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What are wave frequencies in the EEG?

What are wave frequencies in the EEG?


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Here is a typical EEG reading:


(source: frontalcortex.com)

I understand that each row corresponds to the signal read between two sensors on a standard 10-20 (or 10-5) distribution setup (e.g.C3-P3).

What I'm missing here is: what waves are we actually seeing here - alpha, beta, etc.? Or do EEGs not really contain wave types, and instead just show voltage differentials between sensors?


EEGs are often analyzed in the frequency domain, where signals are subjected to spectral analysis, typically by Fast Fourier Transformation, or FFT.

What an FFT basically does is decomposing a signal in the time domain into one in the frequency domain. It does this by decomposing the input signal (any signal, including EEG) into a series of sinusoids. These sinusoids are not present in the original signal; it is a mathematical trick. If you would add all the sinusoids back together, the approximate original signal is restored.

Typically not a single frequency is analyzed, but a band of frequencies, because the frequencies within these bands are associated with similar brain states. The frequency bands that are typically reported on are shown in Table 1, including the brain state they associate with as well as sample EEGs:


Table 1. Typical analyzed EEG frequency bands. Source: Conorrus Somanno

Reference
- Nelson Garcia et al. Device and method for cognitive enhancement of a user (2014). Patent EP 2681729 A1


What are the Different Types of Normal EEG Waves?

The main types of EEG waves are alpha, beta, theta and delta waves. Alpha waves are the most prominent component of the EEG. They are most marked in the parietooccipital area of the scalp when the person is awake, quite and resting with eyes closed. They disappear on opening the eyes and on attentive mind. They disappear entirely during deep sleep. They are fairly regular pattern of waves at a frequency of 8 – 13 / sec and an amplitude of 50 – 100 pV. The mean peak alpha frequency is 10.2 Hz and decreases in old age due to decreased cerebral perfusion leading to decreased cerebral metabolic rate. Frequencies of alpha rhythm are also decreased in conditions like low blood glucose level, low body temperature, low level of adrenal glucocorticoids and high arterial partial pressure of CO2.If there is a consistent difference of 1 Hz or more in alpha frequency between two cerebral hemispheres, the side of lower frequency is likely to be involved in pathological process.

Beta waves have frequency more than 13 cycles per second and may be as high as 25Hz. They have lower voltage than alpha waves. They are frequently recorded from the parietal and frontal region. They are seen during tension or CNS activation. When attentive to external stimulus or thinking hard about anything, the a wave is replaced by (3 rhythm. This transformation is known as EEG arousal. The seniles are found to have significantly less alpha or more beta activity that the young adult group. In infants, there is a fast beta like activity in EEG and occipital rhythm is slow 0.5-2/sec pattern. Barbiturates induce beta activity typically at a frequency of 18-24 Hz.

Theta waves have frequency between 4 – 8 Hz and have larger amplitude than alpha waves. They are seen in parietal and temporal region in children. They are seen in emotional stress in adults particularly during disappointment and frustration, and also occur in many brain disorders. The incidence of transient theta component is about 30% in an alert adult. Amplitude of theta component is greatest at 6 – 9 month (up to 150 pV when eyes are closed) of age. The theta component of EEG often accentuates during crying of children. Theta components persist into adult life in 10 – 15% of normal subject.

Delta waves have frequency of less than 3 Hz. They are seen in deep sleep (stage III and IV NREM) and in infancy. When they occur in awake state, they indicate serious organic brain disease.


The Use of EEG Waves in Brain Activity Monitoring and Measurement

The human brain is an amazing part of the body that’s responsible for very complex functions, such as directing movements, remembering things, controlling emotions, understanding the different signals received by our senses, or controlling our bodily functions. To do all these, brian cells communicate with each other 24/7 through electrical signals. And measuring these signals has become an important tool in the medical field for understanding different neurological conditions. These brain signals are tracked and recorded through an Electroencephalogram (EEG) test, or EEG Waves Test. It is mainly used to detect seizures, monitor, or diagnose any problems in the human brain such as anxiety or even create therapies to address different neurological challenges.

Using EEG Waves to Measure Electrical Activities of the Brain

Despite EEG’s daunting name, knowing the basics of it is surprisingly easy. EEG devices, like the Neeuro SenzeBand, are used to record brain frequencies. In essence, being able to record these EEG Waves can provide you with a visual representation of what is going on in your brain.

Represented by 5 types of EEG waves, each has its importance in brain activity analysis and is triggered by age, brain status:

  1. Gamma waves are responsible for cognitive functioning, learning, memory, and information processing
  2. Beta waves are involved in conscious thought and logical thinking and tend to have a stimulating effect.
  3. Alpha waves are most prominent in occipital derivations and attenuated by eye-opening.
  4. Theta waves are classified as a quiet activity and may appear normally during relaxed wakefulness.
  5. Delta waves are associated with the deepest levels of relaxation and restorative, healing sleep.

Image from Phakkharawat Sittiprapaporn. Read their full academic paper here.

The Importance of EEG Waves and EEG Tests

EEG waves are used to interpret and record brain processes, monitor the brain’s current state, whether it is active or awake, or when it is drowsy or has only limited activities. The EEG waves help experts analyze brain processes, determine possible causes of underlying symptoms caused by a disruption in the brain, measure the capability of a brain, and more.

EEG tests for the brain are readily available in medical institutions for each age bracket. EEG for the brain can diagnose two stages of dementia, seizures, and now there is an EEG device that is found useful for ADHD symptom management. EEG tests can determine the ADHD stage to create a more effective program to help cure it.

However, these tests can be costly for many or they can also be too difficult to interpret. Some people just want simple tools that help them understand their brain, whether they are being attentive or relaxed while doing an activity.

Record your EEG Waves Anywhere

The Neeuro SenzeBand is an EEG device that’s made available to the public. Apart from showing EEG frequency waves, the Neuro SenzeBand also interprets these brain waves into more meaningful data for ordinary users. The Neeuro SenzeBand safely measures brain signals and activities using dry electrodes, which are proven safe and efficient in capturing brain signals. It has been tested with the help of medical professionals to ensure that the data is accurate and of medical grade.

