Firing in Rhythm: Simultaneous Ripples Across Distant Brain Regions Facilitate the Integration of Brain Signals
Post by Rachel Sharp
The takeaway
When high-frequency brain waves, known as “ripples”, occur simultaneously across different brain regions, they help integrate signals between these regions through coordinated neural activity. These findings support the hypothesis that ripples play an important role in helping our brains combine complex information across distant brain regions.
What's the science?
How do individual elements of a neural event occurring at different locations across the brain unify into a cohesive mental experience? The “binding-by-synchrony” hypothesis suggests that high-frequency ripples synchronize across brain regions, forming integrated networks of neural activity. Ripples have been shown to synchronize, and have even been shown to organize cell firing in rodents, but the impact of synchronized rippling across distant brain regions in humans is still unknown. This week in PNAS, Verzhbinsky and colleagues test the impact of simultaneous rippling on the integration of neural signals across different brain areas in humans. To do this, the authors used implanted microelectrodes spanning distances of up to 16mm (a US dime is 17.9 mm in diameter) in regions of the cortex used to execute movements and process language.
How did they do it?
The authors implanted four Utah Arrays (a group of 96 microelectrodes with the ability to record spatially accurate neural activity) into the cortex of three participants. They then recorded activity from these electrodes for each patient, over several hours, during both wake and sleep, and analyzed how selected neurons changed their firing rates in response to ripple activity detected by neighboring electrodes. After collecting the full recordings from each participant, the authors analyzed the rate at which ripples co-occurred, whether or not co-firing increased across brain regions when those regions were co-rippling, and whether co-rippling neurons across different regions were better able to predict the firing patterns of other neurons, compared to non-co-rippling neurons. To examine this, they categorized individual neurons as either “predicting drivers (B)” or “predicted targets (A)”. They measured the extent to which firing patterns in B neurons predicted observed firing patterns in A neurons when the surrounding neurons were co-rippling.
What did they find?
The authors found that brain regions that experienced co-rippling also experienced greater integration of neural firing across patients, brain regions, and brain states (wake vs. sleep). They also found that neurons were more likely to fire in synchrony with co-occurring ripples, which was correlated with the ability of neurons to predict another neuron’s firing patterns. This finding supports the idea that networks of neurons distributed across different brain regions synchronize through simultaneous high-frequency ripples. Neurons in co-rippling regions are more likely to fire together, and to exhibit predictive firing patterns. Ultimately, their findings supported the “binding-by-synchrony” hypothesis, showing evidence for networks of co-firing neurons across brain regions, enhanced by simultaneous ripple activity.
What's the impact?
This study found that simultaneous ripples improve connections between both nearby and distant neurons in the human brain. These ripples were able to organize firing across large groups of neurons, showing the importance co-occurring ripples may have on integrating complex neural events, even when they occur in different brain regions. This integration of neural events helps us achieve complex cognition, organize our thoughts, and make appropriate decisions. Understanding the biological mechanism that drives neural integration gives us another clue to deciphering the puzzle of complex human perception and cognition.
