Above the ocean. Image credit: @parechyn (CC0)
Brain cells known as neurons communicate via electrical and chemical signals to pass on information within and between neurons. When large groups of neurons fire in a coordinated way, they create brain waves. These waves or oscillations occur over many different-sized regions, from a few neurons to the whole cortex, the large, folded structure that sits over the rest of our brain and fills most of our skulls.
These patterns of dynamic activity help the brain coordinate and integrate information across regions and are involved in cognitive performance, such as motor control, memory and perception. The timing or phase of electrical activity in the brain is important for different areas to communicate effectively. However, it is still unclear if these activity patterns mostly happen at small (1mm-1cm), medium (>1cm) or large distances spanning almost the entire cortex (>8cm).
While most studies have focused on small and medium waves, which are implicated in sleep, attention and memory, large waves have been understudied. This is mainly due to technical limitations of sampling large areas of cortex and the blurring that occurs when measurements are made from outside the skull.
Alexander and Dugué used existing brain data from epilepsy patients, recorded with a technique called stereotactic electroencephalogram (sEEG). Local brain activity from a large number of electrodes was measured while participants engaged in a delayed free-recall task. The researchers then applied a new method to estimate the spatial frequency spectrum to quantify waves over all scales, from small to large.
Alexander and Dugué discovered that large ripples covering most of the cortex are the strongest and dominate how electrical activity is structured. As a result, signals recorded from a single point in the brain mostly reflect engagement in global brain activity rather than just nearby neurons. This pattern is consistent across many brain frequencies, from slow to fast oscillations. Since brain activity is expensive in terms of metabolism, the existence of these large ripples could be functionally important.
The study of Alexander and Dugué provides new insight into how the brain coordinates communication across different regions. A deeper understanding of the most important factors involved in neuronal processing will contribute to the development of new, targeted medical treatments and technologies to support people living with a range of cognitive conditions.