Combining EEG Wearables with Apps Can Help Improve the Quality of Life

One popular use of the Neeuro SenzeBand is its ability to help people calm down and sleep better when paired with its stress management and relaxation app called Galini. It allows users to measure, manage, and ultimately lower stress levels via exercises for Listening, Breathing, and Moving.

Real-time feedback from the SenzeBand enables the Galini app to adapt the exercises according to the mental state of the individual to achieve the best results. Galini can also help unlock mindfulness and improve the user’s focus.


What are wave frequencies in the EEG? - Biology

Abnormal waveforms seen on an electroencephalogram (EEG) recording include epileptiform and non-epileptiform abnormalities. In order to identify abnormal waveforms indicative of disease on an EEG, the reader should have a basic understanding of the normal EEG pattern in various physiological states in children and adults. The electroencephalographer is expected to have the significant skills to recognize artifacts, and also have a thorough understanding of normal benign variants. This activity reviews the abnormal waveforms in EEG recordings to help review these abnormalities for the clinical provider and improve patient outcomes.

  • Identify various epileptiform abnormalities noted on EEG recordings.
  • Outline the specific electrographic features of epileptiform abnormalities noted on EEG recordings.
  • Describe non-epileptiform abnormalities noted on EEG recordings.
  • Review the clinical significance of epileptiform abnormalities noted on EEG recordings.

Introduction

Electroencephalography (EEG) was first used in humans by Hans Berger in 1924. The first report was published in 1929. It is a tracing of voltage fluctuations versus time recorded from multiple electrodes placed over the scalp in a specific pattern to sample different cortical regions. It represents fluctuating dendritic potentials from superficial cortical layers, which are recorded in an organized array pattern and require voltage amplification to be captured. Deep electrical activity of the brain is not well sampled in an EEG using extracranial electrode monitoring. 

Abnormal waveforms seen in an EEG recording include epileptiform and non-epileptiform abnormalities. In order to identify abnormal waveforms in EEG, the reader should have a basic understanding of the normal EEG pattern in various physiological states in children and adults. The electroencephalographer is expected to have the significant skills to recognize artifacts, and also an understanding of normal, benign variants. This article reviews the abnormal waveforms in EEG recordings.

Function

EEG has many potential uses:

  • To distinguish epileptic seizures from psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders, and migraine variants
  • To differentiate encephalopathy from psychiatric syndromes like catatonia
  • To provide ancillary brain death testing
  • To determine whether to wean anti-epileptic medications
  • To characterize seizures to determine the most appropriate anti-epileptic medication
  • To localize the region of the brain from which a seizure originates for workup of possible epilepsy surgery

Issues of Concern

Even normal EEG waveforms can be considered potentially abnormal, depending upon various factors. For example, alpha waves are seen over the posterior head regions in a normal awake person and considered as the posterior background rhythm. However, in certain comatose states, there can be diffuse alpha activity (alpha comma) and may be considered pathognomonic. Delta waves can be seen in drowsiness and also in very young children however, the appearance of focal delta activity can be abnormal (see below). Beta activity is present in the frontal regions of the brain and can spread posteriorly in early sleep. Focal beta activity sometimes seen in structural lesions and also in various epilepsies (generalized fast activity/GFA). Medications like sedatives (phenobarbital, benzodiazepines) commonly cause diffuse beta activity.

Triphasic waves: Triphasic waves were initially described in 1950 by Foley, and in 1955 Bickford and Butt gave it the name. Triphasic waves were first believed to be pathognomic of hepatic encephalopathy. However, these are nonspecific and can be seen in any metabolic encephalopathy. They are high amplitude sharp waves, with the duration of each phase longer than the next. They are sharply contoured with three phases. The first phase is always negative, hence the name triphasic waves. Triphasic waves are seen diffusely with bifrontal predominance and are synchronous. They are not seen in an awake state. They are seen in patients with altered levels of consciousness. It is hypothesized that they occur due to structural or metabolic abnormalities at the thalamocortical levels due to the changes in the thalamocortical relays.[1][2][3] 

Interictal Epileptiform Discharges (IED)

Interictal epileptiform discharge is an abnormal synchronous electrical discharge generated by a group of neurons in the region of the epileptic focus.[4] They represent the epileptic focus in patients with seizures. They have a low sensitivity in routine 30 minute EEG recording, and the yield increases with repeat EEG and prolonged EEG recordings. The presence of IED in a routine EEG in children with a new-onset seizure is 18% to 56%, while in adults, it is 12% to 50%.[5] Though uncommon, they can occur in healthy persons without a history of seizures.[6] IEDs can be subdivided into spikes or sharps. 

  • Spike and wave: Spikes are very short in duration, with a sharp-pointed peak duration of 20 to 70 milliseconds. A spike is followed by a wave component, and this is generated by GABA-b mediated currents.[7] 
  • Sharps: Sharps are longer in duration than a spike and last 70 to 200 milliseconds. 

The following patterns of interictal epileptiform discharges may be seen:

  • 3 Hz and spike-wave: These are typical for absence seizures but can also occur in other types of generalized seizures. The waking background EEG activity is normal. The spike-and-wave is a bi-synchronous, symmetric discharge of sudden onset and resolution with a frequency of 3.5 Hz to 4 Hz at the onset, slowing to 2.5 Hz to 3 Hz at resolution. The greatest amplitude is at the superior frontal electrodes. The EEG discharges are reactive and inhibited by eye-opening and alertness. Hyperventilation and hypoglycemia readily activate them. While they are felt to be subclinical, response testing may demonstrate a subtle decline in maximal alertness.[8] These occur secondary to thalamocortical oscillations, which is the same mechanism that results in sleep spindles.[9]
  • Centro-temporal spikes/ Rolandic spikes: These are seen in benign focal epilepsy of childhood with centrotemporal spikes (BECTS). Epileptic spikes characterized by horizontal dipoles are common and usually have maximal negativity in the centrotemporal area and positivity in the frontal area. The EEG discharges may be unilateral, bilateral, or have shifting laterality and often asynchronous between the hemispheres. Hyperventilation and photic stimulation do not affect the EEG discharges, drowsiness and sleep activate these spikes.[10] More than 1 seizure focus may be noted, and occasionally, the spike shifts its location toward or away from the centrotemporal area.[11] Seizures are usually brief focal and also secondarily generalized tonic-clonic seizures and seen in sleep, and infrequently during wakefulness. 
  • Epileptic encephalopathy with continuous spike-and-wave during sleep (CSWS): Continuous spike and wave activity is seen during sleep. This can be seen in many different seizure subtypes and epilepsy syndromes. It can be caused by structural abnormalities of the brain, genetic abnormalities, and metabolic derangements.[12]
  • Slow spike and waves: These bilaterally synchronous discharges occur in the symptomatic generalized epilepsies and are the typical EEG feature of children with Lennox&ndashGastaut syndrome(LGS). The frequency of these discharges is commonly in the range of 1 Hz to 2.5 Hz. Slow spike-and-wave may evolve from a previously normal EEG or patterns of hypsarrhythmia (seen in infantile spasms) or multiple independent sharp-wave foci. The waking background shows generalized slowing. There can be augmentation in sleep to electrical status epilepticus (ESES).[13] The spikes have an amplitude emphasis in the frontal and temporal regions.[14]
  • Poly spike and waves: A complex of repetitive spikes is noted, followed by a wave component. These are seen in generalized epilepsy and less commonly in focal epilepsy. Generalized polyspikes and waves are commonly seen in myoclonic epilepsy. Examples of myoclonic epilepsy include Juvenile myoclonic epilepsy and progressive myoclonic epilepsy. Polyspike and wave discharges have a frequency ranging from 3.5 Hz to 5 Hz and termed fast spikes and waves. They show a bifrontal predominance. Myoclonic epilepsy predominantly involves the upper extremities, though it can involve the lower extremities. Photic stimulation often activates these discharges.
  • Generalized spike and waves: Single spike is noted, followed by a wave component. These are seen in primary generalized epilepsy. When they occur in idiopathic generalized epilepsy, they occur with a normal background, and other epileptiform abnormalities are not seen. 
  • Lateralized periodic discharges (LPDs or PLEDs): LPDs are repetitive focal discharges that occur at regular intervals. LPDs can be seen with focal structural lesions (usually acute) and after the resolution of partial-onset status epilepticus.[15] There is no defined morphology for LPDs, and they can be present as spikes, shapes, polyspikes, and waves, etc., Herpes simplex encephalitis is classically described to have temporal LPDs. Other conditions that can cause LPDs are brain infections, tumors, Creutzfeldt-Jacob disease, and other conditions that cause acute brain injuries like subarachnoid hemorrhage, stroke, or traumatic brain injury.[16]
  • Bilateral independent periodic discharges (BIPDs/ BiPLEDs): BIPDs are LPDs that occur from 2 different locations, each from different cerebral hemispheres. The 2 LPDs are independent and not synchronous and may occur at different frequencies. 
  • Generalized periodic discharges (GPDs): GPDs are synchronous, repetitive discharges that occur at regular intervals. The inter-discharge intervals are usually quantifiable. The morphology of each discharge is similar. They can be seen in multiple conditions, including anoxic brain injury, hypothermia, during or after the resolution of status epilepticus, infectious/toxic/metabolic encephalopathy, etc.[17] They occur secondary to disruption of thalamocortical pathways.[18] Prognosis is often guarded, but this is ultimately dependent on the underlying etiology. They can be seen with nonconvulsive status epilepticus, but they do not represent status epilepticus by themselves.   
  • SREDA (Subclinical EEG discharges of adults): This is a rarely seen pattern some consider as a benign variant but is generally considered epileptiform. This has been reported in children. The appearance can mimic an electrographic seizure as there will be a sudden evolution of high voltage generalized fast (5 Hz to 6 Hz)spike and wave activity and can occur in a recurrent pattern.[19]
  • Brief (potentially ictal) rhythmic epileptiform discharges B(i)RDs/ BERDs: This is rare and mostly described in critically ill patients and neonates. The discharges can be sudden runs of sharply contoured theta activity lasting up to 3 seconds. This can be related to epileptogenic foci in refractory epilepsy and also sites of cerebral injury in critically ill patients.[20] 

Non-epileptiform Abnormalities

  • Slowing: Slowing in the EEG indicates cerebral dysfunction. Slowing can be described as 'polymorphic' based upon the shape of waveforms, and 'rhythmic' based upon the frequency. It is generally accepted that polymorphic slowing is seen in structural dysfunction, and rhythmic slowing may be much more indicative of underlying epileptiform dysfunction. Slowing can be either diffuse or focal, depending on the location or extent of the brain involved.[21] 
  • Diffuse slowing: Diffuse slowing indicates global cerebral dysfunction. The slowing can be in the theta or delta ranges. The slowing can be high or low amplitude. Several etiologies can cause diffuse slowing, including sedative medications, metabolic encephalopathy, toxic encephalopathy, cerebral infections like meningoencephalitis, or deep midline brainstem structural lesions.
  • Focal slowing: Focal slowing indicates focal cerebral dysfunction. This can be continuous or intermittent. 
    • Continuous focal slowing is often indicative of structural abnormalities and can be seen in conditions like brain tumors, stroke, traumatic brain injury, intracerebral hemorrhage, etc.,
    • Intermittent focal slowing can be of the following types based on the location of the slowing:
      • Frontal intermittent rhythmic delta activity (FIRDA)
      • Occipital intermittent rhythmic delta activity (OIRDA)
      • Temporal intermittent rhythmic delta activity (TIRDA)

      Other Diffuse or Focal Abnormal Patterns in EEG

      • Electrocerebral inactivity (ECI): In ECI, no detectable EEG activity is noted at a sensitivity of 2 microvolts. Electrocerebral inactivity can be used as a supportive test in the diagnosis of brain death. It is not specific to brain death and can be seen with deep sedation and severe hypothermia and some metabolic disorders. When performing recording as an ancillary test to determine brain death, certain criteria need to be met, which include 30 minutes of good quality EEG, a complete set of scalp electrodes must be used with the interelectrode impedances between 100 to 10,000 Ohms. Interelectrode distance must be at least 10cms.[22] 
      • Burst suppression pattern: Burst suppression is characterized by brief bursts of electrographic activity. The bursts may be sharp waves, spikes, or slow waves. The bursts are seen intermittently in a background of isoelectric EEG. It represents a state of cortical hyperexcitability due to compromised inhibition.[23] They can be seen as a medication effect of sedative drugs, hypothermia, metabolic disorders, and anoxic brain injury from cardiac arrest. Further deepening of coma from burst suppression results in severe low amplitude slowing with no reactivity, the EEG appears relatively flat.[24] Burst suppression is often medically induced in the medical management of refractory status epilepticus. The goal is to keep the bursts to 1 per page or less. Myoclonic jerks may be seen accompanying the bursts in anoxic brain injury.[25]
      • Breach rhythm: This does not in itself mean any electrical or structural abnormality, but rather a focal abnormal morphology and change in voltage seen over areas of cranial or scalp defects. This is related to decreased impedance in capturing the signal from the cortex, where the overlying bone or tissue is lacking.

