What are brain waves? It’s no wonder the term sounds like science fiction. In the 1920s, a German psychiatrist embarked on a highly personal quest to discover the supposed medium of telepathy. By placing electrodes on the human scalp, Hans Berger found waves of electrical brain activity using a tool called electroencephalography, or EEG. Physicists had recently shown that electromagnetic waves could propagate through space to carry information. If the brain had its own waves, could they transmit thoughts to others like a radio broadcast?
Since Berger’s time, no evidence of telepathy — now regarded as pseudoscience — has been found. While the “brain waves” Berger discovered do not transmit information outside the body, increasing evidence suggests that they are critical for transmitting information within the brain. While the brain does give off radio waves, their energy is far too weak to transmit information outside the body. Instead, the brain waves most neuroscientists study are waves fixed in space yet changing in time. Scientists refer to these waves as neural oscillations.
Like the rhythmic glowing of a firefly swarm seen from afar, neural oscillations generally reflect the synchronous firing of millions of cells in the cortex called pyramidal cells. Because their dendrites all point in the same direction, the dendritic activity of countless synchronized pyramidal cells creates an electric field so large it can be measured from the scalp. Without this happy coincidence, Berger’s EEG would have been impossible. Neural oscillations do not directly reflect action potential events, but they do reflect the simultaneous depolarization of many cells’ dendrites as the cells are excited in unison.
Because the EEG reflects the activity of so many cells, it’s easy to dismiss EEG oscillations as something like the noise of the crowd at a sports arena. However, this thinking ignores the scale-free, complex organization of the brain. Many neural oscillations are believed to be responsible for timing and coordinating communication between populations of neurons. For effective communication, neurons must respond to appropriate inputs and ignore extraneous inputs, like a radio being tuned to the right station. Gamma oscillations, which occur at 30 to 100 cycles per second, are believed to achieve such coordination through a cycle of excitability and inhibition. Pyramidal cells are made excitable during the phase when relevant inputs are expected, allowing them to respond appropriately. Following this excitable phase, pyramidal cells are silenced by inhibitory interneurons and ignore all input. This creates an efficient mechanism by which neuronal populations may easily synchronize with each other even when their synaptic connectivity is weak, like the pendulums of two grandfather clocks synchronizing by weak coupling forces acting through the wall.
Many neuroscientists hold an almost mystical reverence for gamma oscillations due to their associations with working memory, cognition, attention, and even consciousness itself. Neuronal populations oscillating at the same gamma frequency may encode different features of the same stimulus, even when distributed across a large area of the brain. This allows your mind to experience the perception of a passing car holistically without central coordination, a concept known as binding by synchrony. Without gamma synchrony, the redness of the car, its motion, and its shape might all be experienced independently without the perception of the car as a whole!
Because of their importance for consciousness, gamma oscillations are exciting to study, yet they are often frustratingly masked by electrical muscle activity in EEG recorded from the scalp. Easier to observe are the tranquil waves of alpha oscillations, the first brain waves observed by Hans Berger nearly a century ago. These oscillations, occurring at 8 – 12 cycles per second, appear over the back of the head when the eyes are closed and the subject is quiet and restful. Long described as an “idling rhythm,” alpha is now associated with inner focus and suppression of distracting stimuli. During mediation, Zen monks show strong alpha oscillations over much of the scalp, even when the eyes are open. Despite being discovered so long ago, the cellular mechanisms of alpha oscillations are a mystery, but it is hypothesized that such rhythms occur wherever neural computations are being suppressed in the cortex. Without alpha oscillations, it is possible that your visual cortex would still struggle to “see” something even when your eyes are closed!
When the eyes are opened, alpha gives way to a new rhythm, the beta oscillation. Observations of beta oscillations date back as far as Berger, yet our understanding of this 12 – 30 cycle per second rhythm remains surprisingly poor. Beta is associated with arousal and movement. Yet, at the same time, it can be induced by sedating benzodiazepine drugs like Valium or Xanax. This so called “beta buzz” is thought to occur when such drugs slow the frequency of gamma oscillations to beta frequencies by modulating the neurons that “keep time,” like a waltz switching to a slower tempo.
Moving to deeper brain tissue, theta oscillations are 4 – 8 cycle per second rhythms thought to act as a clocking mechanism for the hippocampus, a brain region involved in both memory retrieval and spatial navigation. Keeping precise time allows the hippocampus to perform “dead reckoning” much like a sailor on the ocean using nothing but a clock and the ship’s velocity to track position. Buried inside the medial temporal lobe, theta activity of the hippocampus cannot be detected from the scalp. Instead, theta oscillations in scalp EEG often appear in similar contexts as alpha oscillations and may also indicate drowsiness.
Beyond the drowsiness of theta lies the deep sleep of delta oscillations. At 1 – 4 cycles per second, this slow rhythm is needed for restful sleep and can also be observed during anesthesia. What, then, lies after delta? Many other brain rhythms have many described, some slower than delta, some faster than gamma. The field of neural oscillations is a wild story of dynamic characters and Greek alphabet soup that doesn’t always spell sense. In fact, the frequencies of particular oscillations vary considerably across brain structures, people, and species, making misidentification easy! As neurophysiologist György Buzsáki writes,
“the borders between the different [frequency] bands were evenly and arbitrarily drawn … like the straight-line country borders between the African nations drawn by the colonialists.”
For this reason, Bradley Voytek at the University of California, San Diego is developing an EEG fingerprinting technique which avoids textbook boundaries. Rather, Voytek’s method finds neural oscillations in patients and subjects based on a data driven approach, resulting in identification of rhythms which may better correspond to behavior.
While not a medium for the paranormal, neural oscillations are crucial for healthy brain function. Many disorders — including schizophrenia, autism, epilepsy, ADHD, and Parkinson’s disease — have been associated with abnormal brain rhythms, making such oscillations promising targets for drug treatments. Despite their clear importance, the mechanism and purpose of many neural oscillations remain hotly debated nearly 100 years after Berger’s first EEG recording. If you are a student studying the brain, perhaps one day you could make a breakthrough in this exciting frontier of neuroscience!
Kasamatsu, Akira, and Tomio Hirai. “An electroencephalographic study on the Zen meditation (Zazen).” Psychiatry and Clinical Neurosciences 20.4 (1966): 315-336.
Haller, M. et al. “Automated “spectral fingerprinting” of electrophysiological oscillations” SfN poster #661.07/VV59, abstract (2014).
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