Theta wave
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, and marsupials.
In rats, 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.
When a rat is eating, grooming, or sleeping, the hippocampal EEG usually shows a non-rhythmic pattern known as large irregular activity or LIA.
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.
This theory states that theta waves may act as a switch between encoding and recall mechanisms, and experimental data on rodents [10] and humans [11] support this idea.
Another study on humans has showed that theta oscillations determine memory function (encoding or recall) when interacting with high frequency gamma activity in the hippocampus.
[15] 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).
Their findings provoked widespread interest, in part because they related hippocampal activity to arousal, which was at that time the hottest topic in neuroscience.
In rats, hippocampal theta is seen mainly in two conditions: first, when an animal is running, walking, or in some other way actively interacting with its surroundings; second, during REM sleep.
Numerous studies have shown that the medial septal area plays a central role in generating hippocampal theta (Stewart & Fox, 1990).
Under certain conditions, theta-like oscillations can be induced in hippocampal or entorhinal cells in the absence of septal input, but this does not occur in intact, undrugged adult rats.
It is commonly argued that cholinergic receptors do not respond rapidly enough to be involved in generating theta waves, and therefore that GABAergic and/or glutamatergic signals (Ujfalussy and Kiss, 2006) must play the central role.
For type 1 theta, the picture is still unclear, but the most widely accepted hypothesis proposes that the frequency is determined by a feedback loop involving the medial septal area and hippocampus (Wang, 2002).
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 (Buzsáki, 2002).
There are other complications as well: the hippocampal layers are strongly curved, and theta-modulated inputs impinge on them from multiple pathways, with varying phase relationships.
Direct projections from the septal area to hippocampal interneurons also play a role in generating theta waves, but their influence is much smaller than that of the entorhinal inputs (which are, however, themselves controlled by the septum).
This mechanism is supposed to allow long term potentiation (LTP) to reinforce the connections between neurons of the hippocampus representing subsequent elements of a memory sequence.
[18] 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 (Hyman et al., 2003; Brankack et al., 1993), 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.
Most of the available information on human hippocampal theta comes from a few small studies of epileptic patients with intracranially implanted electrodes used as part of a treatment plan.
In the largest and most systematic of these studies, Cantero et al. (2003) found that oscillations in the 4–7 Hz frequency range could be recorded from both the hippocampus and neocortex.
Research has shown that these oscillations are closely associated with memory encoding and retrieval, emotional regulation, and the maintenance of cognitive tasks.
Research indicates that during cognitive control tasks, increased theta amplitude is observed in frontal brain regions, which are critical for decision-making and behavioral regulation.
Specifically, theta activity is thought to support mechanisms that allow individuals to adapt their behavior based on changing environmental demands and internal goals.
This dynamic interplay between theta oscillations and cognitive control processes highlights how these brain rhythms contribute to efficient task performance.[22].
Moreover, studies using non-invasive brain stimulation have demonstrated the causal role of theta oscillations in the prefrontal cortex during the anticipation of cognitive control demands.
[23] In summary, theta oscillations serve as a fundamental neural mechanism underlying both learning and cognitive control, facilitating the organization of information processing and adaptive behavior in response to environmental challenges.
