion":1,"uuid":"bfe17048-7482-4495-8ccd-9014f7836005","revisionUuid":"e4107afd-16be-4fc1-a1e7-051d5bd8e71c"},{"pluginKey":"select","typeKey":"content","dataKey":"1030","dataValue":{"default":{"question":"Which of the following is seen in a normal pattern reversal VEP?","multiselect":false,"answers":[{"answer":"A peak at approximately 135 ms.
"},{"answer":"A trough at approximately 100 ms.
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"}],"singleAttempt":false,"assessmentType":"instantFeedback","maximumGrade":1.0}},"dataVersion":2,"uuid":"bfe17048-7482-4495-8ccd-9014f7836005","revisionUuid":"e4107afd-16be-4fc1-a1e7-051d5bd8e71c"}],"10106":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10106","dataValue":{"default":{"theText":"It is possible to examine sensory nervous behavior by presenting repetitive stimuli at fixed time intervals and recording from nerves close to the brain, or from the brain itself. Electrical responses of the nervous system that are time-locked to these sensory stimuli (for example, an electrical excitation, a movement, or other identifiable events) are referred to as "event-related" potentials. The term "evoked potential" is used more specifically to designate the responses of the sensory pathways to sensory or electrical stimuli that are recorded from the central nervous system.
An evoked potential is an electrical potential recorded in response to a stimulus. It is thus different from the spontaneous potentials normally recorded in electroencephalograms (EEGs) or electromyograms (EMGs). The amplitudes of evoked potentials are relatively small, sometimes less than a microvolt (μV). In comparison, EEGs are usually tens of microvolts, and EMGs and ECGs are millivolts (mV) in amplitude. Because these ongoing physiological signals (and noise) are much greater in amplitude than the evoked potentials, signal averaging has to be used to extract the evoked potentials from the "background noise". For this reason, a number of repetitive sweeps need to be recorded. As the evoked potential is time-locked to the stimulus (whereas the stimulus-independent EEG, EMG, ECG, and noise, are not), the noise and other signals are cancelled out by the averaging. Therefore, the evoked potential emerges as averaging progresses. Most of the electrical activity seen is from the movement of action potentials down axons. Therefore, the final tracing is the summed action potentials from many axons which were activated synchronously by the stimulus.
A checkerboard pattern is reversed every minute.
"},{"answer":"A checkerboard pattern is reversed every second or less.
"},{"answer":"A checkerboard pattern is reversed when the subject requests this.
"}],"singleAttempt":false,"assessmentType":"instantFeedback","maximumGrade":1.0}},"dataVersion":2,"uuid":"5592993e-fd83-4dc7-966e-07861024d81a","revisionUuid":"aee008dc-ae9c-4ab9-9dac-051aca75c191"},{"pluginKey":"select","typeKey":"modelAnswer","dataKey":"1029","dataValue":{"default":{"answers":[{"answerDescription":"The time frame for pattern reversal VEP is much faster!
","checked":false},{"answerDescription":"Well done! Often the rate of reversal is 2/s.
","checked":true},{"answerDescription":"The pattern is reversed at set time points so that results can be comparable.
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","multiselect":false,"answers":[{"answer":"Noise is cancelled out.
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","checked":true},{"answerDescription":"Would you expect averaging to exaggerate a series of random events?
","checked":false},{"answerDescription":"Would you expect averaging to have no effect on a series of random events?
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","checked":false},{"answerDescription":"Amplitudes of evoked potentials tend to be low, in the range of microvolts (or sometimes even less than this).
","checked":true},{"answerDescription":"Background noise is much larger than the evoked potential itself. This noise is reduced by averaging the recordings.
","checked":false}]}},"dataVersion":1,"uuid":"d590d53c-a313-4f4e-8024-3f5f07464acf","revisionUuid":"61b7fd65-acf5-41fb-94f4-f2885a908994"},{"pluginKey":"select","typeKey":"content","dataKey":"1027","dataValue":{"default":{"question":"How would you describe the amplitude of an evoked potential?","multiselect":false,"answers":[{"answer":"An evoked potential is relatively large (mV).
"},{"answer":"An evoked potential is relatively tiny (μV).
"},{"answer":"It is much greater than any background "noise" in a recording.
