What's the science?
Oscillations in neural activity (sometimes referred to as ‘brain waves’) are important for brain function, as they help to coordinate activity across the brain and help spatially separated brain regions to communicate. The brain oscillates at different frequencies, including ‘alpha’ and ‘theta’ frequencies. Animal studies have shown that brain oscillations can travel across the cortex in the form of a wave, however this has not been investigated in humans. Recently in Neuron, Zhang and colleagues examine whether oscillations in neural activity can travel across the human cortex in the form of a wave and if wave propagation correlates with behavior and cognition.
How did they do it?
Electrocorticography (ECoG) - which is the measurement of neural activity from electrodes placed on the brain’s surface - was performed on 77 patients undergoing brain surgery. Neural activity was recorded while participants performed a working memory task where participants tried to memorize a list of stimuli, followed by a retrieval cue where they recalled presented stimuli. The authors used a technique designed to test whether oscillations in the brain travel across the cortex. They did this by locating electrodes where neuronal oscillations were present at the same frequency simultaneously, and showed a timing (i.e. phase) gradient across space (i.e. the cortex). Neural activity was recorded while participants performed a working memory task. They used a clustering approach to identify clusters of spatially contiguous electrodes that showed the same frequency of oscillations. They then examined the timing of the activity across each cluster to look for patterns of phase synchrony to see whether the oscillations travelled in the form of a wave. They did this by calculating the phase of the oscillations at each electrode across space. Lastly, they tested whether travelling waves in the cortex were associated with behavior.
What did they find?
Most patients (96%) had ‘clusters’ of electrodes that showed the same frequency of oscillations (with a phase gradient) across space. They found a total of 208 clusters in the 77 patients. Clusters were in frequencies ranging from 2-15 Hz (alpha and theta). Clusters at given electrodes within one patient did not necessarily have the same frequency as the (spatially) identical clusters in another patient, suggesting that neuronal oscillations vary from individual to individual. They found that frequencies within patients showed a strong spatial correlation. They also found that many of the clusters had oscillation cycles that varied systematically with the electrode location within the cluster, indicating a traveling wave. They used a circular-linear model to examine the relationship between electrode location and phase of the wave to demonstrate the direction and the robustness of the travelling wave. 140 of the clusters (67%) showed consistent travelling waves and a consistent propagation direction across multiple trials. The clusters with travelling waves were found across all lobes of the cortex. The direction of travelling waves was most consistent in the frontal and temporal lobes, and most waves demonstrated a posterior-to-anterior directionality in these regions. Direction was more varied in the parietal and occipital lobes. They authors tested whether these travelling waves were related to the working memory task and found that directional consistency (how consistently the wave propagated in one direction) was higher in the frontal and temporal lobes after the retrieval cue onset (where they were required to recall previously shown stimuli) in the working memory task. Directional consistency was also positively correlated with performance, suggesting that waves travelling in a consistent direction are associated with better working memory efficiency.
What's the impact?
This is the first study to show that oscillations in neural activity at particular frequencies travel across the human cortex in waves. The consistency of propagation of these waves was related to an individual’s working memory. Understanding how neural activity is coordinated and associated with behavioral factors like working memory is crucial for understanding how the brain works. Future research will be need to further understand the importance of travelling waves in the brain.
Zhang et al., Theta and Alpha Oscillations Are Traveling Waves in the Human Neocortex. Neuron (2018). Access the original scientific publication here.