The rhythmic electrical activity in our brains, known as brain waves, has fascinated researchers and the public alike for over a century. The primary question remains: Do Brain Waves Travel Outside The Skull? While the existence of brain waves is well-established, their precise function and whether they are merely a byproduct of brain activity or serve a more significant purpose are still under investigation. Recent studies suggest that many brain waves are not just synchronous oscillations but “traveling waves” that physically propagate through the brain.
A groundbreaking study from Columbia University, spearheaded by neuroscientist Joshua Jacobs, indicates that traveling waves are prevalent in the human cortex, the area responsible for higher cognitive functions. Moreover, the study reveals that these waves become more organized as the brain performs tasks more efficiently, demonstrating their relevance to behavior. This finding supports previous research suggesting that traveling waves are a vital but often overlooked brain mechanism involved in memory, perception, attention, and even consciousness.
Electrocorticography (ECoG) electrode array positioned directly on the brain’s surface for improved spatial resolution in detecting brain wave activity.
Brain waves were initially detected using electroencephalogram (EEG) techniques, which involve placing electrodes on the scalp. EEG recordings capture activity across a range of frequencies, from delta (0.5 to 4 Hz) to gamma (25 to 140 Hz). Slower frequencies are associated with deep sleep, while higher frequencies correlate with increased consciousness and concentration. However, interpreting EEG data can be challenging due to its limited ability to pinpoint the location of activity and signal distortion as the waves pass through the skull.
The Columbia University study, published in Neuron, utilized electrocorticography (ECoG), a more advanced technique. ECoG involves placing electrode arrays directly on the brain’s surface, minimizing signal distortions and significantly improving spatial resolution. This allowed researchers to gain a more accurate understanding of brain wave activity and the propagation of traveling waves.
Scientists have proposed several potential roles for brain waves. One prominent hypothesis suggests that synchronous oscillations “bind” information from different brain regions, associating them with the same object or concept, such as the shape, color, and movement of a visual object. Another idea proposes that brain waves facilitate information transfer between brain regions.
These hypotheses typically assume that brain waves are synchronous, producing standing waves similar to two people swinging a jump rope. Traveling waves, on the other hand, propagate through the brain like a crowd performing “the wave” at a sporting event. Traveling waves have unique properties that could represent information about the past states of different brain locations. The fact that they physically move through the brain suggests they could be a mechanism for transmitting information from one area to another.
While these ideas have been around for decades, they have often been overlooked by neuroscientists. One reason is that earlier reports of traveling waves primarily focused on describing the waves without establishing their functional significance.
Terry Sejnowski, a computational neuroscientist, highlights the need for techniques capable of monitoring multiple neurons simultaneously to understand traveling waves.
Computational neuroscientist Terry Sejnowski of the Salk Institute for Biological Studies, who was not involved in the study, notes that many neuroscientists view these waves as an epiphenomenon, similar to the hum of an engine, lacking direct connection to behavior or function.
The tools used by researchers may also have contributed to this lack of attention. Traditional neuroscience has focused on studying individual neuron behavior using microelectrodes. Researchers observed variability in the timing of neuron firing across different experimental trials. They concluded that this timing was not important and began averaging responses from multiple trials to determine a “firing rate.” However, this approach may ignore the timing information necessary to reveal traveling waves, as the variability may stem from where neurons are in oscillation cycles.
Sejnowski explains that the conceptual framework has grown out of understanding individual neuron activity, while the brain functions through populations of interacting neurons. Because traveling waves involve the activity of numerous neurons spread across the brain, they are not detectable using single-neuron techniques. Recent technological advancements have enabled the simultaneous monitoring of many neurons, providing a more comprehensive view of brain activity.
Optical methods, such as voltage-sensitive dyes, allow researchers to visualize electrical changes in thousands of neurons simultaneously but are not safe for use in humans. ECoG, however, is commonly used in epilepsy patients to investigate seizures, making it a valuable tool for studying traveling waves in the human brain.
The researchers in the Columbia University study recruited 77 epilepsy patients with implanted ECoG arrays to investigate traveling waves. They first identified clusters of electrodes exhibiting oscillations at the same frequency. Nearly two-thirds of all electrodes were part of these clusters, which were present in 96 percent of patients at frequencies ranging from 2 to 15 Hz, spanning the theta (4-8 Hz) and alpha (8-12 Hz) bands.
Next, the researchers determined which clusters represented genuine traveling waves by analyzing the timing of oscillations. In a traveling wave, consecutive oscillations are slightly delayed or advanced depending on the direction of travel, similar to how people in a crowd follow each other during a wave. Two-thirds of the detected clusters were traveling waves moving from the rear to the front of the cortex, involving nearly half of all electrodes and occurring in all lobes and both hemispheres of the brain.
Brain activity patterns during a working-memory task, illustrating the coordination of traveling waves in the frontal and temporal lobes.
The team then tasked participants with a working-memory exercise and discovered that traveling waves in their frontal and temporal lobes became more organized half a second after the prompt to recall information. The waves shifted from moving in various directions to moving in a more coordinated manner. Notably, the extent of this coordination correlated with the participants’ response speed.
Jacobs states that more consistent waves correspond to better task performance, suggesting a new method for measuring brain activity to understand cognition, potentially leading to improved brain-computer interfaces (BCIs). BCIs are devices that connect the human brain to a machine, enabling tasks such as controlling a prosthetic limb.
These findings address some of the skepticism surrounding the importance of these waves. Psychologist David Alexander of the University of Leuven in Belgium, who was not involved in the study, recognizes the work as a strong contribution to the study of cortical traveling waves, building upon previous research on their role in human cognition.
Alexander claims that the team made unjustified claims about the novelty of their research and failed to acknowledge previous work. For instance, he cites a 2002 EEG study that found a correlation between the timing of a reversal in the direction of theta waves and memory performance, and a 2009 study that found fewer waves moving from the front to the back of the head during a working-memory task in people who had experienced their first episode of schizophrenia compared with healthy individuals, implying a link between traveling wave behavior and psychiatric symptoms.
Jacobs argues that Alexander’s findings may not involve the same signals as his team’s research, noting that Alexander reported patterns involving the entire brain, while his team’s findings were limited to specific regions. He also points out differences in recording techniques and the nature of the recorded signals.
The confirmation of the importance of traveling waves opens up new avenues in neuroscience. Jacobs emphasizes that discovering that a wide range of oscillations are traveling waves demonstrates their involvement in coordinating activity across different brain regions.
He believes that these waves propagate information, at least in the context of the current study. Another idea suggests that waves may repeatedly move across patches of cortex, modulating neuron sensitivity to sweep a “searchlight” of attention across areas like the brain’s visual processing center.
Sejnowski concludes that the concept of a traveling wave is closely linked to maintaining the cortex in an optimal state of sensitivity to other inputs, enabling optimal function. He anticipates that interest in traveling waves will continue to grow, marking a transformation from an old conceptual framework to a completely new one, a paradigm shift.