"}],"singleAttempt":false,"assessmentType":"instantFeedback","maximumGrade":1.0}},"dataVersion":2,"uuid":"d590d53c-a313-4f4e-8024-3f5f07464acf","revisionUuid":"61b7fd65-acf5-41fb-94f4-f2885a908994"}],"10625":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10625","dataValue":{"default":{"theText":"Recommended electrode placement for measuring visual evoked potentials
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","multiselect":false,"answers":[{"answer":"An evoked potential arises as a result of evocative thoughts.
"},{"answer":"An evoked potential arises spontaneously.
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","checked":false},{"answerDescription":"Hint: to "evoke" is to "summon" or "call forth", hence the term evoked potential does not imply a spontaneous response.
","checked":false},{"answerDescription":"Good job! A specific stimulus will elicit a stereotyped electrical response (an evoked potential) in the cortex.
","checked":true}]}},"dataVersion":1,"uuid":"f95a8087-73ad-482e-abd7-148186ed0c69","revisionUuid":"5d7853e7-c309-4d32-ba36-c511857cc187"}],"10627":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10627","dataValue":{"default":{"theText":"There are two common ways of generating visual evoked potentials in the lab and clinic: pattern reversal and flash. Pattern reversal testing is primarily used because of its ability to detect minor visual pathway abnormality with much greater sensitivity and accuracy. Flash testing is used when the subject (for example, an infant or child with hyperactivity) cannot focus on the pattern reversal for an extended period of time. Flash VEPs can also be used after a pattern reversal, if the subject did not elicit a response to the pattern reversal.
","boxStyle":"defaultStyle","scrollable":false,"height":0}},"dataVersion":1,"uuid":"56e73aaf-8ac4-4c06-b7ae-aaf6ba7c7111","revisionUuid":"cfc21eba-8be6-4cef-b7d1-86c416682330"}],"10629":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10629","dataValue":{"default":{"theText":"In the flash test, the subject sits about 1 m from the screen and flashes of light are shown every second. This is similar to watching a strobe light.
The generated waveform is different from the one seen during a pattern reversal test. The first peak is typically seen at 30 ms poststimulus, but this waveform is much more variable between subjects. There is also a large positive peak near 100 ms, (but it is not referred to as P100). The nomenclature of the flash VEP is different to that of the pattern reversal VEP so they can be distinguished quickly and easily. The most common way to label flash VEP waveform peaks is to use Roman numerals, but some researchers and clinicians also choose to distinguish between the positive and negative peaks using the nomenclature N1, P1, N2, P2, N3, and P3. These six major peaks occur in the first 250 ms. There are also peaks seen after this time frame, but they are considered rhythmic after-discharge and are not important.
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The human eye
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The process of repeating a measurement multiple times so as to maximise a signal whilst minimising the noise. The process works on the assumption that noise, which is random, will tend to cancel itself out. Whilst the signal is generally constant in time and tends to accumulate.
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The waveform is characterized by three peaks. The first peak occurs around 75 ms poststimulus and has a negative polarity; therefore it is called N75. The second peak occurs around 100 ms and has a positive polarity. It is referred to as P100 and is the primary waveform researchers and clinicians look for. The third peak occurs around 135 ms and has a negative polarity. This peak is called N135. In clinical testing, the clinician usually covers one of the subject's eyes and completes the test using each eye individually.
","boxStyle":"defaultStyle","scrollable":false,"height":0}},"dataVersion":1,"uuid":"f5c1cce8-0f1f-473c-b120-ed129fda75ed","revisionUuid":"d9041d89-1342-4d31-ad6c-4e227db974c0"}],"10633":[{"pluginKey":"kcTextArea","typeKey":"content","dataKey":"10633","dataValue":{"default":{"theText":"P100 is in the normal range if the peak occurs between 90 ms and 117 ms. Women typically have shorter P100 latencies than men, but the difference is so small that the same P100 representation is used for both genders.
Clinically, VEPs are used to assess the visual pathways and can help determine if there are lesions affecting the visual system. Such lesions are caused by stroke, trauma, and tumors. VEPs can also be used to help diagnose multiple sclerosis, an autoimmune disorder which destroys the nerve fibers’ myelin sheaths. This destruction prolongs the VEP latencies significantly as the demyelinated axons can no longer conduct action potentials efficiently, hence delaying the signal.
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The nervous pathways from retina to visual cortex (occipital lobe of cerebral cortex)
In order to properly understand what a VEP is, it is important to first know the basic pathway involved in the generation of a VEP, as well as what constitutes an evoked potential.