Atlas of Neonatal Electroencephalography, 3rd Edition

Chapter 3

Artifacts

The differentiation of true brain electrical activity from extraneous artifacts is critical to the interpretation of the neonatal electroencephalogram (EEG). Traditionally, the sources of artifacts are considered in four broad categories: environment, instrumentation-patient interface, instrumentation, and physiologic potentials of noncerebral origin (Brittenham, 1990; Kellaway and Crawley, 1964; Saunders, 1979, 1985; Scher, 1985).

The identification of artifacts in a given EEG is a two-step process. The first occurs during the actual recording and is dependent on the electroneurodiagnostic technologist's (ENDT's) recognition of possible artifact sources. With this recognition, it is the ENDT's responsibility to isolate the source and resolve the problem. If this proves impossible, the ENDT will make appropriate notations on the record and on the accompanying log to characterize the activity and suggest the source. The second step of artifact identification occurs during interpretation of the EEG. Because artifacts can mimic true brain-generated waveforms (Tables 3-1, 3-2, 3-3, 3-4 and 3-5), the challenge lies with the clinical neurophysiologist to make the accurate and appropriate distinctions.

ENVIRONMENT

Although the EEG laboratory is a relatively controlled electrical environment, the neonatal intensive care unit is relatively uncontrolled. This is because of the large number of instruments used to monitor or care for infants, including phototherapy lights, ultrasound instrumentation, pumps for intravascular infusions in the umbilical and scalp vessels, and extracorporeal membrane oxygenation (ECMO) pumps (Figs. 3-1, 3-2 and 3-3). Electrical artifacts due to external currents from these instruments may appear in all EEG channels to the same degree or may appear focally. When all of the electrode impedances are equal, the more likely the environmental current will be expressed in all channels; unequal impedances will cause the current to be expressed to a greater degree or exclusively in those channels with the highest impedance.

Additional environmental artifacts may be created or enhanced by factors such as capacitatively induced potentials from electrode wires that sway or electrostatic potentials resulting from movement of personnel around the recording area. Malfunctions and/or improper operation of equipment such as monitors connected to the patient also can result in artifactual signal induced into the EEG machine.

New ventilators recently introduced to neonatal care allow a very fast rate of respiration, and these may induce relatively high frequency electrical artifacts. Thus rate alone cannot be used to exclude potentials suspected of being generated by a ventilator.

Radio transmitters, including those used by dispatch personnel or cellular telephones, can sometimes produce artifactual signals in the EEG tracing, especially if these devices are operated within a few feet of the patient.

RECORDING INSTRUMENTATION-PATIENT INTERFACE

The interface between the recording instrument and the patient is at the electrode site. Inadequate or unstable contact between the electrode surface and the skin may result in a sudden change in the junction potential and/or impedance that can produce extraneous potentials in affected channels. These may appear as single or repetitive rapid, spike-like waves with an abrupt upward initial phase (the so-called

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electrode “pop”) (Figs. 3-4 and 3-5). In addition, asymmetry of the site of placement of homologous electrodes may result in significant voltage asymmetries in the EEG.

TABLE 3-1. Sources of artifacts that can mimic EEG focal asymmetries

Instrument-patient interface

 

Asymmetric electrode placement

Instrumentation

 

Settings (analog and digital)

 

Pen damping (analog only)

Physiological

 

Asymmetric scalp edema

 

Skull defects

TABLE 3-2. Sources of artifacts that can mimic focal slow activity that is either random or rhythmic

Instrument-patient interface

 

Inadequate electrode contact

 

Head positioning

 

Spontaneous or passive body, limb or head movements

 

Pulse

 

Respiration

Physiological

 

Endogenous electrical potentials

   

Tongue movements

   

Eye movements

TABLE 3-3. Sources of artifacts that can mimic sharp waves or spikes that are either random or rhythmic

Instrument-patient interface

 

Inadequate electrode contact (“pop”)

 

Spontaneous or passive movements

   

Sucking, burping, hiccupping

Instrumentation

 

Loosely adjusted pens (analog)

 

Physiological

 

Electrocardiogram

 

Electrooculogram

 

Facial electromyogram

Environment

 

Infusion pumps

 

Ventilator

 

Extracorporeal membrane oxygenation pump

 

Radio transmitters

TABLE 3-4. Sources of artifacts that can mimic generalized activity that is either paroxysmal or sustained

Instrumentation-patient interface

 

Spontaneous and passive movements

   

Rocking, patting

Instrumentation

 

Swaying of electrode leads

Physiologic

 

Sweating

Environment

 

60-Hz interference from adjacent instruments

The electrode interface also may be altered by the degree to which the infant may perspire. Diffuse sweating may result in long-duration potentials that initially appear as generalized or regional slow activity (Figs. 3-6 and 3-7). Very slow potentials may occur because of changes caused by alterations in surface electrolyte compositions—these potentials are similar to the galvanic skin response.