      Clinical Significance

      Understanding abnormal EEG waveforms and differentiating them from normal EEG variations is very important. A normal EEG does not rule out epilepsy, as the sensitivity of an EEG to identify epilepsy is less than 50%. Further, it is also important to understand that even healthy volunteers may have interictal discharges and other EEG abnormalities. Hence unneeded EEG testing can lead to unnecessary and erroneous diagnoses and cause potential harm from treatments if not interpreted properly.

      Furthermore, abnormalities like breach rhythm (normal rhythm seen with skull defects) can have focal, sharply contoured morphology.[26] While several EEG characteristics differentiate breach rhythm from epileptiform abnormalities, clinical information of a prior craniotomy or magnetic resonance imaging (MRI) abnormalities showing a skull defect can help with accurate interpretation. Hence, the abnormalities noted on the EEG must always be clinically correlated.

      Enhancing Healthcare Team Outcomes

      An interprofessional team approach involving EEG technicians, nurses, and physicians will provide the best care for patients with abnormal EEGs. Education of the caregivers and health professionals managing patients who have an abnormal EEG is important. Adequate training in the interpretation of EEG reports and abnormal waveforms will help the clinical team to provide optimal care for the patient. [Level 5]


      Effects of high-frequency electromagnetic fields on human EEG: a brain mapping study

      Cell phones emitting pulsed high-frequency electromagnetic fields (EMF) may affect the human brain, but there are inconsistent results concerning their effects on electroencephalogram (EEG). We used a 16-channel telemetric electroencephalograph (ExpertTM), to record EEG changes during exposure of human skull to EMF emitted by a mobile phone. Spatial distribution of EMF was especially concentrated around the ipsilateral eye adjacent to the basal surface of the brain. Traditional EEG was full of noises during operation of a cellular phone. Using a telemetric electroencephalograph (ExpertTM) in awake subjects, all the noise was eliminated, and EEG showed interesting changes: after a period of 10-15 s there was no visible change, the spectrum median frequency increased in areas close to antenna after 20-40 s, a slow-wave activity (2.5-6.0 Hz) appeared in the contralateral frontal and temporal areas. These slow waves lasting for about one second repeated every 15-20 s at the same recording electrodes. After turning off the mobile phone, slow-wave activity progressively disappeared local changes such as increased median frequency decreased and disappeared after 15-20 min. We observed similar changes in children, but the slow-waves with higher amplitude appeared earlier in children (10-20 s) than adults, and their frequency was lower (1.0-2.5 Hz) with longer duration and shorter intervals. The results suggested that cellular phones may reversibly influence the human brain, inducing abnormal slow waves in EEG of awake persons.


      Short answer
      Brain waves are not electromagnetic waves.

      Long answer
      Measured brain activity, as you already mentioned, is the result of individual neurons firing. The activity exists, in fact, of two parts. First of all, there are the action potentials (APs). APs are current flow within a neuron from one end to the other. The magnitude of these APs (and the summation of many) is so low however, that it is barely measurable.

      The actual brain activity we can measure is the result of the second way of signal conduction: post-synaptic potentials as a result of neurotransmitters. (Pyramidal) Neurons communicate with each other through neurotransmitters, which are released from multiple synapses and flow to the axon of the next neuron. The release of the neurotransmitters causes a much larger potential difference that is conducted through different tissues (e.g. bones and skin). The activity that we measure with EEG is thus only the result of potential difference of the pyramidal neurons. Due to how electrical fields work, we are only able to measure the neurons oriented in right angles to the surface of the scalp (see the right picture).

      A magnetic field cán also be measured though, but this is in fact the result of the flow in current. If electricity flows through a loop, a magnetic field is generated. Moreover, if there is a magnetic field, electrical current will be generated. This is how MEG works. If there is an electrical current, and you place these loops around the head, the magnetic field will be "caught". Then, in turn, this magnetic field will generated electricity in the MEG recording equipment, thereby recording electrical activity in the brain (See left part of the picture, there are two loops where the magnetic field goes through). The magnetic fields are orthogonal to the electrical fields (look for the Right-hand rule) and neurons that lie parallel to the scalp are more easily measurable. EEG and MEG complement each other thus, and combining them greatly improves localization of activity.

      This is a quick and dirty explanation. For a better one, you may want to read the book of Luck: An Introduction to the Event-Related Potential Technique (2014), which explains it really nicely.

      Short answer
      Brainwaves are typically associated with the electroencephalogram, which is a signal mainly composed of potential differences generated in the superficial layers of the brain. Potential differences represent electric fields and do not represent electromagnetic (EM) radiation. EM radiation is build up of packets of energy (photons). EM radiation types are characterized and classified by their specific wavelengths, but this has nothing to do with brain waves.

      Background
      In addition to Robin Kramer's excellent answer I wish to approach this question from a more terminological approach, namely what are brainwaves?

      Brainwave is a bit of a colloquial term. It is typically associated with the electroencephalogram (EEG). The EEG measures electrical potential differences, typically across the scalp (Fig. 1). This electrical activity emanating from the brain is displayed in the form of brainwaves. There are four categories of these brainwaves. These categories are based on frequency bands. The term frequency bands is a more formal term and refers to the way EEGs are typically analyzed, namely via Fourier transformation. Fourier transformation dissects any time-based signal into a number of well-defined sine waves, each with a characteristic frequency, expressed in cycles per second (i.e., Hz).

