July 18, 2018
The electrical oscillations we call brain waves have intrigued scientists and the public for more than a century. But their function—and even whether they have one, rather than just reflecting brain activity like an engine’s hum—is still debated. Many neuroscientists have assumed that if brain waves do anything, it is by oscillating in synchrony in different locations. Yet a growing body of research suggests many brain waves are actually “traveling waves” that physically move through the brain like waves on the sea.
Now a new study from a team at Columbia University led by neuroscientist Joshua Jacobs suggests traveling waves are widespread in the human cortex—the seat of higher cognitive functions—and that they become more organized depending on how well the brain is performing a task. This shows the waves are relevant to behavior, bolstering previous research suggesting they are an important but overlooked brain mechanism that contributes to memory, perception, attention and even consciousness.
Brain waves were first discovered using electroencephalogram (EEG) techniques, which involve placing electrodes on the scalp. Researchers have noted activity over a range of different frequencies, from delta (0.5 to 4 hertz) through to gamma (25 to 140 Hz) waves. The slowest occur during deep sleep, with increasing frequency associated with increasing levels of consciousness and concentration. Interpreting EEG data is difficult due to its poor ability to pinpoint the location of activity, and the fact that passage through the head blurs the signals. The new study, published earlier this month in Neuron, used a more recent technique called electrocorticography (ECoG). This involves placing electrode arrays directly on the brain’s surface, minimizing distortions and vastly improving spatial resolution.
Scientists have proposed numerous possible roles for brain waves. A leading hypothesis holds that synchronous oscillations serve to “bind” information in different locations together as pertaining to the same “thing,” such as different features of a visual object (shape, color, movement, etcetera). A related idea is they facilitate the transfer of information among regions. But such hypotheses require brain waves to be synchronous, producing “standing” waves (analogous to two people swinging a jump rope up and down) rather than traveling waves (as in a crowd doing “the wave” at a sports event). This is important because traveling waves have different properties that could, for example, represent information about the past states of other brain locations. The fact they physically propagate through the brain like sound through air makes them a potential mechanism for moving information from one place to another.
These ideas have been around for decades (pdf), but the majority of neuroscientists have paid little attention. One likely reason is that until recently most previous reports of traveling waves—although there are exceptions—have merely described the waves without establishing their significance. “If you ask the average systems neuroscientist, they’ll say it’s an epiphenomenon [like an engine’s hum],” says computational neuroscientist Terry Sejnowski of the Salk Institute for Biological Studies who was not involved in the new study. “And since it has never been directly connected to any behavior or function, it’s not something that’s important.”