Movement of the head against the bed due to respirations or other body movements may produce sharp and/or slow potentials arising from that particular electrode (Figs. 3-8 and 3-9). Pulse also may cause a recorded artifact by production of movement in a region adjacent to an electrode site. The head also may be moved by mechanical devices such as a ventilator or ECMO pump (Figs. 3-10 and 3-11). This is owing to a mechanical, or ballistic, movement induced by the instrument—a cause of artifact from these devices different from electrical interference described earlier.

Other body movements also may alter the patient-electrode interface and result in artifacts. These include limb movements that may be random, purposeful, or associated with clinical seizures and other limb or body movements (Figs. 3-12, 3-13, 3-14 and 3-15).

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In addition to the movements that may be caused by the infant, the infant may be moved or manipulated during comforting, feeding, medical procedures, and in the course of treatments (Figs. 3-16, 3-17 and 3-18). Movements created to comfort the infant, such as rocking and patting the infant, may be particularly troublesome.

TABLE 3-5. Sources of artifact that can mimic generalized voltage depression

Instrumentation

 

Instrumentation settings

Physiological

 

Diffuse scalp edema

RECORDING INSTRUMENTATION

The recording instrument itself can be a source of artifacts. These may be the result of malfunction at any recording level. Analog-type EEG instruments may be subject to pen misalignment and excessive damping. Digital recordings may have problems relating to malfunction of the operating system. The potential for human error also occurs in the use of either recording device. Settings for EEG channels may not be uniform, electrodes may not be correctly plugged in, and montages may not be accurately selected.

NONCEREBRAL PHYSIOLOGIC POTENTIALS

Alterations in Electrical Properties of Scalp or Skull

Differences may be found in impedance and volume-conduction properties over various regions of the scalp because of scalp edema. The edema may be the result of transit through the birth canal, more significant birth or other trauma, placement of intravenous lines with or without extravasation of fluid, the placement of a ventriculoperitoneal shunt, or the presence of a surgical wound. Diffuse edema may lead to a pattern of background activity that is low in amplitude in all regions. Regional or asymmetric edema may lead to a pattern of focal depression, suggesting a focal lesion if the edema is not noted. Conductive properties may be altered because of the absence of underlying skull, typically (although rarely) in the case of cranial surgery. A skull defect creates a preferential pathway for electrical current, resulting in an increased amplitude of EEG activity over the affected region.

Vital Signs Monitoring

Heart rate and respirations are important sources of artifact on EEG. The electrocardiogram (EKG) in an infant may appear as a contaminant in one, some, or all of the EEG channels (Figs. 3-19 and 3-20). It may be constant or intermittent. Respirations also may appear as artifacts, whether they are spontaneous or driven by a ventilator. These artifacts may be unilateral or bilateral, depending on body and head position.

Movements

Some movements by the infant can produce a number of endogenous electrical potentials that can be reflected in EEG channels. These movements include oral-buccal-lingual movements, such as sucking and tongue thrusting (i.e., glossopharyngeal potential); paroxysmal pharyngeal movements, such as hiccupping, burping [i.e., pharyngeal muscles and diaphragmatic electromyogram (EMG)], and jaw tremor (Figs. 3-21, 3-22, 3-23, 3-24, 3-25, 3-26 and 3-27); ocular movements, such as eye deviations, blinking, and repetitive eye opening and closure [i.e., electrooculogram (EOG)] (Figs. 3-28, 3-29, 3-30 and 3-31); and chewing or other facial movements (i.e., temporalis, frontalis, or other facial muscle EMG) (Figs. 3-32, 3-33, 3-34, 3-35, 3-36 and 3-37).

