Brain waves often capture the imagination when it comes to explaining what happens between our ears. Different frequency waves, such as delta, theta, alpha, beta and gamma, have been associated with various states and functions, from sleep and calm wakefulness to learning and problem-solving. However, the precise functions of such neural oscillations are not yet clear. 

We spoke with 14 neuroscientists who shared their thoughts on the role of rhythms in the brain and cognition, including coordinating brain areas, influencing memory and sleep, and contributing to synchrony. 

Coordinating brain areas

“There is a lot of debate about this, but many, including myself, think that these neural oscillations temporally coordinate when cells are receiving or transmitting information. So I think the primary role of network oscillations is to organise systems of neurons to compute certain things and send it to other areas. I think of it as a metronome that says, you can compute now and transmit now and receive information now. It’s kind of a way to temporally organise complex neural circuits.” Andrew Alexander, UCSB

“Theta rhythms play a fascinating role in many of our brains’ functioning processes; coordinating communication between regions, integrating information, consolidating memory… the list goes on. If you prefer to think in analogies, rhythms are like the maestros of our brains’ symphonies, coordinating the activity of different brain regions and shaping our cognitive processes.” Kanaka Rajan, Harvard University

“I feel like rhythms may be like different states of the brain. They may help coordinate activity in the brain in specific scenarios.” Teresa Guillamón Vivancos, Instituto de Neurociencias de Alicante

"I may have a more unorthodox opinion about theta because we don’t see theta oscillations in the bird hippocampus, at least not in chickadees. It is possible that either theta oscillations or non-rhythmic patterns such as saccades can potentially organise neural activity in an analogous way." Hannah Payne, Columbia University

“One of the fascinating developments in neuroscience today is that circuits define what systems can do. That’s true, however, what many neural systems, especially in an area like the hippocampus, can do, is more than one kind of thing. The hippocampus is involved in space, memory and cognitive behaviour and at any moment in time, the hippocampal system might need to know where you currently are, an encoding of your current location, and in the very next moment it might need to remember that, or to remember some object or event from the past. 

One possibility is that there are different neural systems for anything that you can think of. There are different neurons connected to each other in different ways and so on. This to me seems absurd. What seems a much more sensible system is to build a system that can do multiple jobs computationally. And so when we think about how to do that, without undoing or interfering with job A or job B such as encoding and recollection for example, a great way is through neuronal oscillations because those oscillations reflect the population synaptic activity. 

Oscillations like theta and gamma in particular, allow you to take what is a static system from its connectivity and say during this time you are going to be in state A, that does encoding for example, and during this other time, the same system is going to do recollection or some other kind of process. The theta cycle for example could be a reflection of the excitatory drive onto the system, or conversely the inhibitory drive of the system. And so, it is allowed to have this multi-stable set of possibilities when it is under the control of such things that emerge as things we recognise as neural oscillations but is really just population synaptic activity.” André Fenton, New York University

A sample of human EEG with prominent resting state activity - alpha-rhythm. Left - EEG traces (horizontal - time in seconds; vertical - amplitudes, scale 100uV). Right - power spectra of shown signals (vertical lines - 10 and 20Hz). Credit: Andrii Cherninskyi, CC BY-SA 4.0, via Wikimedia Commons

Memory, navigation and sleep

"The theta rhythm is known to play a role in certain cognitive processes, including memory, and navigation. It is also thought to play an important role in how different brain areas communicate with each other. It is not something we have focused on specifically in our lab, but it is a fundamental construct in cognition." Ziv Williams, Harvard University

“There's very good evidence from John O'Keefe and many other colleagues that theta waves help memory formation. However, in sleep, we mainly use rhythms as a way of classifying the phases of sleep. But just because something has a rhythm (in the sleep EEG) doesn't necessarily mean it has a specific meaning. Many people believe these rhythms contain information, but it's also quite complicated to interpret.

Other rhythms and processes may have different meanings. Circadian rhythms, for example, span 24 hours.” William Wisden, Imperial College London

“Brain oscillations have obviously been implicated in a number of functions. Theta oscillations are very important in the hippocampus in learning and memory, for example. Alpha and beta oscillations have been implicated in other functions including those related to sensorimotor processing, but there is growing evidence for their involvement in more cognitive processes too.” Andrew King, University of Oxford

"There is a lot to be discovered! I think that there are very convincing studies showing the importance of those oscillations and rhythms in things like memory. There is a lot of beautiful work in the hippocampus that has to do with memorizing things and replays, which is extremely fascinating.

