Answer
Answer
Myogenic potentials are the most common artifacts (see images below). Frontalis and temporalis muscles (eg, clenching of jaw muscles) are common causes. Generally, the potentials generated in the muscles are of shorter duration than those generated in the brain and are identified easily on the basis of duration, morphology, and rate of firing (ie, frequency). Particular patterns of electromyogram (EMG) artifacts can occur in some movement disorders. Essential tremor and Parkinson disease can produce rhythmic 4- to 6-Hz sinusoidal artifacts that may mimic cerebral activity.
Another disorder that can produce repetitive muscle artifacts is hemifacial spasm. The photomyoclonic response is a special type of EMG artifact that occurs during intermittent photic stimulation. Some subjects contract the frontalis and orbicularis muscles. These contractions occur approximately 50-60 milliseconds after each flash, disappear after eye opening and use of paralyzers, are located mostly frontally, and have no concomitant EEG changes.
Electromyogram (muscle) artifact best observed in the left temporal region. ECG artifact also is present, best observed in the posterior region.
Electromyogram (muscle) artifact. These waveforms represent motor unit potentials as typically observed on needle electrode examination during electromyogram, with a frequency of 20-100 Hz. Distribution varies, and in this case it is more prominent on the left side. Such artifact can be diminished by the judicious use of the high-frequency filter. (This sample has the default setting of high-frequency filter 70 Hz.)
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Electromyogram (muscle) artifact best observed in the left temporal region. ECG artifact also is present, best observed in the posterior region.
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Electromyogram (muscle) artifact. These waveforms represent motor unit potentials as typically observed on needle electrode examination during electromyogram, with a frequency of 20-100 Hz. Distribution varies, and in this case it is more prominent on the left side. Such artifact can be diminished by the judicious use of the high-frequency filter. (This sample has the default setting of high-frequency filter 70 Hz.)
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Eye movements such as these usually are observed in frontal electrodes and not further posteriorly than the midtemporal region. The phase reversals at lateral frontal electrodes F7 and F8 are of opposite polarity, indicating lateral eye movements. Because the cornea is charged positively and the retina negatively, the side of the positivity indicates the direction of eye movement. Thus, the first one here is to the right.
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Left frontal artifact in the fourth second. This is not limited to a single electrode and has the morphology of an eye movement, but it is unilateral. This is an eye movement in a patient who has a glass right eye.
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Regular (periodic) slow waves best observed at midtemporal and posterior temporal electrodes T4-T6 and T3-T5. These clearly are related to ECG. The duration and morphology are those of pulse artifact, but as demonstrated by the marker, no delay occurs between the ECG and the artifact. Thus, this is an ECG artifact with broad QRS complexes.
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Sweat artifact. This is characterized by very low-frequency (here, 0.25- to 0.5-Hz) oscillations. The distribution here (midtemporal electrode T3 and occipital electrode O1) suggests sweat on the left side. Note that morphology and frequency are also consistent with slow rolling eye movements, but distribution is not.
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Electrode artifact at frontal pole electrode Fp1. The duration is too short ("narrow") for any cerebral potential, and the distribution is limited to a single electrode (Fp1). In general, activity that affects a single electrode (ie, without the expected drop off and activity at neighboring electrodes or "plausible field") should be considered an artifact until proven otherwise.
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Electrode artifact at occipital electrode O1. The morphology is very unusual for any cerebral waveform, and the distribution is limited to a single electrode. In general, activity that affects a single electrode (ie, without the expected drop off and activity at neighboring electrodes or "plausible field") should be considered an artifact until proven otherwise.
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Electrode artifact at frontal electrode F3. This should not be misinterpreted as a spike. This sharply contoured transient clearly occurs at only one electrode, as confirmed on the referential montage.
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Electrode (impedance) artifact at parietal electrode P3. Initially, a slow artifact is followed by a more abrupt one at the seventh second. This commonly is referred to as an electrode pop. Note again the unusual morphology of the sharp component and that it is at a single electrode. Also note an eye blink in the third second and slight electromyogram artifact in the frontal regions in the first 2 seconds.
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Just as electrode artifacts can simulate interictal spikes, they also can mimic an ictal pattern. This rhythmic artifact may be mistaken for an electrographic seizure or subclinical rhythmic epileptiform discharges of adults (SREDA). However, this is confined to a single electrode (posterior temporal electrode T6), as can be confirmed on a referential montage. This artifact often is confirmed by the presence of other definite electrode pops in the same electrode.
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Ground recording artifact. This is a somewhat less common electrode artifact, also related to accidentally high impedance. The high impedance at posterior temporal electrode T6 results in this electrode recording from the ground on the forehead, thus picking up eye movements (which normally should not be observed at T6).
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