As we experience the world, our brain manages to continually absorb new information even as it calls up memories and thoughts from within. The two processes seem to happen simultaneously. Thus, we are able to drive to the grocery store, recalling a familiar route, while processing fresh input about road conditions and that pedestrian who suddenly darted across the street. Now a team at the University of Texas at Austin has found evidence that in the brain's spatial system, this balancing act is accomplished via distinct electrical frequencies. The results also offer hints about how the brain compresses memories—that is, how we can recall an hours-long event in mere seconds.
The group, led by neuroscientist Laura Colgin, studied rats as the animals navigated a maze, recording electrical activity in the hippocampus, an area crucial for memory formation. The experiment focused on a type of hippocampal cell called place cells, which correspond to specific locations in space. In a rat, researchers can tell by which place cells are firing where the rat is in the maze—or what part of the maze the rat is thinking of.
As with all the brain's neurons, place cells produce electrical signals that oscillate in waves. In particular, past research suggests that when place cells encode spatial memories they produce theta waves, which operate on a relatively slow, long-wave frequency. Yet these theta oscillations do not work alone. They also contain shorter and more frequent gamma rhythms nested within them like folded accordion bellows. As each wave of electrical activity pops up at the gamma frequency, it conveys information nuggets to the interacting theta wave, effectively presenting a highlights reel relative to the longer theta wave.
In a 2009 study, Colgin and her colleagues described an additional level of theta-gamma complexity in the rat hippocampus. When the hippocampus communicated with a brain area relaying as-it-happens sensory information, the team saw theta signals supported by “fast” (60- to 100-hertz) gamma frequencies. A second, previously unappreciated set of “slow” (25- to 55-hertz) gamma rhythms seemed to be interacting with theta waves when the hippocampus swapped messages with brain areas that may replay memories or plan future movements.
In their current analysis, Colgin and her team found further evidence that fast gamma waves code new information and slow gamma waves retrieve memories. The researchers recorded place cell activity in seven rats as they negotiated a short track during three 10-minute sessions daily for several days. They found fast gamma signals when place cell activity matched the rats' actual location. Slow gamma activity showed up when place cell activity aligned with locations ahead of the rats' current position—perhaps reflecting the animals' memory and anticipation of the upcoming route.
The team also noticed that the length of track represented by place cells during each millisecond seemed to skyrocket when slow gamma rhythms took over, prompting speculation that another level of memory compression may exist within the theta-gamma code. This could explain how the brain is able to replay long events over mere seconds.
Not all experts are convinced by this interpretation. Brandeis University researcher John Lisman, an expert on the theta-gamma code, explains that such compression would require cells to fire faster than current biophysical estimates allow—although he praised Colgin's team for uncovering distinct functional roles for slow and fast gamma frequencies in the hippocampus.
Other scientists think the brain might indeed be capable of faster and more complex signaling than many models predict. Loren Frank, a neuroscience researcher at the University of California, San Francisco, is in this camp. He says the new finding reveals that “things associated with memory may be going on very, very quickly.”