Theta rhythm
The theta rhythm is an oscillatory pattern in electroencephalography(EEG)
signals recorded either from inside the brain or from electrodes glued
to the scalp. Two types of theta rhythm have been described. The "hippocampal theta rhythm" is a strong oscillation that can be observed in the hippocampus and other brain structures in numerous species of mammals including rodents, rabbits, dogs, cats, bats, and marsupials. "Cortical theta rhythms" are low-frequency components of scalp EEG, usually recorded from humans.
In rats, the most frequently studied species, theta rhythmicity is
easily observed in the hippocampus, but can also be detected in numerous
other cortical and subcortical brain structures. Hippocampal theta,
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 and other primates,
hippocampal theta is difficult to observe at all.
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. The most popular theories, however, link the theta rhythm
to mechanisms of learning and memory.(Hasselmo, 2005)
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, 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.