"Each of these neurons fires only in a restricted region of space," Lubenov says. "Furthermore, the spikes don't just happen any timethey 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."
When the data about neuronal firing, oscillation phase, and rat location were combined, the researchers were able to show 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," notes Lubenov. "It was believed that the firing of hippocampal neurons encodes the physical location of the rat in its environmentin other words, a point of physical space. Our findings suggest that what is encoded is actually a portion of the rat's trajectorythat is, a segment of physical space."
"Such segments may be the elementary unit of hippocampal computation," adds Siapas. "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."
This finding may also have significant implications for 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 a
|Contact: Lori Oliwenstein|
California Institute of Technology