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EEG Basics: Waveform Morphology

Updated: Jun 24


EEG

Libenson (2024) observes that we can describe EEG activity by its location, voltage (amplitude), shape (morphology), frequency, rhythmicity, continuity, and the extent of the wave observed in specific clinical states such as wakefulness, drowsiness, and sleep. This post covers EEG morphology, which provides critical insights into the brain's electrical activity, distinguishing normal patterns from abnormalities.

Waveform Morphology

A wave is a change in the potential difference between two EEG electrodes. Waveform and morphology refer to the signal's shape generated by oscillating potential differences. Neurofeedback professionals examine the raw waveform's morphology before considering the filtered and quantified EEG (Demos, 2019). EEG activity means a single wave or series of waves (Fisch, 1999).


We can distinguish between regular and irregular activity. A regular or monomorphic series of waves is rhythmic with the same frequency and morphology. Rhythmic waves that resemble sine waves are called sinusoidal, as in the graphic below.


sinusoidal

Regular waves may be arch-shaped and resemble wickets. They are waves or complexes with an approximately constant period and a relatively uniform appearance (Kane et al., 2017). Graphic © eegatlas-online.com.

wickets

Saw-tooth waves resemble asymmetrical triangles. They comprise short bursts of rhythmic sharp waves at 4-7 cycles per second, often reaching high amplitudes of up to 125 µV (Kane et al., 2017). Graphic © eegatlas-online.com.

saw-tooth wave


Irregular waves continuously change shape and duration. They comprise EEG waves and complexes with irregular periods and/or uneven shapes or contours (Kane et al., 2017). The graphic below shows runs of irregular high-amplitude delta waves.


delta


EEG waves may be monophasic or polyphasic. Monophasic waves possess a single upward or downward deflection. Two polyphasic waveforms containing waveforms with two or more elements are diphasic and triphasic waves. Diphasic waves have two elements--one positive and one negative. Triphasic waveforms contain three elements with alternating directions.

waves

A transient is a single wave or series of waves distinct from background EEG activity.


transient


Sharp transients are waves with steep peaks that are not produced by epilepsy.


sharp transient


In contrast, epileptiform activity consists of spikes and sharp waves. An epileptiform spike has a sharp appearance and lasts 20-70 ms. Epileptiform activity comprises transients distinguishable from background activity with a characteristic morphology typically, but neither exclusively nor invariably, found in interictal EEGs of people with epilepsy (Kane et al., 2017). Epileptiform patterns have to fulfill at least 4 of the following 6 criteria: (1) Di- or tri-phasic waves with sharp or spiky morphology (i.e. pointed peak). (2) Different wave-duration than the ongoing background activity, either shorter or longer. (3) Asymmetry of the waveform: a sharply rising ascending phase and a more slowly decaying descending phase, or vice versa. (4) The transient is followed by an associated slow after-wave. (5) The background activity surrounding epileptiform discharges is disrupted by the presence of the epileptiform discharges. (6) Distribution of the negative and positive potentials on the scalp suggests a signal source in the brain, corresponding to a radial, oblique, or tangential orientation of the source (see dipole). This is best assessed by inspecting voltage maps constructed using common-average reference.


Epileptiform graphic © eegatlas-online.com.


epilepsy


Less steeply shaped sharp waves last 70-200 ms. They are epileptiform transients distinctly separated from the background activity, with varying amplitudes (Kane et al., 2017). It features a pointed peak on a conventional time scale, typically with a steeper ascending phase compared to the descending phase. The main component is generally negative relative to other areas and may be followed by a slow wave of the same polarity.


A complex is a sequence of two or more waveforms with a characteristic composite morphology (Kane et al., 2017). When recurring, they display a fairly consistent form and are distinguishable from the background activity. See the K-complex in the EEG record below. Graphic © eegatlas-online.com.


complex


Summary


Libenson (2024) provides a comprehensive overview of the various ways EEG activity can be described, emphasizing the significance of understanding waveform morphology. EEG activity, which captures the brain's electrical activity, is crucial for distinguishing normal brain function from abnormalities. The key characteristics of EEG activity include location, voltage (amplitude), shape (morphology), frequency, rhythmicity, continuity, and the extent of the wave observed in clinical states such as wakefulness, drowsiness, and sleep. This detailed analysis helps identify different patterns vital for clinical diagnoses and neurofeedback training.


