The alpha rhythm is typically predominant in the awake-resting state, either relaxed and comfortable (desynchronised tonic slow alpha) ( 3) or concentrated ( 4) (phasic desynchronised alpha), as well as in the case of alpha coma ( 1). This means that, roughly speaking, the bigger the alpha power, the less active the brain ( 1). It is important to stress that alpha power and brain activity are inversely related. Third, the tau rhythm originates from the auditory cortex. Second, there is the mu rhythm, which is dominant in central electrodes and is related to the somatosensory cortex and movement. First, the posterior alpha rhythm, originating from the parietooccipital cortex, is dependent on the alertness and attentional factors. The alpha rhythm is typically seen in at least three different types, which are different in topography and function. A very specific type of theta referred to as “frontal midline theta” can be observed during various tasks such as mental calculation, working memory, error processing, and meditation ( 2). The theta rhythm is usually observed in drowsiness and the states of low-level alertness. Another category of very high (30–40 Hz) frequencies is referred to as gamma waves ( 1).Įvery type of a brainwave can roughly indicate distinct states of the brain although this connection varies strongly depending on various factors.įor example, very low frequencies (delta activity) are dominant during deep sleep, coma, and anaesthesia. Absolute brainwave power describes the power of certain frequency components present in the time period whereas relative brainwave power reflects the role of a specific frequency in the context of all frequencies, e.g., the percentage of the absolute power of a certain frequency in the whole power spectrum.ĭepending on the frequency, waves can be categorised as delta (1–4 Hz), theta (4–8 Hz), alpha (8–13 Hz) (which is sometimes divided into alpha 1 (8–10 Hz) and alpha 2 (11–13 Hz)) and beta (more than 13 Hz). This unit derives from the spectral analysis of the electroencephalogram. Another important unit describing brainwaves in an EEG is brainwave spectral power expressed in Volts-squared per Hz (V 2/Hz). They are defined in terms of three parameters: amplitude, frequency, and phase. Voltage traces recorded by EEG electrodes are described as waves. Also, although brain activity is spontaneous and does not depend on specific sensory stimuli, it might be easily altered by them, and these changes can also be seen in the findings of EEGs ( 1). They reflect and identify baseline brain electrical activity as well as the changes that happen during distinct deep sleep stages, anaesthesia, epileptic seizures, and cognitive events. Is widely used to evaluate the synaptic actions in the cerebral cortex, which are moderately to strongly connected to the state of the brain. An electroencephalogram (EEG) is a recording of the oscillations of electric potentials in the brain, sometimes referred to as a “window of the mind”.
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