      When the brain is aroused and actively engaged in mental activities, it generates beta waves. These beta waves are of relatively low amplitude, and are the fastest of the four different brainwaves (15 to 40 Hz frequency band). Alpha waves (9 - 14 Hz) represent non-arousal, are slower, and higher in amplitude. A person who has completed a task and sits down to rest is often in an alpha state. The next state, theta brainwaves (5 - 8 Hz), are typically of even greater amplitude and slower frequency. This frequency range is normally between 5 and 8 cycles a second. A person who has taken time off from a task and begins to daydream is often in a theta brainwave state. A person who is driving on a freeway, and discovers that they can't recall the last five miles, is often in a theta state induced by the process of freeway driving. The final brainwave state is delta (1.5 - 4 Hz). Here the brainwaves are of the greatest amplitude and slowest frequency. A deep, dreamless sleep is characterized by this frequency band. When we go for a night's sleep, brainwaves typically descend from beta, to alpha, to theta and finally, when we fall asleep, to delta (source: Sci Am, 1997).

      EEG activity is measured via electrodes and these pick up a potential difference, or electric field. An electric field is not electromagnetic (EM), because it is not (necessarily) accompanied by a magnetic component. An electric field is generated everywhere where charge is separated. If no current flows, there is still an electric field, namely a static electric field. Only when current starts to flow a magnetic component is introduced (source: WHO). In the brain, static electric fields may exist, but EEG activity is typically evoked by repetitive, synchronized neural firings. Within the tissue, hence, current flows during action potential generation and hence there is definitely a magnetic component involved, this is measured with a magnetoencephalogram (MEG).

      MEG measures magnetic fields and is typically not analyzed in the form of brainwaves but in the form of brain images (Fig. 2).


      Fig. 2. MEG analysis. source: NYU Cognitive Neurophysiology Lab

      MEG signals are also not EM radiation, but magnetic signals.

      Finally, then what is EM radiation? EM radiation is a form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter. The electric and magnetic fields come at right angles to each other and combined wave moves perpendicular to both magnetic and electric oscillating fields thus the disturbance. Electron radiation is released as photons, which are bundles of light energy that travel at the speed of light as quantized harmonic waves. This energy is then grouped into categories based on its wavelength into the electromagnetic spectrum. These electric and magnetic waves travel perpendicular to each other and have certain characteristics, including amplitude, wavelength, and frequency (Fig. 3).

      Importantly, EM radiation can either act as a wave or a particle, namely a photon. As a wave, it is represented by velocity, wavelength, and frequency. As a particle, EM is represented as a photon, which transports energy. Photons with higher energies produce shorter wavelengths and photons with lower energies produce longer wavelengths.

      If "brain waves" produce a time-varying electric potential as shown on the EEG, then as far as I know electromagnetic waves are present. I was taught that you cannot have a time varying electric potential without creating an electromagnetic wave. You can try browsing wiki explanation https://en.wikipedia.org/wiki/Maxwell%27s_equations, but the main idea is that a time varying electric field cannot exist without the presence of a time-varying magnetic field. I admit I have basically zero background knowledge on brainwaves, however after reading the two previous thorough answers I was left wondering why a brain wave would not fall into the category of electromagnetic waves.

      "An electric field is not electromagnetic (EM), because it is not (necessarily) accompanied by a magnetic component." This is theoretically true for static electric fields, but I think static electric fields are similar to a "vacuum state" in the sense that they don't exist in real life or even if they did it would be really hard to measure without perturbing the system.

      Waves are not static and, therefore, the EEG certainly shows a time-varying electric field.

      Strictly from a point of view in physics, there are only 4 fundamental interactions: gravitation, electromagnetic, weak interaction and strong interaction.

      The weak and strong interactions only exist in sub-atomic, so they won't contribute anything to brainwave. The gravitation interaction, while theoretically affects, is extremely tiny to the point that it can be neglected either. Therefore, everything the brain does is electromagnetic. In fact, every chemical process can also be said to be purely electromagnetic.

      I must emphasize this is strictly a physics point of view, because I know in other fields, like biology or neuroscience, it is impractical to group every form of electromagnetic interaction in one basket. Electric field, magnetic field, radiation, Van de Waals interaction, you name it, are different forms of electromagnetic interaction.

      What can be quite confusing is that in biology or neuroscience, the term electromagnetic can be used for a form of such interaction: the co-existence of electric field and magnetic field. This is why we can say that electric field is not electromagnetic. This is, strictly from a physics point of view, wrong. However, this is just different interpretations of the term, so biologists and neuroscientists can safely use that statement.

      This is an important question for a number of reasons, not the least of which is the pervasive conflation of "brain waves" with EM or radio waves in popular media and even in some articles in Scientific American. The three top-voted answers at this point (June 2019) by Robin Kramer, AliceD, and bobby although apparently inconsistent, are all correct, but lack some detail that can resolve the apparent inconsistency.

      To begin, as Robin states and AliceD implies, Brain waves are NOT electromagnetic (EM) waves brain waves are the term given to the patterns of voltage differences measured between two electrodes connected to the three dimensional extracellular fluid matrix surrounding the brain (as shown beautifully by Robin). This matrix includes the skull and scalp of the subject, and since the skull has a high resistance, the current that eventually makes it to the scalp is quite small and produces a very small voltage as it flows through the somewhat resistive scalp between the two electrodes. During open skull surgery, the EEG recorded from the brain surface is 10-100 time larger as the current does not have to flow out through the skull to reach the electrodes and then back again. These voltage patterns of course go up and down, thus producing "waves" in the EEG record of voltage versus time as AliceD explains.

      This is not the same sense of the term "wave" that is used in physics to describe wave phenomena generally physicists talk about waves as solutions to differential wave equations, including Maxwell's equations. Only in the broadest sense of some possible periodicity of the phenomenon producing ups and downs in a graph of the phenomenon versus time can the commonality of these two senses of the word "wave" be identified. Note, however, that physicist's solutions to wave equations can be quite general, and include any combination of solution functions that take as arguments (ax+bt) and (ax-bt) representing forward and backwards traveling solutions. Hence, a square pulse will solve wave equations, and given that any realistic signal has a Fourier representation, any signal can be said to be comprised of a weighted sum of sine and cosine "waves" as described by AliceD, even if the signal itself is not periodic.