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LIST OF FIGURES

Environmental Interference

Fig. 3-1. Electrical interference due to mechanical device

Fig. 3-2. Electrical interference due to intensive care unit instrumentation

Fig. 3-3. Periodic electrical interference due to mechanical device

Alternations of Electrode Impedance

Fig. 3-4. Electrode “pop”

Fig. 3-5. Irregular, sustained waveforms due to unstable electrode

Fig. 3-6. Intermittent, high-amplitude, long-duration potentials due to sweat

Fig. 3-7. Sustained, high-amplitude, long-duration potentials due to sweat

Induced by Movements

Fig. 3-8. High-amplitude, long-duration, asynchronous potential due to head movement

Fig. 3-9. Moderately high-amplitude, short-duration, repetitive potentials due to head movement associated with sobbing

Fig. 3-10. Slow periodic waves due to movements induced by mechanical ventilation

Fig. 3-11. Periodic waves due to movement induced by ECMO pump

Fig. 3-12. High-amplitude generalized spike-like artifact associated with generalized myoclonic movement

Fig. 3-13. Rhythmic potentials due to tremors

Fig. 3-14. Rhythmic sharp potentials due to tremors

Fig. 3-15. Rhythmic slow activity due to sneezing

Fig. 3-16. Rhythmic polymorphic activity induced by patting

Fig. 3-17. Rhythmic slow activity induced by patting

Fig. 3-18. Rhythmic sharp wave activity induced by patting

Endogenous Noncerebral Potentials

Fig. 3-19. EKG

Fig. 3-20. EKG altered with change in head position

Fig. 3-21. Isolated, focal EMG potential due to sucking

Fig. 3-22. Wide reflection of EMG potentials due to sucking

Fig. 3-23. Periodic EMG potentials due to sucking

Fig. 3-24. Periodic bursts of EMG potentials due to sucking

Fig. 3-25. Multifocal, repetitive EMG potentials due to jaw tremor

Fig. 3-26. Rhythmic slow activity associated with sucking

Fig. 3-27. Rhythmic slow activity and EMG associated with sucking

Fig. 3-28. Slow activity due to eye movements

Fig. 3-29. Intermittent sharp activity due to eye movements

Fig. 3-30. Semirhythmic slow activity due to eye movements

Fig. 3-31. Slow activity associated with repetitive eye opening and closure

Fig. 3-32. Random EMG potentials from facial muscles

Fig. 3-33. Multifocal and complex EMG potentials from facial muscles

Fig. 3-34. Brief rhythmic EMG potentials from facial muscles compared with occipital sharp waves

Fig. 3-35. Build-up of sustained EMG in temporal regions

Fig. 3-36. Build-up of sustained EMG in frontal region

Fig. 3-37. Build-up of sustained, slow EMG in temporal region associated with arousal

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FIG. 3-1. Electrical interference due to mechanical device. Electrical interference is present in all leads when an infusion pump for intravenous fluids is activated at the bedside. The interference is modified in this instance by the use of a 60-Hz filter. The EEG background activity is depressed and undifferentiated in this term infant.

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FIG. 3-2. Electrical interference due to intensive care unit (ICU) instrumentation. Electrical artifact is present in channels involving the C3 electrode, which has relatively high impedance compared with others. This 40-week CA infant was cared for in an ICU with monitors, ventilator, and infusion pump. The background EEG activity is undifferentiated.

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FIG. 3-3. Periodic electrical interference due to mechanical device. Periodic bursts of electrical interference are due to an extracorporeal membrane oxygenation pump used to support this term infant, whose background EEG is depressed and undifferentiated. The bursts correlate with rotations of the pump.

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FIG. 3-4. Electrode “pop.” An unstable electrode at C4 has resulted in a characteristic waveform with a steep initial component, sharp morphology, and more gradual return to baseline. This occurred in a 36-week CA infant whose background EEG is characterized by a suppression-burst pattern.

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FIG. 3-5. Irregular, sustained waveforms due to unstable electrode. The Pz electrode in this recording has become unstable, resulting in irregular, sustained, low-voltage, relatively fast activity. Some of the waveforms have a nonphysiologic angular and square morphology. This occurred in a 42-week CA infant whose background EEG activity was normal.

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FIG. 3-6. Intermittent, high-amplitude, long-duration potentials due to sweat. Low-voltage, long-duration waveforms are present, predominantly in the right central region, because of excessive sweating in this 40-week CA infant with normal background EEG activity.

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FIG. 3-7. Sustained, high-amplitude, long-duration potentials due to sweat. The high-voltage, long-duration waveforms are predominantly in frontal and central regions and are sustained. This activity is due to excessive sweating of the infant. The electrocardiogram also is reflected in leads from the left central region and there is electromyographic activity in the anterior leads. This infant is 40-week CA with a background EEG that is depressed and undifferentiated.

