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Theta rhythm
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The theta rhythm is an oscillatory EEG pattern that can be observed in the hippocampus and other brain structures in numerous species of mammals including rodents, rabbits, dogs, cats, bats, and marsupials. Whether a theta rhythm exists in primates is a controversial issue.
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The theta rhythm is an oscillatory EEG pattern that can be observed in the hippocampus and other brain structures in numerous species of mammals including rodents, rabbits, dogs, cats, bats, and marsupials. Whether a theta rhythm exists in primates is a controversial issue. Two types of theta rhythm have been described, with different behavioral correlates. Type 1 theta occurs during active motor behaviors, especially walking or running, and also during REM sleep. Type 2 theta occurs during states of still alertness. There are species differences in the prevalence of these two varieties: for example, type 2 theta occurs frequently in rabbits and cats, but only rarely in rats; whereas type 1 theta occurs frequently in rats, but rarely in rabbits. There is a strong tendency for rhythmic hippocampal activity to occur together with a desynchronized EEG pattern in the neocortex.
In rats—the most frequently studied species—theta rhythmicity is most easily observed in the hippocampal formation, but can also be detected in numerous other brain structures, including the medial and lateral septum, entorhinal cortex, retrosplenial cortex, prefrontal cortex, amygdala, nucleus accumbens, several nuclei of the hypothalamus and thalamus, and parts of the brainstem reticular formation. The medial septal area serves as an essential coordinating center: if it is destroyed, theta rhythmicity disappears throughout the brain.
Neurons in many brain areas show firing rates that are modulated by the theta rhythm. In the hippocampus and entorhinal cortex of rats, a particularly interesting type of modulation occurs, commonly known as phase precession. This pattern shows up in the firing of hippocampal pyramidal cells, which in rats are inactive most of the time during exploratory behavior, but show brief surges in activity when the rat passes through a small portion of the environment called the place field of the cell. During such a surge of activity, the firing of the cell is usually theta-modulated, but the phase of the theta cycle during which the cell fires tends to advance from cycle to cycle.
The functional significance of the theta rhythm is not clearly understood at present. A number of investigators have suggested that it may be related to learning and memory. Observations that support this concept include the fact that theta modulates long term potentiation, a type of synaptic plasticity that is widely believed to form the physical basis for memory; and that the presence of theta correlates with enhanced performance in several learning tasks.
Terminology
It is important to understand that because of a historical accident, the term "theta rhythm" is used in the literature to mean two quite different things. 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 3–6 cycles per second (Hz). In the 1930s-1950s, a very strong rhythmic oscillation pattern was discovered in the hippocampus of—originally—cats and rabbits. In these species, the hippocampal oscillations fell mostly into the 3–6 Hz frequency range, so they were often referred to as "theta" oscillations, although other names were also used, such as "rhythmic slow activity" (RSA). Later, hippocampal oscillations of the same type were observed in rats and mice, which turned out to be better experimental subjects than cats or rabbits in many respects. In rats, however, the frequency of the hippocampal oscillations averaged about 8 Hz and rarely fell below 6 Hz. Thus the hippocampal oscillation pattern should not, strictly speaking, have been called a "theta rhythm" in rats—however the term "theta" had become so strongly associated with hippocampal oscillations that it continued to be used even for rodents, and over the years this association has come to be stronger than the original association with a specific frequency range, although the original association also persists.
The upshot of all this is that "theta" can mean either of two things: (1) a specific type of regular oscillation seen in the hippocampus and several other brain regions that are connected to it, or (2) oscillations in the 3–6 Hz frequency range, regardless of where in the brain they occur or what their functional significance is. The first meaning is usually intended in literature that deals with rats or mice, while the second meaning is usually intended in studies of human EEG recorded using electrodes glued to the scalp. In general, it is not safe to assume that experimental studies of "theta" in the human EEG have any relationship to the "hippocampal theta rhythm". Scalp EEG is generated almost entirely by the cerebral cortex, and even if it falls into a certain frequency range, this cannot be taken to indicate that it has any functional dependence on the hippocampus.
Research findings in theta-wave activity
Theta-frequency EEG activity is also manifested during some short term memory tasks. Studies suggest that they reflect the "on-line" state of the hippocampus; one of readiness to process incoming signals. Conversely, theta oscillations have been correlated to various voluntary behaviors (exploration, spatial navigation, etc.) and alert states (piloerection, etc.) in rats, suggesting that it may reflect the integration of sensory information with motor output (for review, see Bland & Oddie, 2005). A large body of evidence indicates that theta rhythm is likely involved in spatial learning and navigation.
Theta rhythms are very strong in rodent hippocampi and entorhinal cortex during learning and memory retrieval, and are believed to be vital to the induction of long-term potentiation, a potential cellular mechanism of learning and memory. Based on evidence from electrophysiological studies showing that both synaptic plasticity and strength of inputs to hippocampal region CA1 vary systematically with ongoing theta oscillations, it has been suggested that the theta rhythm functions to separate periods of encoding of current sensory stimuli and retrieval of episodic memory cued by current stimuli so as to avoid interference that would occur if encoding and retrieval were simultaneous.
Underlying large-scale synchronization which results in rhythmic slow activity of field EEG are theta-frequency membrane potential oscillations, typically sodium-dependent voltage-sensitive oscillations in membrane potential at near-action potential voltages. Specifically, it appears that in neurons of the CA1 and dentate gyrus, these oscillations result from an interplay of dendritic excitation via a persistent sodium current (INaP) with perisomatic inhibition.
It is likely that human sources of theta rhythm are similar to those found in other mammals, and thus it is likely that cholinergic projections from the basal forebrain drive the theta rhythm seen in human EEG patterns. Similarly, humans show hippocampal theta rhythms that are probably mediated by inputs from the ascending brainstem synchronizing system via the medial septum (see diagram).
History
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 Berlin by Jung and Kornmüller in 1938. They were not able to follow up on these initial observations, and it was not until 1954 that further information became available, in the form of 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. 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, 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. 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.
See also
Brain waves
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