Post by Shireen Parimoo
What Are Brain Rhythms?
The brain is never silent. Across the brain, hundreds of millions of neurons are firing at any given moment. While any individual neuron can be firing (‘on’ state) or silent (‘off’ state), populations of neurons collectively generate a rhythmic pattern of electrical activity known as neural oscillations. Most neurons in a given population alternate between the on and off states in a synchronized and rhythmic manner.
Brain rhythms were first discovered by Hans Berger in 1924, who invented electroencephalography (EEG) by placing metal electrodes on the scalp and amplifying the recorded electrical signal. When the participants’ eyes were closed, Berger noticed a dominant pattern of brain waves oscillating in the 7.5-12 Hz frequency range that are now known as the “alpha” rhythm. Interestingly, as soon as participants opened their eyes, the alpha rhythm became suppressed and was replaced by the higher frequency “beta” rhythms (12 - 30 Hz). Alpha suppression also occurred when participants engaged in mental tasks, leading Berger to speculate that alpha rhythms represent the resting brain while higher frequency rhythms are important for active cognitive processing. Berger went on to study brain rhythms in different populations, making important discoveries about their role in cognitive processes and conditions like epilepsy, but it took several years before his work gained widespread attention in the scientific community.
Studying Brain Rhythms
Today, the study of brain rhythms and their role in cognitive processes is an active area of research. While EEG is used to non-invasively record electrical signals from the scalp, more invasive methods such as electrocorticography are also used to directly measure activity from the brain regions of interest, usually in patients undergoing brain surgery. Similarly, magnetoencephalography is used to record the magnetic fields arising from the electrical currents produced by the populations of neurons in the brain. Structural brain imaging such as MRI is often used together with EEG to localize the source of electrical signals recorded from the scalp within the brain.
Recordings of brain rhythms look primarily like sine waves of different sizes (and sometimes, different shapes). There are three key properties of neural oscillations:
1. Phase: the position or angle of an oscillation. This can be the peak (top of the wave), trough (the bottom of the wave), or the rising or falling phase of the wave.
2. Power: the amplitude or size of an oscillation.
3. Frequency: the number of oscillations per second, usually represented in Hertz.
It is also possible to study the interactions between brain rhythms, either within a single region or between brain regions, as a form of functional connectivity. The interaction between oscillations of different frequencies is typically investigated through cross-frequency coupling analyses. This includes:
1. Phase-amplitude coupling: the phase of a slower oscillation is coupled or synchronized with the amplitude of a higher-frequency oscillation. The idea is that the higher frequency oscillations represent information content, while the slower frequency oscillations control the flow of information between brain regions by influencing the amplitude of those high-frequency oscillations.
2. Phase-phase coupling or phase synchronization: the synchrony or alignment between the phases of brain rhythms, which can occur within the same frequency band or across different frequency bands:
a. Cross-frequency phase synchronization: the higher-frequency oscillations begin at the same phase as the slower, lower-frequency oscillation. As with phase-amplitude coupling, phase-phase coupling is thought to be important for information transfer and neural plasticity.
b. Inter-regional phase synchronization: phase alignment within the same frequency band across different brain regions provides a measure of communication between different brain regions. For example, the peaks and troughs of alpha rhythms in two phase-synchronized regions would typically occur with a small but consistent time lag.
The Role of Brain Rhythms in Cognition
Our understanding of the functional significance of brain rhythms has come a long way since Berger discovered them a century ago. Now, brain rhythms are generally divided into five primary frequency bands, each of which has been associated with a distinct set of cognitive processes, some of which are described below. Note that the functions of different brain rhythms depend not only on their properties such as frequency, but also on the interactions between different frequency bands and the brain regions where they are observed.
Delta (1 – 4 Hz). Delta rhythms are lowest frequency oscillations and are most commonly linked to brain activity during deep or ‘slow wave’ sleep. However, delta rhythms are also important for internally directed attention, as delta power increases when people are concentrating on a mental task or meditating. Phase-amplitude coupling between delta and beta oscillations in prefrontal regions is also important for working memory, suggesting that delta rhythms may play a role in blocking interference from irrelevant sources while maintaining a ‘train of thought’. Other cognitive functions associated with delta rhythms include arousal, emotional and motivational processing, and memory consolidation during slow-wave sleep.
Theta (4 – 7.5 Hz). Theta oscillations, which are slightly faster than delta rhythms, are also known as “hippocampal rhythms” and are heavily linked to memory-related processes and spatial navigation. The hippocampus is a key structure in the medial temporal lobe that is involved in the formation and retrieval of memories. Early animal studies recorded theta rhythms in the rat hippocampus and found that neuronal spiking activity during specific phases of the theta oscillation was crucial for forming new memories. In humans, the power of theta rhythms in the hippocampus increases during memory tasks while phase synchronization between theta and gamma rhythms increases when people successfully learn new information. Similarly, theta synchronization between the hippocampus and prefrontal regions also increases during the successful formation and retrieval of memories.
Cortical theta rhythms also play a role in working memory and cognitive control (i.e., the ability to perform goal-directed actions and flexibly adapt our behavior). In the prefrontal cortex, theta rhythms coordinate the activity of other brain regions that are needed for a specific task. For example, theta phase synchronization between prefrontal areas and the visual cortex is observed during visual attention tasks. Similarly, theta rhythms in the medial prefrontal cortex are linked to error-related processing, like adjusting behavior after a mistake is made.
Alpha (7.5 – 12 Hz). Early EEG studies indicated that the alpha rhythm reflected a brain at rest, as alpha power was suppressed when people were engaged in a task but increased when people had their eyes closed. However, research from the past several decades suggests that alpha rhythms are involved in nearly every cognitive process, including attention, memory, learning, working memory, and language, just to name a few.
There is controversy regarding exactly what role alpha rhythms play in cognition, but one of the more prominent theories is that alpha rhythms have an inhibitory function and the power of alpha oscillations plays a role in gating the flow of information, often through phase-amplitude coupling with higher-frequency gamma rhythms. Alpha activity leads to brief periods of inhibition on the neuronal population through inhibitory interneurons. Slower rhythms like alpha and theta create ‘optimal’ windows of time during which bursts of high-frequency activity can take place. When alpha power is high, those windows are short because of the inhibitory period that follows. When alpha power decreases, however, the window of time is lengthened, making neuronal firing more likely to occur.