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FIG. 3-8. High-amplitude, long-duration, asynchronous potential due to head movement. Spontaneous head movement has resulted in moderate-amplitude slow waves appearing in leads from both hemispheres, with some asynchronous components on the two sides in this 41-week CA infant with normal background EEG activity.

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FIG. 3-9. Moderately high-amplitude, short-duration, repetitive potentials due to head movement associated with sobbing. This infant experienced a brief sobbing episode characterized by shuddering that involved respiration and truncal muscles as well as head, which was turned to the right. The rapid, rhythmic movement of the head resulted in brief rhythmic theta-like activity in the right central region in this 40-week CA infant with normal background EEG activity. The simultaneous body movements are indicated by waveforms in the electromyogram channel.

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FIG. 3-10. Slow periodic waves due to movements induced by mechanical ventilation. Mechanical ventilation may produce movements of the body and head, which in turn may result in EEG artifact, as in this recording with periodic, very slow waveforms that are lateralized to left and are aligned with deflections in the respiration channel. The background EEG in this 40-week CA infant is depressed and undifferentiated.

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FIG. 3-11. Periodic waves due to movement induced by extracorporeal membrane oxygenation (ECMO) pump. Low-voltage, periodic waves primarily in the right occipital region are due to head movement induced by the action of the ECMO instrument used to support this 36-week CA infant. The background activity is characterized by a suppression-burst pattern.

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FIG. 3-12. High-amplitude generalized spike-like artifact associated with generalized myoclonic movement. A generalized myoclonic event in this 38-week CA infant resulted in a high-amplitude generalized spike-like waveform. Movement recorded as electromyographic (EMG) activity precedes the waveforms from scalp electrodes. There is sustained low-voltage EMG in the anterior leads.

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FIG. 3-13. Rhythmic potentials due to tremors. High-voltage rhythmic theta activity is present with variable localization and is preceded and followed by high-voltage, very slow activity. This is due to this 44-week CA infant's tremulousness or jitteriness, preceded and followed by slow random movements of the body. Sustained electromyographic activity is superimposed on the background EEG activity.

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FIG. 3-14. Rhythmic sharp potentials due to tremor. Rhythmic theta activity with a sharp morphology is present in the right central region because of the tremors and jitteriness of the 40-week CA infant. Low-voltage electromyographic activity appears primarily in the temporal regions.

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FIG. 3-15. Rhythmic slow activity due to sneezing. High-voltage, rhythmic slow activity is present in the left temporal region associated with repetitive sneezing or coughing of this 39-week CA infant with normal EEG background activity. Superimposed electromyographic (EMG) activity occurs in the EEG channels, and movement also is suggested by activity in the EMG channel.

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FIG. 3-16. Rhythmic polymorphic activity induced by patting. Polymorphic rhythmic activity in the left temporal region is induced by patting or comforting this 40-week CA infant with a normal background EEG.

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FIG. 3-17. Rhythmic slow activity induced by patting. High-voltage, rhythmic, monomorphic, delta-like activity is induced by patting this 40-week CA infant with normal EEG background activity.

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FIG. 3-18. Rhythmic sharp wave activity induced by patting. An apparent build-up of rhythmic sharp waves is present in the occipital regions bilaterally, with some lateralization to leads on the left, associated with patting of this 40-week CA infant. The background EEG activity is normal.

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FIG. 3-19. Electrocardiogram (EKG). The EKG is present in several channels in this 39-week CA infant with suppression-burst EEG background activity.

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FIG. 3-20. Electrocardiogram (EKG) altered with change in head position. The EKG is present more prominently in the early portion of this recording, with high-amplitude spike-like activity in leads from the left occipital and temporal regions. The amplitude of the waves is reduced, and they are less prominent in the temporal region when the infant's head is moved to the midline by the technician. This movement is marked by the generalized high-amplitude slow activity in the middle of this segment. The infant is 40 weeks CA, and the background EEG activity is depressed and undifferentiated.

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FIG. 3-21. Isolated, focal electromyogram (EMG) potential due to sucking. Isolated EMG potentials in the right temporal region result from sucking motions of this 40-week CA infant. The background EEG is within the range of normal variation.

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FIG. 3-22. Wide reflection of electromyogram (EMG) potentials due to sucking. The EMG due to sucking arises from the left temporal region but has variable amplitude and a wide reflection on the two sides in this 40-week CA infant with normal EEG background activity.

