Brain Waves Travel in Both Directions in the Hippocampus

Post by D. Chloe Chung

What's the science?

The hippocampus, a brain region important for memory formation and processing, flexibly integrates various information from multiple interconnected brain systems. It can function in a substantially complex and flexible manner, processing emotional and visuospatial information at the same time. Previous studies have reported that brain waves in the hippocampus travel in one direction (specifically, from the back to the front of the hippocampus), but researchers question how the simple, unidirectional path of brain waves is sufficient to support the highly integrative function of the hippocampus. This week in Nature Communications, Kleen and colleagues utilized a novel two-dimensional array system and found that brain waves in the hippocampus can travel in both directions.

How did they do it?

The authors recruited six participants who required open brain surgery, which would expose the surface of the hippocampus. To detect and measure neural activity in the hippocampus, the authors used a novel, two-dimensional microgrid array system that contains 32 electrodes. This new device looks like a thin film and, when placed on the hippocampus surface, can conform to the contour of the hippocampus and uniformly record brain waves at a higher density. During the surgery, two participants stayed awake and were asked to verbally speak out the name of the objects that appeared on a screen. Their neural activity in the hippocampus was simultaneously recorded and time-locked with the verbal response detected by the microphone. This visual naming task was performed to test whether the direction and consistency of brain waves that travel through the hippocampus would change upon behaviors.

What did they find?

From the neural activity recording, the authors first found that hippocampal brain waves at different frequencies travel in both directions unlike popular belief: from the back of the hippocampus to the front, and also from the front of the hippocampus to the back. For instance, the authors observed that the brain waves that were first traveling from the back to the front would reverse their route and move in the opposite direction, potentially integrating information from different interconnected brain regions. This bi-directional traveling pattern of brain waves was maintained even when recording electrodes were re-adjusted on the hippocampus surface, which indicates that the observation occurs independently of the location of brain wave measurement. Interestingly, during the visual naming task, the authors observed that the low-frequency brain wave (1.9 Hz, meaning 1.9 cycles per second) travels from the back to the front when the participant was recalling the object name and reversed its direction when the participant was at rest. On the contrary, high-frequency brain waves (13.8 Hz, meaning 13.8 cycles per second) tend to travel from the back to the front when the participant was recalling the name of the object. These findings suggest that cognitive activity can modulate the traveling pattern of brain waves in the hippocampus in a frequency-dependent manner.

What’s the impact?

This study is the first work to show that brain waves can travel back and forth in the hippocampus, expanding our understanding of how memory and cognition are processed in the hippocampus. This new piece of information may be valuable in evaluating how hippocampal function is affected in neurological diseases such as epilepsy. Also, this work highlighted the value of the new thin-film microgrid array system that can record real-time neural activity in a high-density manner, which can be similarly used in studying brain waves in other brain regions. As the authors suggested, it will be worthwhile for future studies to evaluate how hippocampal brain waves travel during more complex cognitive tasks.

Kleen et al. Bidirectional propagation of low frequency oscillations over the human hippocampal surface. Nature Communications (2021).Access the original scientific publication here.