Routing information through the brain

Brain signals continuously shift and change. (CC Image from

One big missing piece in the puzzle of how brains work is how information is routed throughout the brain. There are so many connections between neurons in the brain (up to 1 quadrillion – that’s 1,000,000,000,000,000!). With connection numbers like this, you might think that every part of the brain must be connected to just about every other part.

But when we monitor brain activity, what we see is that different parts of the brain somehow manage to totally disconnect from others at certain times. What it means to disconnect is that brain activity in one region becomes completely independent of activity elsewhere. This disconnection happens despite the actual physical connections still being present.

An article published 3 days ago in the prestigious journal Science has given us some clues about how these connections and disconnections may occur, by looking at a part of the brain called the hippocampus. The hippocampus is involved in language (in humans), in the formation of memories (in all mammals) and in our sense of location and direction.

With all these functions, the hippocampus is connected to many different parts of the brain. The scientists recorded from the hippocampus of rats while they performed different tasks, like searching for food (which rats like, especially when they find it!) or having to run through a wide open space (which rats don’t like as it makes them anxious).

What the scientists found was that different neurons in the hippocampus became more active in the different situations. What was most intriguing was that the neurons that became more active in any given situation all tended to connect to only certain other parts of the brain.

So when the hippocampus needed to send information to one part of the brain, it used mostly those neurons that connected to that particular part. For sending information to a different part of the brain, it would activate a different set of neurons.

This solves a small part of the puzzle of how the brain controls where information goes. The big question now is – How does the hippocampus activate just those neurons that have the right connections?

Since all the information starts in the hippocampus, somehow the hippocampus itself is making the decision about where the information needs to go, and then preferentially activating the appropriate neurons. HOW? No-one knows!

Using a different set of neurons with different connections to different parts of the brain, as discussed in this article, is just the last stage of a complex routing process that is somehow occurring in the brain. The BIG question – How does the brain choose and activate the right neurons in the first place? – still needs to be answered!


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