      EM waves are solutions to Maxwell's equations that carry energy through space by means of changing electric and magnetic fields that can travel long distances from where they are launched and are associated with far-field energy. This far-field energy is no longer affected by its source, nor does its fate affect its source. This is different than the energy in the electric and magnetic fields related to the current flow in the extracellular matrix this is called the near-field, and it comprises the motive power that drives the current flow. Attention to details is important here EEGs do not record electric fields, they record differences in potential. Potential is a scalar field with a single numerical value at each point in space and no absolute zero point - hence having to always measure the difference in voltage (potential) between two points and to have connections to the extracellular fluid matrix circuit, whereas the electric field is a vector field with a magnitude and direction at each point in space. The electric field is the gradient of the potential, and this is the direction that the current will flow in isotropic extracellular fluid. Changing the potential at points in the extracellular matrix will change the near-field electric field and thus the three dimensional pattern of current flow and any recorded potential differences. Brain waves are these latter potential differences due to the near-field energy in the electric and magnetic fields, and separate from the far field effects of radiated energy in the form of EM waves.

      Now, bobby points out that changing potential differences representing brain waves imply changing electric fields that, as Maxwell says, produces changing magnetic fields, which, in turn generates a changing electric field, etc - and we're off to the races: an EM wave is launched! Or is it?

      One needs a device called an antenna to transduce a changing voltage/current into and EM wave, and a very basic rule for antennas is that they only start converting significant amounts of energy when the size of the antenna approaches 1/4 the wavelength of the signal being radiated. So let's see how big our antenna would need to be for a 10 Hz alpha wave to be launched out of our scalp. Since EM waves travel at the speed of light, or 300,000,000 m/s, our scalp would have to be 75,000,000 meters in size! I don't have the equations here, but it's pretty obvious that essentially zero energy at 10 Hz is going to be radiated. And if one wanted to pick up that signal, the receiving antenna would have to be equally large! Seventy five Megameters is pretty damn big.

      This is why the EEG electrodes have to touch the scalp or otherwise connect to the actual circuit in which current is flowing rather than than just be placed nearby to pick up radiated EM energy from the brain. And while it's true a number of tricks can be pulled (as is done in cell phones dielectric antennas) to reduce this size by maybe a factor of ten, even for 100Hz or 1000Hz signals, virtually no energy is going to radiate from the scalp, nor will EM waves be picked up and converted into changing potentials on the scalp from the EM milieu around us. Cell phones can be small because they utilize signals in the range of 3 GHz where 1/4 of a wavelength is about 2.5 cm, or an inch.

      So, even though there could be EM waves produced by brain "waves", practically speaking, it doesn't happen, and looking in detail at how EM wave are radiated reveals that the brain "wave" is, in fact, a different phenomenon from any EM wave that it might be associated with or generate.

      Perhaps the most succinct way to pinpoint the difference is to note that EM waves consist of packets of energy propagating through space via self-regenerating changing electric and magnetic fields that have units of volts/meter and amps/meter, while brain "waves" are difference in voltages between two points on the scalp measured in Volts - note that they have different units. With brain "waves", essentially no energy is leaving the scalp and radiating into space because the frequencies are too low and the scalp is far to small to act as an effective antenna to convert them into EM waves.


      Understanding brain waves

      Neurofeedback training is based on the principle of operant conditioning, which involves rewarding an individual for inhibiting certain brain waves and increasing others, depending on their levels of cortical arousal. An audio or visual stimulus is used for reinforcement during most NF training protocols.

      Certain frequencies of brain waves are inhibitory, whilst others are excitatory. This means that the stimulation of certain wave bands may be responsible for characteristics associated with over-arousal (e.g. fidgeting, hyperactivity and feelings of agitation), whilst others lead to features of under-arousal (e.g. poor concentration, spaciness, and day-dreaming)

      As mentioned, different brain waves are associated with different states. Brain waves are measured in Hertz (Hz) cycles per second, and can change across a wide range of variables. When slower brain waves are dominant we can feel sluggish, inattentive and scattered, and can feel depressed or develop insomnia. When higher frequencies abound, we are engaged in critical thinking, hyper-alertness or anxiety, but can also result in nightmares, hyper-vigilance and impulsive behaviour.

      Delta Waves (1-4 Hz) are slow brainwaves, which begin to appear in stage 3 of the sleep-cycle, and by stage 4 dominate almost all EEG activity. At this stage, healing and regeneration are stimulated, and are considered essential for the restorative properties of sleep. An excess of delta waves when a person is awake may result in learning disabilities and ADHD, and make it extremely difficult to focus. It has been found that individuals with various types of brain injuries produce delta waves in waking hours, making it extremely difficult to perform conscious tasks. Sleep walking and talking tend to occur while delta production is high.

      Research suggests that cortical circuits generate delta <1Hz, whereas higher-frequency delta rhythms are an intrinsic property of thalamacortical cells and intracortical network interactions. Importantly, delta may also reflect general neurotransmitter activity, specifically dopamine and acetylcholine. Because delta is active within brain networks that connect the cortex and insula with the hypothalamus and the brainstem, delta is closely involved with the physiological interface between the brain and the body. During delta wave sleep, neurons are globally inhibited by gamma-aminobutyric acid (GABA).

      Theta waves (4-8 Hz) are particularly involved in day-dreaming and sleep. Cortical theta is observed frequently in young children, but in older children and adults, it tends to appear during meditative, drowsy, or sleeping states (but not during the deepest stages of sleep). When we are awake, excess theta levels can result in feeling scattered or day-dreamy, and is commonly reported in ADHD. Too much theta in the left hemisphere is thought to result in lack of organisation, whereas too much theta on the right results in impulsivity. Theta in people with attention disorders is often seen more towards the front of the brain.

