The Hippocampus Maps More Than Space

New research shows that the hippocampus can map sound, supporting a broader role for this brain region in organizing information.

Some cells in the hippocampus respond to specific frequencies. Credit: David Tank and Dmitriy Aronov

The hippocampus and the neighboring entorhinal cortex are well known for their striking ability to map space. But new research published in Nature suggests a much broader function. The study, from Simons Collaboration on the Global Brain director David Tank and his collaborators, found that cells in this area can also map sound.

As we described in a story published in December 2016, Dmitriy Aronov, first author on the study, trained rats to use a joystick to move through a sort of sound maze — a defined sequence of frequencies. The researchers discovered a set of cells that act very much like place cells, which fire when the animal is in a specific location. These ‘sound cells’ fire when the animal hears a specific tone. As we noted in the December piece:

“The findings point to a more general function of the hippocampus — it’s representing the whole experience,” says Aronov, [who recently joined the faculty at Columbia University.] “Anatomically, it makes no sense for the hippocampus to be only about space, because it receives information from everywhere in the brain.”

Mapping sound space might seem like an arbitrary task not particularly relevant to real-world needs. But Tank, Aronov and others think that the hippocampal circuit might be doing something much more profound: mapping experience more broadly.

A number of scientists have found that cells in the hippocampus and entorhinal circuit produce a predictable sequence of neuronal activity during tasks. Tank and Aronov, for example, found that cells in the hippocampal circuit fired in a predictable pattern during the entire experiment, not just in response to sound. “From pressing the lever to traversing the frequency to receiving the reward and starting a new trial — there is a sequence of activation throughout the entire period of behavior,” Tank says. The sequence probably represents not just sound but other aspects of the task as well, perhaps even abstract concepts, Tank says.

 

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