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Theta waves are neural oscillations that generate the theta rhythm, a neural oscillatory pattern observed in electroencephalography (EEG) signals, which are recorded either from macro-electrodes inside the brain or from electrodes glued to the scalp. These oscillations can also be observed in local field potential (LFP) signals, which are recorded with micro-electrodes embedded deep within the brain.
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The term theta rhythm can be used to designate two different phenomenon. The "hippocampal theta rhythm" is a strong oscillation recorded in hippocampal LFP or EEG signals. It has been observed in numerous species of mammals including rodents, rabbits, dogs, cats, bats and marsupials. In rats, they usually have a frequency range of 6-10 Hz and appear when the animal is engaged in active motor behavior such as walking or exploratory sniffing, and also during REM sleep. Theta waves with a lower frequency range, usually around 6–7 Hz, are sometimes observed when a rat is motionless but alert.
"Cortical theta rhythms" are low-frequency components of scalp EEG, usually recorded from humans. Cortical theta rhythms can be quantified using Quantitative electroencephalography (qEEG) and freely available toolboxes, such as, EEGLAB or the Neurophysiological Biomarker Toolbox (NBT).
In rats, the most frequently studied species, theta wave rhythmicity is easily observed in the hippocampus, but can also be detected in numerous other cortical and subcortical brain structures. Hippocampal theta waves, with a frequency range of 6–10 Hz, appears when a rat is engaged in active motor behavior such as walking or exploratory sniffing, and also during REM sleep. Theta waves with a lower frequency range, usually around 6–7 Hz, are sometimes observed when a rat is motionless but alert. When a rat is eating, grooming, or sleeping, the hippocampal EEG usually shows a non-rhythmic pattern known as Large irregular activity or LIA. The hippocampal theta rhythm depends critically on projections from the medial septal area, which in turn receives input from the hypothalamus and several brainstem areas. Hippocampal theta rhythms in other species differ in some respects from those in rats. In cats and rabbits, the frequency range is lower (around 4–6 Hz), and theta is less strongly associated with movement than in rats. In bats, theta appears in short bursts associated with echolocation. In humans, hippocampal theta rhythm has been observed and linked to memory formation[1][2] and navigation.[3]
The function of the hippocampal theta rhythm is not clearly understood. Green and Arduini, in the first major study of this phenomenon, noted that hippocampal theta usually occurs together with desynchronized EEG in the neocortex, and proposed that it is related to arousal. Vanderwolf and his colleagues, noting the strong relationship between theta and motor behavior, have argued that it is related to sensorimotor processing. Another school, led by John O'Keefe, have suggested that theta is part of the mechanism animals use to keep track of their location within the environment. Another theory links the theta rhythm to mechanisms of learning and memory (Hasselmo, 2005). These different theories have since been combined, as it has been shown that the firing patterns can support both navigation and memory[4]
Cortical theta rhythms observed in human scalp EEG are a different phenomenon, with no clear relationship to the hippocampus. In human EEG studies, the term theta refers to frequency components in the 4–7 Hz range, regardless of their source. Cortical theta is observed frequently in young children. In older children and adults, it tends to appear during meditative, drowsy, hypnotic or sleeping states, but not during the deepest stages of sleep. Several types of brain pathology can give rise to abnormally strong or persistent cortical theta waves.