      Frontal Midline Theta: Sinusoidal and high in amplitude (1-10 second bursts), generally occurs in response to events (ie. an ERP). This midline theta is associated with opening the sensory gate to the hippocampus for intermediate storage of episodic information. The frequency of frontal midline theta varies from 5-7.5 Hz, with an average of 6Hz. This rhythm is associated with working memory, episodic encoding and retrieval. It also appears during hypnosis and deep meditation. Frontal midline theta is thought to originate from the anterior cingulate. It mainly appears when one is performing a task requiring focused concentration, and its amplitude increases with the task load. It is mainly concentrated around Fz. When anxious and restless, the signal is reduced or even eliminated. When anxiety is medicated, the signal is restored. This suggests that the anterior cingulate cortex is involved in regulating the emotional state from restless anxiety to focused relaxation.

      Hippocampal Theta : Has been found in the posterior cingulate, entorhinal cortex, hypothalamus and amygdala. Often more tonic and diffuse, and elicits and coordinates memory.

      Alpha waves (8-12 Hz) dominate during moments of quiet thought, and similar meditative states. Alpha is considered the “power of now”, being here and in the present of the moment. It is the resting state for the brain, not unlike a car idling at a stoplight. Alpha waves aid overall mental co-ordination, calmness and alertness, mind/body integration and learning. Alpha tends to be highest in the right hemisphere, and too little alpha in the right hemisphere correlates with negative behaviours such as social withdrawal. This is also seen in people with depression, particular with too much alpha frontally. Alpha is involved in active and adequate inhibition of the irrelevant sensory pathways.

      Alpha is related to resource allocation in the cortex, and is produced as a result of a resonance process between the thalamus and the cortex. If we consider the thalamus the gateway to the cortex, alpha can be thought of as the mechanism by which the sensory gate to the cortex can be closed.

      Alpha appears to be closely involved with reticular activation, and participates in binding mechanisms and resource allocation in regards to orientation and task sequences.

      Alpha diminishes during sleep onset, while focusing on tasks, and is also a normal consequence of ageing. When alpha slows and theta increases in frequency, it is often an indicator of pathologically slowed high-amplitude alpha, which is associated with Parkinson’s disease and cognitive decline. This indicates degradation of myelination and cell death in the cortex, and reflects growing metabolic inefficiency.

      After completing a task and given feedback, the high functioning brain shows increased levels of alpha. This is associated with consolidation of the task events, called post reinforcement synchronisation (PRS). This represents and alpha burst in the brain when the brain is consolidating information.

      Beta waves (12-38 Hz) represent our normal waking state of consciousness when attention is directed at cognitive tasks and the outside world. Beta is ‘fast wave‘ activity and dominated when we are alert, attentive and engages in problem-solving, decision making and focussed mental activity. Low beta (12-15 Hz) is thought to be ‘fast idle’, or musing thought, Beta (15-22 Hz) is high-engagement and actively figuring things out, and finally, High Beta (22-38 Hz) is highly complex thought, integrating new experiences, high anxiety or excitement. Continual high frequency processing is not an efficient way to run our brains , and can result in tension and difficulties relaxing, and if present at night, can result in difficulties settling the mind and falling asleep. Beta waves tend to dominate in the left hemisphere, and too much beta on the right can be correlated with mania.

      There are discrepancies regarding how the three levels of beta and gamma divide their territory in the brain. While it is widely agreed that higher beta frequencies are more correlated with arousal, some convincingly suggest that they are mostly a result of muscle artefact. For example, Helleter et al. found that anxiety was highly correlated with elevated right hemisphere beta, and more recent work has found that insomnia is correlated with higher temporal lobe frequencies of beta, and migraines are associated with central high beta.

      Gamma brainwaves have the highest frequencies of any brainwave, oscillating between 30 (ish) to 100 Hz. They are associated with peak concentration and high levels of cognitive functioning. Low levels of gamma acitivity have been linked with learning difficulties, impaired mental processing and limited memory, while high gamma activity is correlated with a high IQ, compassion, excellent memory, and happiness.

      Gamma is currently of limited clinical value, as it is argued that it cannot be effectively measured using current EEG technology, due to muscle contamination. While promising research has suggested that Gamma training can be successfully implemented to enhance intelligence, it will not be of proper clinical use until this issue of technology is resolved.

      Gamma and theta work together to recruit neurons which stimulate local cell column activity. As such, it is associated with cortical processing related to cognitive functions, and is also potentially related to meditative states, although research on this relationship is vague.

      The EEG is recorded from the surface of the head. Measurable surface potentials (microvolts) are produce by neurons in the brain in the top layer of the cortex. The cortex contains the outer information processes of the brain. The main EEG signals are produced by pyramidal cells as these are oriented in a manner than produced measurable voltage. The brains electrical sources are dipoles, which is a charged entity that had a positive and negative side (similar to a battery). The EEG is a epiphenomenon (side effect) of the brain’s activity, but is not a direct measure of information processing such as a recording of action potentials.

      Common brain imaging techniques such as MRIs & CAT scans are built to measure brain structure. An EEG measures brain activity. A QEEG brain map enables us to see areas of the brain where there is too little or too much activity, and areas that are not coordinating their activity the ways in which they should. These maps are created by comparing the values to a normative database (ie. the scores are compared to people the same age). An EEG uses surface sensors to detect the brain’s electrical patterns (known as brainwaves). A qEEG (Quantitiave EEG) can identify not only brainwaves, their amplitude, location and whether these patterns are typical or dysregulated, but also Coherence (quality of communication between regions), and Phase (thinking speed). These are all crucial patterns involved in optimum mental functioning.

      Absolute Power: How much brain power is available?

      Absolute Power represents the electrical power in each band of EEG and it is compared to all other individuals in the database, which determines whether the results are typical or atypical. The voltage produced by the brain is measured at each of the sites. It aids in determining whether enough brainpower within a particular frequency range is present at each recording site. The colour coding represents the intensity of the difference between the client and the normative group. The scale ranges from negative to positive values (measured in Z-scores/values).

      Relative Power: Who is in charge here?

      Relative Power can be understood as the power in one frequency band compared to all other bands, or the distributed total amount of power at each site. It is compared to all other similar measurements of other individuals in the database to determine whether a particular frequency is overpowering other vital brain frequencies, or if the power is low.

      Amplitude Asymmetry: The Brain’s Balancing Act

      Amplitude Asymmetry shows whether the brain waves between various parts of the brain are balanced by telling us the difference in power between the left side and the right side of the brain. Excessive activity may indicate an over-firing of brain cells, while insufficient activity may suggest brain cells are not firing sufficiently to maintain proper brain function. Both will lead to inefficient brain function.

