Theta rhythm

Theta waves generate the theta rhythm, a neural oscillatory pattern that can be seen on an electroencephalogram (EEG), recorded either from inside the brain or from electrodes attached 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. Theta rhythms can be quantified using quantitative electroencephalography (qEEG) using freely available toolboxes, such as, EEGLAB or the Neurophysiological Biomarker Toolbox (NBT). Theta waves generate the theta rhythm, a neural oscillatory pattern that can be seen on an electroencephalogram (EEG), recorded either from inside the brain or from electrodes attached 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. Theta rhythms can be quantified using quantitative electroencephalography (qEEG) using 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, appear 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 and navigation. 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. 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. Although there were a few earlier hints, the first clear description of regular slow oscillations in the hippocampal EEG came from a paper written in German by Jung and Kornmüller (1938). They were not able to follow up on these initial observations, and it was not until 1954 that further information became available, in a very thorough study by John D. Green and Arnaldo Arduini that mapped out the basic properties of hippocampal oscillations in cats, rabbits, and monkeys (Green and Arduini, 1954). Their findings provoked widespread interest, in part because they related hippocampal activity to arousal, which was at that time the hottest topic in neuroscience. Green and Arduini described an inverse relationship between hippocampal and cortical activity patterns, with hippocampal rhythmicity occurring alongside desynchronized activity in the cortex, whereas an irregular hippocampal activity pattern was correlated with the appearance of large slow waves in the cortical EEG. Over the following decade came an outpouring of experiments examining the pharmacology and physiology of theta. By 1965, Charles Stumpf was able to write a lengthy review of 'Drug action on the electrical activity of the hippocampus' citing hundreds of publications (Stumpf, 1965), and in 1964 John Green, who served as the leader of the field during this period, was able to write an extensive and detailed review of hippocampal electrophysiology (Green, 1964). A major contribution came from a group of investigators working in Vienna, including Stumpf and Wolfgang Petsche, who established the critical role of the medial septum in controlling hippocampal electrical activity, and worked out some of the pathways by which it exerts its influence. Because of a historical accident, the term 'theta rhythm' is used to refer to two different phenomena, 'hippocampal theta' and 'human cortical theta'. Both of these are oscillatory EEG patterns, but they may have little in common beyond the name 'theta'. In the oldest EEG literature dating back to the 1920s, Greek letters such as alpha, beta, theta, and gamma were used to classify EEG waves falling into specific frequency ranges, with 'theta' generally meaning a range of about 4–7 cycles per second (Hz). In the 1930s–1950s, a very strong rhythmic oscillation pattern was discovered in the hippocampus of cats and rabbits (Green & Arduini, 1954). In these species, the hippocampal oscillations fell mostly into the 4–6 Hz frequency range, so they were referred to as 'theta' oscillations. Later, hippocampal oscillations of the same type were observed in rats; however, the frequency of rat hippocampal EEG oscillations averaged about 8 Hz and rarely fell below 6 Hz. Thus the rat hippocampal EEG oscillation should not, strictly speaking, have been called a 'theta rhythm'. However the term 'theta' had already become so strongly associated with hippocampal oscillations that it continued to be used even for rats. Over the years this association has come to be stronger than the original association with a specific frequency range, but the original meaning also persists.