Waveform morphology is particularly important in EEG analysis. A wave in EEG terms is defined by the change in potential difference between two electrodes. This signal's shape, generated by oscillating potential differences, is referred to as its morphology. Neurofeedback professionals closely examine the raw waveform's morphology before considering the filtered and quantified EEG (Demos, 2019). Regular or monomorphic waves are rhythmic and maintain a consistent frequency and shape, often resembling sinusoidal waves. Other regular waves may be arch-shaped, similar to wickets, and display a uniform appearance. Conversely, irregular waves continuously change shape and duration, comprising EEG waves and complexes with uneven contours (Kane et al., 2017).


Further, EEG waves can be categorized based on their shapes and phase. Monophasic waves have a single upward or downward deflection, while polyphasic waves, including diphasic and triphasic waves, contain two or more elements with alternating directions. Transients are distinct waves or series of waves that stand out from the background EEG activity. Sharp transients have steep peaks but are not necessarily associated with epilepsy. In contrast, epileptiform activity consists of spikes and sharp waves, often found in the interictal EEGs of individuals with epilepsy. These epileptiform patterns must meet specific criteria, such as having a sharp morphology, differing wave duration, asymmetry, an associated slow wave, and a characteristic distribution of potentials on the scalp (Kane et al., 2017). Understanding these detailed characteristics and patterns in EEG morphology is essential for accurate diagnosis and effective neurofeedback therapy.



Glossary


EEG activity: a single wave or series of waves.


epileptiform activity: spikes and sharp waves associated with seizure disorders.


irregular waves: EEG waves that continuously change shape and duration.


monomorphic waves: series of waves that are rhythmic with the same frequency and morphology.


morphology: the shape of the signal generated by oscillating potential differences.


polyphasic waves: waveforms with two or more elements; diphasic and triphasic waves.


regular waves: rhythmic waves with the same frequency and morphology.


saw-tooth waves: waves that resemble asymmetrical triangles.


sharp transients: waves with steep peaks that are not produced by epilepsy.


sharp waves: steeply shaped waves with 70-200 ms duration.


Sheer rhythm: a 38-42 Hz rhythm associated with learning and problem-solving, meditation, mental acuity, and peak brain function in children and adults.


sinusoidal: rhythmic waves that resemble sine waves.


spike: a waveform with a sharp appearance that lasts 20-70 ms.


spike-and-wave complexes: spikes that are succeeded by slow waves.


transient: a single wave or series of waves distinct from background EEG activity.


triphasic wave: a wave that consists of three elements with alternating directions.


wave: a plot of voltage using a bipolar (positive/negative) scale with zero in the middle; the analog form of the signal in which voltage continuously varies.


waveform: the shape of the signal that is generated by oscillating potential differences between two electrodes.



References


Demos, J. N. (2019). Getting started with neurofeedback (2nd ed.). W. W. Norton & Company.


Fisch, B. J. (1999). Fisch and Spehlmann's EEG primer (3rd ed.). Elsevier. Kane, N., Acharya, J., Benickzy, S., Caboclo, L., Finnigan, S., Kaplan, P. W., Shibasaki, H., Pressler, R., & van Putten, M. J. A. M. (2017). A revised glossary of terms most commonly used by clinical electroencephalographers and updated proposal for the report format of the EEG findings. Revision 2017. Clinical Neurophysiology Practice, 2, 170–185. https://doi.org/10.1016/j.cnp.2017.07.002


Libenson, M. H. (2024). Practical approach to electroencephalography. Elsevier.


Thompson, M., & Thompson, L. (2015). The biofeedback book: An introduction to basic concepts in applied psychophysiology (2nd ed.). Association for Applied Psychophysiology and Biofeedback.





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