      Coherence: How efficient is my brain’s ability to communicate with itself?

      This tells us about the brain’s efficiency, or lack thereof, to connect/disconnect with different parts of the brain and shows how much one part of the brain is communicating with the other part. Different parts of the brain must share information in order for us to make sense of our complex world and execute decisions. Good coherence readings are said to show that a brain is flexible.

      Areas of high coherence show over-communication and suggest that the brain has become overly dependent on those centres instead of efficiently processing and executing information. This often results in poor day to day performance. Areas of low coherence show under-communication. In both cases, plasticity and function suffer. The more extreme the coherence readings, the more disordered the brain. If coherence is extremely high (measured with Z scores), there is limited regional communication, division of labour, connectivity and regional cooperation. If coherence is extremely low, there is limited to no connection occurring between regions. It may be worthwhile to note that noise from volume conduction and thalamic input may confound the validity of the connectivity measure hence it needs to be interpreted with caution.

      Phase Lag: Is the brain’s electrical energy moving at the optimal speed for adequate to superior performance?

      This is the measurement for the energy from one part of the brain arriving at another area at just the right moment to perform a specific task. High phase means the signals arrive too early. Low phase means the signals arrive too late. In both cases, the brain is not operating at optimal efficiency. Phase is particularly meaningful in relationships to coherence measures.


      Beta waves were discovered and named by the German psychiatrist Hans Berger, who invented electroencephalography (EEG) in 1924, as a method of recording electrical brain activity from the human scalp. Berger termed the larger amplitude, slower frequency waves that appeared over the posterior scalp when the subject's eye were closed alpha waves. The smaller amplitude, faster frequency waves that replaced alpha waves when the subject opened his or her eyes were then termed beta waves. [2]

      Low-amplitude beta waves with multiple and varying frequencies are often associated with active, busy or anxious thinking and active concentration. [3]

      Over the motor cortex, beta waves are associated with the muscle contractions that happen in isotonic movements and are suppressed prior to and during movement changes. [4] Bursts of beta activity are associated with a strengthening of sensory feedback in static motor control and reduced when there is movement change. [5] Beta activity is increased when movement has to be resisted or voluntarily suppressed. [6] The artificial induction of increased beta waves over the motor cortex by a form of electrical stimulation called Transcranial alternating-current stimulation consistent with its link to isotonic contraction produces a slowing of motor movements. [7]

      Investigations of reward feedback have revealed two distinct beta components a high beta (low gamma) component [8] and low beta component. [9] In association with unexpected gains, the high beta component is more profound when receiving an unexpected outcome, with a low probability. [10] However the low beta component is said to be related to the omission of gains, when gains are expected. [9]

      Beta waves are often considered indicative of inhibitory cortical transmission mediated by gamma aminobutyric acid (GABA), the principal inhibitory neurotransmitter of the mammalian nervous system. Benzodiazepines, drugs that modulate GABAA receptors, induce beta waves in EEG recordings from humans [11] and rats. [12] Spontaneous beta waves are also observed diffusely in scalp EEG recordings from children with duplication 15q11.2-q13.1 syndrome (Dup15q) who have duplications of GABAA receptor subunit genes GABRA5, GABRB3, and GABRG3. [13] Similarly, children with Angelman syndrome with deletions of the same GABAA receptor subunit genes feature diminished beta amplitude. [14] Thus, beta waves are likely biomarkers of GABAergic dysfunction, especially in neurodevelopmental disorders caused by 15q deletions/duplications.


      Drowsiness

      This patient is awake but very drowsy. Recall that drowsiness is marked by diffuse attenuation and possibly mild slowing of the background, but you can still see a clear PDR in the posterior leads here. In the frontal and frontopolar regions, opposing slow undulations are seen in polarity, indicative of lateral roving eye movements. This occurs because the cornea is positively charged, and thus when you look to the right, the right eye's cornea gets closer to F8 and it sees a positive charge at the same time, the left cornea moves away from F7 and thus it sees a negative charge. So, lateral eye movements lead to a frontal positive charge on the side to which you're looking, and a negative charge on the opposite side.


      THE EPILEPSIES 3

      RACHEL THORNTON , LOUIS LEMIEUX , in Blue Books of Neurology , 2009

      DATA QUALITY

      If EEG is acquired by standard methods in the MRI scanner, in the majority of cases the signal becomes uninterpretable during image acquisition due to the presence of repetitive artifact waveforms superimposed on the physiological signal due to the switching of gradients during EPI sequence acquisition 2,9 ( Figure 6-3 ). The first attempts at recording EEG inside MR scanners revealed the presence of significant pulse artifacts (often referred to as the BCG [ballistocardiogram] artifact). 77 This effect has been shown to be common across subjects. 78 The pulse artifact amplitude can reach 50 μV (at 1.5T) and may resemble epileptic spikes introducing an obvious complication in the study of epilepsy. The precise mechanism through which the circulatory system exposed to a strong magnetic field gives rise to these artifacts remains uncertain, but it is thought to represent a combination of the motion of the electrodes and leads (induction) and the Hall effect (voltage induced by flow of conducting blood in proximity of electrodes). 79 This effect is proportional to the scanner's main field strength.

      In addition to artifacts on EEG, interaction between EEG and MRI systems results in artifacts caused by electrodes and leads on the images acquired, 77 and this has affected the choice of EEG component materials. 5,80,81 Radio-frequency fields radiating from the EEG recording equipment placed in the vicinity of the scanner can cause severe image degradation and may therefore require shielding.

      Various EEG-fMRI data acquisition strategies have been employed to minimize the impact of EEG artifacts.

      Interleaved EEG-fMRI. 80,82 This method requires a gap in the acquisition of fMRI where EEG features can be reliably observed and is most useful for studying evoked responses or slow variations in brain activity.

      EEG-triggered fMRI. This involves the identification of EEG events online to trigger a burst of fMRI scanning and is of particular relevance to epilepsy research. 2,5,8,13

      Continuous EEG-fMRI acquisition, which requires specially designed amplifiers (with adequate dynamic range, bandwidth, and sampling rate), enables image acquisition artifact correction on- or off-line. 9,83



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