I think there are a lot of theories about the role of these rhythms when it comes to the coherence and organisation of different brain areas all trying to work in synchrony. I don’t have a deep knowledge about this, but it is something I’m very curious about. There is a lot of interesting literature coming out in those fields, and I think that it might be very important to explain some elements of cognition that are very poorly understood in terms of the synchrony between different brain areas." Marino Pagan, University of Edinburgh

Synchrony

“There is a lot of mystery around physiological synchrony, a biological marker of closeness. It’s measurable and observable at multiple different spatial and temporal scales. We can see it in the brain at a very local scale, as well as in patterns of activity across the brain in different areas. And it depends on the kind of measure you’re using, such as fMRI versus EEG versus other kinds of measures.

What you are seeing in these rhythms is synchrony, or resonance. Sometimes they are aligned and sometimes they are not. Literature has all been focused on conditions under which they align, like friends engaged in a task that requires cooperation. Their brains begin to synchronise because they attend to the same things, make similar decisions, and coordinate their behaviour. That requires emotional synchrony, which is then observable in facial expressions and maybe changes in arousal, like heart rate.

It’s interesting that people typically pick out different rhythms to look for synchrony. You can also just look at synchrony in the raw EEG signal. Most studies that looked at EEG synchrony have looked at alpha waves. Alpha waves typically go down when we are highly attentive to something or engaged, and go up when we’re distracted or exploring, opening our attentional filter wide. This synchrony in alpha waves makes sense in the context of engagement with another individual.

There are many fascinating open questions about synchrony: how it actually happens in our brains. Synchrony, this process that starts in the brain then percolates through your body, can also run in reverse.

Moving in synchrony somehow gets your heart beating in synchrony, then gets your brains to synchronise. This has been observed multiple times – whether you’re engaging in mirroring behaviour as improv actors do, an exercise or warm-up that gets them dialled into each other, or marching in the military, banging on a drum, singing in unison, all these things are a part of every human culture. 

Every culture engages in kinds of rhythmic rituals and anthropologists have speculated for a while that this is designed to generate coordinated behaviour and cooperation. And the intervening variable that I’m suggesting is physiological synchrony – perhaps it’s magic, I don’t know. Those are the moments when we connect.” Michael Platt, University of Pennsylvania

"A number of studies show they do play an important role. Firstly, gamma oscillations have been shown to be important in studies in both humans and animals. Research has shown stimulation of the hippocampus by gamma frequencies also reduces amyloid load in the early stages of Alzheimer’s disease. Why this is so is a different question. This could just be a feature of the system where the neurons fire together, or neurons could potentially stay healthy for a longer period of time if they are embedded in this synchronisation system. 

If you think about how many neurons there are in cortex, you have to organise the activity in a meaningful way. For many processes, there isn’t an alternative explanation. For example, the fact that we can recognise visual stimuli so fast. Visual stimuli have so many different features and gamma synchronisation is very plausible as a mechanism that then enables this very fast synchronisation of different cell types that encode those different features. You have to be able to recall all these different features at the same time." Tatiana Korotkova, University of Cologne 

It's still an open question

"We don't know if all the different types of oscillations are doing similar things, or if they are all doing completely different things from each other. 

Theta waves are super interesting because they have probably been the most studied of all the oscillations, and people really believe that they have a strong functional role. 

This has been most well established in the hippocampus, thanks to the work of lots of people, including John O'Keefe, here at UCL. They have shown that theta organises the spiking of neurons within the hippocampus, such that neurons which fire at different times relative to theta encode different types of information.

I think that that idea that neuronal spiking at different phases encodes different information is one which is really interesting. We’d like to understand how these are relevant to other oscillations. 

It’s something that we're thinking about. For example, theta oscillations interact with gamma oscillations, though they are much slower. So we see a coupling, for example, when there are theta oscillations nested within gamma oscillations, and there seems to be a strong relationship between those two frequencies." Dr Vikaas Sohal, University of California, San Francisco

"If you silence them then the brain functions as usual to a great extent. It might be that they connect some areas, or are important for large-scale organisation. They might be another level of modulation across areas, but it is not clear to me yet what role they play. It will be important to measure them with new techniques so we can understand what role they play during behaviour and also to perturb them. It may be that they are just epiphenomena of the fact there is an electrical field." Ahmed El Hady, Max Planck Institute of Animal Behaviour

"There are theories that the oscillation can coordinate different brain areas, but currently we have not considered that factor yet because for us the underlying mechanism for oscillation is still unclear. They may be contributed by interneurons but interneurons are driven by complex long-range and local connections. What exactly is the impact of information processing by this kind of oscillation is also unclear. Therefore, I think it is still an open question." Ning-long Xu, Chinese Academy of Sciences