Source: https://www.medicine.mcgill.ca/physio/vlab/biomed_signals/eeg_n.htm
The electroencephalogram (EEG) is a recording of the electrical activity of the brain from the scalp. The recorded waveforms reflect the cortical electrical activity.
Signal intensity
EEG activity is quite small, measured in microvolts (mV).
Signal frequency
The main frequencies of the human EEG waves are:
- Delta: has a frequency of 3 Hz or below. It tends to be the highest in amplitude and the slowest waves. It is normal as the dominant rhythm in infants up to one year and in stages 3 and 4 of sleep. It may occur focally with subcortical lesions and in general distribution with diffuse lesions, metabolic encephalopathy hydrocephalus or deep midline lesions. It is usually most prominent frontally in adults (e.g. FIRDA - Frontal Intermittent Rhythmic Delta) and posteriorly in children e.g. OIRDA - Occipital Intermittent Rhythmic Delta).
- Theta: has a frequency of 3.5 to 7.5 Hz and is classified as "slow" activity. It is perfectly normal in children up to 13 years and in sleep but abnormal in awake adults. It can be seen as a manifestation of focal subcortical lesions; it can also be seen in generalized distribution in diffuse disorders such as metabolic encephalopathy or some instances of hydrocephalus.
- Alpha: has a frequency between 7.5 and 13 Hz. Is usually best seen in the posterior regions of the head on each side, being higher in amplitude on the dominant side. It appears when closing the eyes and relaxing, and disappears when opening the eyes or alerting by any mechanism (thinking, calculating). It is the major rhythm seen in normal relaxed adults. It is present during most of life especially after the thirteenth year.
- Beta: beta activity is "fast" activity. It has a frequency of 14 and greater Hz. It is usually seen on both sides in symmetrical distribution and is most evident frontally. It is accentuated by sedative-hypnotic drugs especially the benzodiazepines and the barbiturates. It may be absent or reduced in areas of cortical damage. It is generally regarded as a normal rhythm. It is the dominant rhythm in patients who are alert or anxious or have their eyes open.
Frequency
Frequency refers to rhythmic repetitive activity (in Hz). The frequency of EEG activity can have different properties including:
- Rhythmic. EEG activity consisting in waves of approximately constant frequency.
- Arrhythmic. EEG activity in which no stable rhythms are present.
- Dysrhythmic. Rhythms and/or patterns of EEG activity that characteristically appear in patient groups or rarely or seen in healthy subjects.
Voltage
Voltage refers to the average voltage or peak voltage of EEG activity. Values are dependent, in part, on the recording technique. Descriptive terms associated with EEG voltage include:
- Attenuation (synonyms: suppression, depression). Reduction of amplitude of EEG activity resulting from decreased voltage. When activity is attenuated by stimulation, it is said to have been "blocked" or to show "blocking".
- Hypersynchrony. Seen as an increase in voltage and regularity of rhythmic activity, or within the alpha, beta, or theta range. The term implies an increase in the number of neural elements contributing to the rhythm. (Note: term is used in interpretative sense but as a descriptor of change in the EEG).
- Paroxysmal. Activity that emerges from background with a rapid onset, reaching (usually) quite high voltage and ending with an abrupt return to lower voltage activity. Though the term does not directly imply abnormality, much abnormal activity is paroxysmal.
Morphology
Morphology refers to the shape of the waveform. The shape of a wave or an EEG pattern is determined by the frequencies that combine to make up the waveform and by their phase and voltage relationships. Wave patterns can be described as being:
- Monomorphic. Distinct EEG activity appearing to be composed of one dominant activity.
- Polymorphic. distinct EEG activity composed of multiple frequencies that combine to form a complex waveform.
- Sinusoidal. Waves resembling sine waves. Monomorphic activity usually is sinusoidal.
- Transient. An isolated wave or pattern that is distinctly different from background activity.
1 Spike: a transient with a pointed peak and a duration from 20 to under 70 msec.
2 Sharp wave: a transient with a pointed peak and duration of 70-200 msec.
Synchrony
Synchrony refers to the simultaneous appearance of rhythmic or morphologically distinct patterns over different regions of the head, either on the same side (unilateral) or both sides (bilateral).
