How neuronal activity is timed in brain’s memory-making circuits

May 30th, 2009 - 11:38 am ICT by ANI  

London, May 30 (ANI): Researchers at the California Institute of Technology (Caltech) have challenged the long-held assumption that theta oscillations-a type of prominent brain rhythm that orchestrates neuronal activity in the hippocampus-remain “in sync” across this key area for the formation of new memories.

In a new study, the researchers have found that instead of being in sync, theta oscillations actually sweep along the length of the hippocampus as travelling waves.

“It was assumed that activity in the hippocampus is synchronized throughout. 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 travelling waves that propagate across the hippocampus in a consistent direction, along its long axis,” Nature magazine quoted Evgueniy Lubenov, a postdoctoral scholar at the Center for Biological Circuit Design at Caltech, as saying.

Athanassios Siapas, associate professor of computation and neural systems and Bren Scholar at Caltech, added: “In other words, the hippocampus has a series of local time zones, just like we have on Earth.”During the study, the researchers analysed the theta oscillations generated as rats move around and explore their environment.

The observed how and when rats’ neurons fired relative to their positions and to the phase of the theta oscillations.

They did so using multiple electrodes with recording sites, which enabled them to simultaneously isolate the spiking of many individual neurons.

“Each of these neurons fires only in a restricted region of space. Furthermore, the spikes don’t just happen any time-they pay attention to the phase of the ongoing theta oscillation. If you have access to the phase at which the neuron fired, you have additional information about where the rat was in space,” Lubenov said.

Upon combining the data about neuronal firing, oscillation phase and rat location, the researchers observed that neuronal activity indeed sweeps across the hippocampus in a wave, with its peak appearing in one region, then another, then another, rather than hitting the entire hippocampus in one synchronized pulse.

“This changes our notion of how spatial information is represented in the rat brain. It was believed that the firing of hippocampal neurons encodes the physical location of the rat in its environment-in other words, a point of physical space. Our findings suggest that what is encoded is actually a portion of the rat’s trajectory-that is, a segment of physical space,” Lubenov said.

Siapas added: “Such segments may be the elementary unit of hippocampal computation. Assume the path a rat takes in an environment is represented and stored as a sequence of point locations. If the rat visits the same location more than once, the representation becomes ambiguous. Representing the rat trajectory as a sequence of segments oriented in space resolves such ambiguities.”

The researchers say that the significance of their findings lies in the fact that they may prove helpful in understanding how information is transmitted from the hippocampus to other areas of the brain.

“Different portions of the hippocampus are connected to different areas in other parts of the brain. The fact that hippocampal activity forms a traveling wave means that these target areas receive inputs from the hippocampus in a specific sequence rather than all at once,” said Siapas.

The researcher also dismissed the suggestion that this behaviour is found only in rat brains, insisting that theta oscillations are ubiquitous in mammalian brains.

“I would expect the traveling-wave nature of theta oscillations to be a general finding, applicable to humans as well,” he said.

And while it is not known whether human hippocampal cells function as place cells, as they do in rats, “it may turn out to be the case that the human hippocampus plays a role in providing spatial cues that are important to episodic memory,” Lubenov said.

What we do know is that, by showing that theta oscillations travel across the hippocampus, the Caltech team will likely change the way neuroscientists think about how the hippocampus works. (ANI)

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