New research reveals that holding information in your mind, a process known as working memory, involves intricate, sweeping waves of coordinated neural activity traveling across the brain’s cortex. These waves, which move in circular, rotating patterns, provide a mechanism for the brain to read out and manage short-term memories. The findings challenge a long-held view of brain oscillations as stationary phenomena, showing instead that their spatial propagation is a key feature of how the brain handles immediate cognitive tasks.
Scientists discovered that these traveling waves are not random but change their direction and organization in response to cognitive demands. By recording activity in the prefrontal cortex of monkeys performing a working memory task, researchers observed that waves in specific frequency bands, especially the beta band, became more directionally focused. This suggests the waves play an active role in organizing the neural activity required to maintain and access information. This dynamic, moving pattern of oscillations helps control the timing of neural communication, which is critical for encoding and retrieving memories on the fly.
Unveiling the Cortex’s Moving Signals
For over a century, scientists have studied the rhythmic electrical activity of brain cells, known as brain waves or neural oscillations. Traditionally, these oscillations were often analyzed by averaging signals across multiple electrodes, a method that captures their frequency and power but overlooks any spatial organization. This approach treated the oscillations as “standing waves,” where entire regions of brain tissue would oscillate in unison. However, mounting evidence shows that these waves often travel across the cortical surface in coherent patterns.
This phenomenon is analogous to a “stadium wave” at a sports event. No single person runs around the stadium; instead, individuals stand and sit in sequence, creating the appearance of a moving wave. Similarly, in the brain, groups of neurons fire in a sequential, coordinated manner, causing a wave of electrical activity to propagate across a region. These waves are of particular interest because they can establish precise timing relationships between different neural groups, a process fundamental to learning and memory through spike-timing-dependent plasticity.
Mapping Waves in the Prefrontal Cortex
The prefrontal cortex (PFC) is a brain region critically important for higher cognitive functions, including working memory. To investigate the spatial structure of oscillations in this area, researchers used microarray recordings to capture neural activity in monkeys as they performed a delayed-match-to-sample task. This task required the animals to see an object, hold it in their working memory during a brief delay, and then identify it again.
Patterns of Propagation
The recordings revealed that oscillations in several low-frequency bands—specifically the theta (4–8 Hz), alpha (8–12 Hz), and beta (12–30 Hz) bands—consistently organized themselves into traveling waves. While some of these waves were planar, moving in a straight line from one point to another, the majority were found to be rotating waves. These circular patterns revolved around central anatomical points within the PFC, much like a wave rotating around the field in a stadium. This rotational dynamic is a significant departure from the simpler, linear waves observed in other brain regions and contexts.
Task-Dependent Changes
Crucially, the behavior of these waves was not static. During baseline conditions, the waves flowed bidirectionally. However, when the monkeys were engaged in the working memory task, the waves became more organized and flowed predominantly in one direction over the other. This change was most pronounced in the beta frequency band, suggesting its particular importance for the cognitive demands of the task. The ability of the waves to change their direction and flow indicates a flexible mechanism for managing neural communication based on current cognitive needs.
A Link Between Brain Regions
Further research using human electroencephalography (EEG) has extended these findings, showing that traveling waves also facilitate communication between different cortical regions during memory-guided actions. This work identified waves moving between the visual cortex, where sensory information is processed, and the frontal cortex, which is involved in planning and executing behaviors. This inter-regional communication is essential for using stored information to guide actions.
The study found two distinct waves that occurred around the time of a memory-guided response. First, a forward-propagating wave in a low-frequency range (2–6 Hz) traveled from the occipital (visual) to the frontal cortex. The timing of this wave’s peak correlated with the participant’s response time, suggesting it plays a role in initiating the memory-guided action. Following the response, a second, higher-frequency wave (14–32 Hz) traveled in the opposite direction, from the frontal to the occipital cortex. This feedback wave may be involved in processes that follow the action, such as resetting the visual system or consolidating the memory trace.
Functional Implications of Traveling Waves
The discovery that neural oscillations operate as traveling waves provides a new framework for understanding cortical function. Rather than simply indicating that a brain region is active, the movement of these waves adds a layer of computational complexity. Their propagation allows for the precise control of information flow and timing across the cortex.
This mechanism could serve several purposes. Traveling waves can help maintain network status and carry information about the recent history of local network activation. By creating specific phase relationships between distant neural populations, they can foster the synaptic changes that underlie memory formation. Furthermore, the direction and speed of the waves can be modulated to control interactions between brain regions, flexibly coordinating everything from sensory perception to motor responses. The absence of these organized waves when a planned behavior is withheld underscores their direct role in the execution of cognitive tasks.