Caltech scientists reveal how neuronal activity is timed in brain's memory-makin...( PASADENA Calif.Theta oscillations ar...)

PASADENA, Calif.Theta oscillations are a type of prominent brain rhythm that orchestrates neuronal activity in the hippocampus, a brain area critical for the formation of new memories. For several decades these oscillations were believed to be "in sync" across the hippocampus, timing the firing of neurons like a sort of central pacemaker. A new study conducted by researchers at the California Institute of Technology (Caltech) argues that this long-held assumption needs to be revised. In a paper published in this week's issue of the journal Nature, the researchers showed that instead of being in sync, theta oscillations actually sweep along the length of the hippocampus as traveling waves.

"It was assumed that activity in the hippocampus is synchronized throughout," says Evgueniy Lubenov, a postdoctoral scholar at the Center for Biological Circuit Design at Caltech. "But when we looked simultaneously at many different anatomical locations across the hippocampus, we found instead a systematic delay in neuronal activity from site to site. Instead of the whole structure oscillating at once, we see traveling waves that propagate across the hippocampus in a consistent direction, along its long axis."

"In other words, the hippocampus has a series of local time zones, just like we have on Earth," adds Athanassios Siapas, associate professor of computation and neural systems and Bren Scholar at Caltech.

The hippocampus has long been known to be critical for the formation and maintenance of episodic memoriesi.e., memories of experiences. In the rat, hippocampal neurons also function as "place cells," only firing when the animal is in a particular spot in its environment. Lubenov and Siapas began to analyze the theta oscillations generated when rats move around and explore their environment. They watched howand whenthe rat's neurons fired relative to the rat's position and to the phase of the theta oscillations. They did these studies using multi
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