Scientists have long pondered how the 16 billion neurons of the human cortex integrate or bind the many different types of information they encode into a single coherent experience or memory. A main hypothesis is that such binding involves high-frequency oscillations or “ripples” which promote neural interactions, in a similar way that rhythm operates in music or dance. Now, a team of researchers from the University of California, San Diego has provided the first empirical evidence that such ripples actually occur.
“Think about the experience of petting your cat: its form, location, surroundings, color, feel, movement and sound, plus your own responding emotions and actions. They are all bound together in a coherent whole,” said study senior author Eric Halgren, a professor of Radiology at UC San Diego.
“These different aspects of the experience are encoded in locations distributed across the cortical surface of the brain, and the experience is sub-served by their spatiotemporal firing pattern. The mystery has been how activities in those different locations get connected.”
Previous studies – focused mainly on rodents – have found that ripples in the hippocampus organize the replay of these spatiotemporal patterns during sleep, which is essential for making memories permanent. Now, by analyzing week-long recordings of the brain activity of 18 patients monitored to locate the origin of their epileptic seizures, Professor Hagan and his colleagues discovered that ripples also occur in all areas of the human cortex, both during sleep and wakefulness. The ripples were brief – about one-tenth of a second – and had a consistent narrow frequency close to 90 cycles per second. A typical such ripple event may involve up to 5,000 small modules that are distributed across the cortical surface, and are activated simultaneously.
“Remarkably, the ripples co-occurred and synchronized across all lobes and between both hemispheres, even at long distances,” said study lead author Charles Dickey, a doctoral student in Neurophysiology at UC San Diego. “Cortical neurons increased firing during ripples, at the ripple rhythm, potentially supporting interaction between distant locations.”
“There were more co-occurrences preceding successful memory recall. All of which suggests that distributed, cortical co-ripples promote the integration of different elements that may comprise a particular experiential memory.”
The scientists observed that cortical ripples were often coupled with hippocampal ripples and embedded in slower oscillations (one to 12 cycles per second). These slower ripples are controlled by the thalamus, a central structure that modulate neuronal firing and is crucial for memory consolidation.
“As our experience is organized hierarchically in time, so too are the rhythms that organize our cortical activities that create that experience,” Professor Halgren explained.
These findings could have major implications in better understanding psychiatric illnesses such as schizophrenia, which is characterized by a high degree of mental fragmentation.
“Our findings and those of others indicate that a particular type of inhibitory interneuron is crucial for the generation of ripples, and these cells are known to be selectively affected by schizophrenia, as are high frequency oscillations. Perhaps we are a little closer to finding a mechanism for one aspect of this tragic disease,” he concluded.
The study is published in the journal Proceedings of the National Academy of Sciences.