Periodicity
Periodicity refers to the distribution of patterns or elements in time (e.g., the appearance of a particular EEG activity at more or less regular intervals). The activity may be generalized, focal or lateralized.
Source: http://www.psych.westminster.edu/psybio/BN/Labs/Brainwaves.htm
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Alpha
The four basic rhythms have been associated with various states. In general, the alpha rhythm is the prominent EEG wave pattern of an adult who is awake but relaxed with eyes closed. Each region of the brain had a characteristic alpha rhythm but alpha waves of the greatest amplitude are recorded from the occipital and parietal regions of the cerebral cortex. In general, amplitudes of alpha waves diminish when subjects open their eyes and are attentive to external stimuli although some subjects trained in relaxation techniques can maintain high alpha amplitudes even with their eyes open. -
Beta
Beta rhythms occur in individuals who are alert and attentive to external stimuli or exert specific mental effort, or paradoxically, beta rhythms also occur during deep sleep, REM (Rapid Eye Movement) sleep when the eyes switch back and forth. This does not mean that there is less electrical activity, rather that the “positive” and “negative” activities are starting to counterbalance so that the sum of the electrical activity is less. Thus, instead of getting the wave-like synchronized pattern of alpha waves, desynchronization or alpha block occurs. So, the beta wave represents arousal of the cortex to a higher state of alertness or tension. It may also be associated with “remembering” or retrieving memories. -
Delta and Theta
Delta and theta rhythms are low-frequency EEG patterns that increase during sleep in the normal adult. As people move from lighter to deeper stages of sleep (prior to REM sleep), the occurrence of alpha waves diminish and is gradually replaced by the lower frequency theta and then delta frequency rhythms.
Although delta and theta rhythms are generally prominent during sleep, there are cases when delta and theta rhythms are recorded from individuals who are awake. For example, theta waves will occur for brief intervals during emotional responses to frustrating events or situations. Delta waves may increase during difficult mental activities requiring concentration. In general, the occurrence and amplitudes of delta and theta rhythms are highly variable within and between individuals.
Source: Emotiv
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Theta (4 - 8 Hz)
Theta activity is seen in drowsiness, arousal and often during meditation. Dominant Theta activity is associated with relaxed, meditative, and creative states, memory recall and 'flow' states. -
Alpha (8 - 12 Hz)
Alpha waves are the default 'relaxed and alert' mode of the brain. High Alpha values are often observed in the rear channels (occipital and parietal sensors) when the eyes are closed, indicating that the visual processing system is not currently engaged but is available and waiting for input. Similar conditions apply to other parts of the brain. High Alpha levels appear in the frontal lobes during relaxation and are suppressed when other activities take place.
It is quite common in EEG analysis to compare the Alpha suppression between different regions in order to determine the functional areas which are currently in use. For example, linguistic processing tends to depress Alpha activity in the left frontal lobe, while abstract spatial thinking can suppress Alpha in the right frontal lobe. Trained meditators often produce much higher levels of Alpha activity during normal activities, especially in the frontal lobes.
Similar rhythms in the motor cortex (called Mu-rhythms, around the same frequency range) indicate muscle relaxation. Suppression of Mu-rhythms in the motor cortex in specific regions corresponds with activation of particular muscle groups. For example, clenching your right fist is directly associated with a dip in Mu-rhythm near the F3 sensor on the left side of the head. -
Beta (12 - 25Hz)
Beta activity of multiple and varying frequencies is often associated with active, task-oriented, busy or anxious thinking and active concentration. -
Gamma (greater than 25Hz)
Gamma rhythms occur when different populations of neurons network together to carry out demanding cognitive or motor functions. Generally Gamma waves are observed in the frontal regions when fast, coupled processing is required, such as in fight/flight mode and when task switching during multi-tasking. In task switching, Gamma bursts are clearly evident when the current task is archived to short term memory and a new task is retrieved for 'concurrent' processing.