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FIG. 3-23. Periodic electromyogram (EMG) potentials due to sucking. This EMG activity due to sucking has a complex morphology. It is periodic, primarily in the left temporal region. There is normal EEG background activity in this 40-week CA infant.

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FIG. 3-24. Periodic bursts of electromyogram (EMG) potentials due to sucking. Brief bursts of repetitive EMG potentials occur periodically in the temporal regions bilaterally in this 41-week CA infant with normal EEG background activity. Low voltage, sustained EMG is present in anterior leads.

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FIG. 3-25. Multifocal, repetitive electromyogram (EMG) potentials due to jaw tremor. Bursts of repetitive EMG due to tremulousness of the jaw are present independently in the left and right temporal regions in this 42-week CA infant with normal EEG background activity.

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FIG. 3-26. Rhythmic slow activity associated with sucking. Rhythmic slow activity is associated with sucking in this 43-week CA infant. Although associated with sucking, this activity is not produced by endogenous potentials, but rather by movement of the head that occurs in conjunction with the vigorous sucking movements. The background EEG activity is within the range of normal variation. The electromyogram channel reflects increased activity associated with sucking.

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FIG. 3-27. Rhythmic slow activity and electromyogram (EMG) associated with sucking. Rhythmic EMG potentials are found in the temporal regions, and rhythmic slow activity is induced by head movement in the central regions associated with sucking in this 40-week CA infant with normal EEG background activity.

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FIG. 3-28. Slow activity due to eye movements. A brief run of slow activity in the right frontal region aligns with the activity recorded in the electroculogram channel. The infant is 40 weeks CA with normal EEG background activity.

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FIG. 3-29. Intermittent sharp activity due to eye movements. Rhythmic, slow, sharp waves are present in the frontal regions bilaterally, higher in amplitude on the left, aligned with the recorded electrooculogram and occurring in association with clinically observed nystagmus. The infant is 40 weeks CA with normal EEG background activity.

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FIG. 3-30. Semirhythmic slow activity due to eye movements. Slow, semirhythmic activity, with a variable geometric morphology, is present in the frontal regions associated with eye movements that also are represented in the electrooculogram channel in the 40-week CA infant with normal EEG background activity.

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FIG. 3-31. Slow activity associated with repetitive eye opening and closure. High-voltage, slow activity is present in the frontal regions bilaterally associated with rhythmic eye opening and closure. This activity also is present in the electrooculogram channel. The EEG background activity is within the range of normal variation in this 39-week CA infant.

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FIG. 3-32. Random electromyogram (EMG) potentials from facial muscles. Random low-voltage EMG potentials are present in the left and right temporal regions and, rarely, in the right frontal region, associated with twitches of facial muscles. The background activity is depressed and undifferentiated in this 41-week CA infant.

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FIG. 3-33. Multifocal and complex electromyogram (EMG) potentials from facial muscles. Spike-like potentials are present in the left and right temporal regions, and a more complex burst of EMG activity is present later in the left temporal region in this 39-week CA infant with normal EEG background activity.

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FIG. 3-34. Brief rhythmic electromyogram (EMG) potentials from facial muscles compared with occipital sharp waves. A burst of EMG activity is present in the right temporal region in the middle portion of this sample. In the latter portion of the recording, a burst of spike and sharp-wave activity of cerebral origin is present in the right occipital region (see Chapter 5) in this 39-week CA infant with otherwise normal EEG background activity.

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FIG. 3-35. Build-up of sustained electromyogram (EMG) activity in temporal regions. A build-up of sustained EMG activity is seen in the temporal regions bilaterally, higher in amplitude on the left. The background EEG is depressed and undifferentiated in the 41-week CA infant.

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FIG. 3-36. Build-up of sustained electromyographic (EMG) activity in frontal region. A brief build-up of EMG activity is seen primarily in the right frontal region in this 37-week CA infant with normal EEG background activity.

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FIG. 3-37. Build-up of sustained, slow electromyographic (EMG) activity in temporal region associated with arousal. A burst of EMG activity is seen in the right temporal region with a relatively slow rate of firing. This activity occurs with the spontaneous arousal of the infant, associated on EEG with generalized voltage attenuation (see Chapter 4). The infant is 40 weeks CA with normal EEG background activity but with multifocal sharp waves of cerebral origin (see Chapter 5).