A new type of rhythm: eta

February 15, 2022

Written by: Vanessa B. Sanchez

With technology advancing before our eyes, many of us can’t help but want to try everything and keep up! But, have you ever considered playing or living in a virtual reality? Virtual reality (VR) technology allows us to escape reality and explore a simulated experience that can either be similar or completely different from our real world – kind of like the movie The Matrix.

There are many different types of virtual realities and each one is created through advanced computer technology that can generate a three-dimensional environment and provide all the sensory (e.g., visual, auditory, touch and motion, etc.) stimuli to make you feel you are really existing in that world. To do so, you would have to wear a headset and/or hold a controller (view image) to help you navigate a whole new reality.

Given the rising use of VR for games, neuroscientists are interested in learning more about how our brains respond differently to real and virtual environments. To do this, a group of scientists at UCLA studied how rats’ brains responded differently to virtual reality environments versus their real environments^,3.

First, rats were trained to walk or run on a tiny treadmill either in regular box to mimic the real world or a VR enriched environment that was visually identical – see this video for an idea!^2,4.  These rats had tiny electrodes embedded in their brains so that their neural activity could be recorded². More specifically, these electrodes were touching the CA1 region of the hippocampus. The hippocampus can be divided into three distinct regions called CA1, CA2, and CA3 (“CA” stands for Cornu Ammonis) which all contain two types of neurons: pyramidal neurons and interneurons⁵. Interneurons are inhibitory neurons that can “turn off” or “shush” their neighbors the pyramidal neurons who are excitatory “turn on” neurons when they get too excited! Within the hippocampus, all three regions (and their neurons) form a loop and communicate with each other⁵. Since the hippocampus is heavily involved in spatial learning and memory, researchers were curious how it responded when rats navigated a virtual rather than real environment.

When brain regions (and their neurons) communicate, they can sometimes synchronize with each other at different frequencies resulting in what is called a brain wave. Brain waves can be divided into different frequencies from low (1 – 3 Hz) to high (38 – 42 Hz), which can be associated with various brain states or functions. For example, when rats ran through either a real or VR environment, neuroscientists could analyze the electrical activity of pyramidal and interneurons via electrodes in CA1 of the hippocampus and figure out how it related to behavior. It turns out that these neurons sync up their electrical activity at a rate of 8 pulses per second, or 8 Hz². This type of synchronization or brain wave is associated with theta oscillations, which are known to underlie relaxation, creativity, learning, and memory⁶.  Like this study, other scientists have found that our brains in a VR exhibit theta oscillations¹¹. This suggests that it is the VR itself that alters the way our brains respond to a VR, such that our brains work harder to integrate all the perceptual and sensory information from a VR that we do not normally require in the real-world¹¹.

Interestingly, when rats ran at faster-speeds (> 15 cm s^-1) in a VR, scientists picked up electrical activity from CA1 interneurons at a lower frequency (2 – 5 Hz) and discovered a new type of oscillation called, eta^2,3. At first, scientists thought that eta was unique to VR and not real-world environments². Because the location of the electrodes can pick up different signals or frequencies, they decided to carefully move the electrodes to see if they could detect eta oscillations while mice ran in a real-world environment^2-4. While not as obvious in a VR, eta oscillations can be detected in a real-world environment!

Why are eta oscillations stronger in a VR?

Although, eta oscillations are much slower (2-5 Hz) than theta (6-10 Hz), eta might be trying to coordinate all hippocampal neuronal activity into one stream of information throughout the brain². Neuroscientists believe that this enhanced eta rhythmicity in a VR can increase the connections and communication between the hippocampus and other brain regions like the cerebral cortex, which is involved in many functions like planning, fear, personality, and behavior². If so, then this new eta rhythm holds the ability to influence neural plasticity which is vital for building and shaping new memories^2,3,9,10. This means that virtual realities have the capacity to boost or slow brain rhythms, ultimately altering neuronal activity and neuroplasticity^2,3,10. Would you try stepping into a VR knowing that it might help your learning and memory?

References:

1. Drew, L. (2019). The mouse in the video game. Nature, 567(7747), 158-160.
2. Safaryan, K., & Mehta, M. R. (2021). Enhanced hippocampal theta rhythmicity and emergence of eta oscillation in virtual reality. Nature neuroscience, 24(8), 1065-1070.
3. Caroline, S. (2021, June 29). Virtual reality boosts brain rhythms crucial for neuroplasticity, learning and Memory ” ucla health connect. https://connect.uclahealth.org/. Retrieved February 4, 2022, from https://connect.uclahealth.org/2021/06/29/virtual-reality-boosts-brain-rhythms-crucial-for-neuroplasticity-learning-and-memory/
4. https://www.youtube.com/watch?v=2hvuw3FmXyQ
5. Andersen, P. (2007). The hippocampus book. Oxford University Press.
6. Buskila, Y., Bellot-Saez, A., & Morley, J. W. (2019). Generating brain waves, the power of astrocytes. Frontiers in neuroscience, 13, 1125.
7. Bekkers, J. M. (2011). Pyramidal neurons. Current biology, 21(24), R975.
8. Karalis, N., Dejean, C., Chaudun, F., Khoder, S., Rozeske, R. R., Wurtz, H., … & Herry, C. (2016). 4-Hz oscillations synchronize prefrontal–amygdala circuits during fear behavior. Nature neuroscience, 19(4), 605-612.
9. Kumar, A., & Mehta, M. R. (2011). Frequency-dependent changes in NMDAR-dependent synaptic plasticity. Frontiers in computational neuroscience, 5, 38.
10. Boeldt, D., McMahon, E., McFaul, M., & Greenleaf, W. (2019). Using virtual reality exposure therapy to enhance treatment of anxiety disorders: Identifying areas of clinical adoption and potential obstacles. Frontiers in psychiatry, 773.
11. Kanayama, N., Hara, M., & Kimura, K. (2021). Virtual reality alters cortical oscillations related to visuo-tactile integration during rubber hand illusion. Scientific reports, 11(1), 1-13.

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