01 April 2011

Why everyone should know about the hippocampus, Part II

Yesterday, I talked about the hippocampus and how it’s critical for the formation and recall of event memories (that bit about recall is contentious, by the way).  Today I want to talk a little bit about the nature of some of the cells making up the hippocampus.  Because they’re mind-numbingly awesome.

Quick background:  Cells called neurons (see below picture) make up the electrical circuitry of the brain.  When I say a neuron ‘fires’, I mean that it sends an electrical signal down its axon to the next neurons to which it’s connected.  This is how information travels through the brain.  End quick background.

 I stole part of this image from here, probably. 
I can’t find the ADAM citation, sorry.

So when recording from the hippocampus in rats that were moving freely around an open arena, researchers in the ’70s noticed that some neurons in the hippocampus only fired when the rats entered certain parts of the arena.  Without fail, when a rat entered one region one cell would fire, and when it went to another region a different cell would fire, and so forth.  It didn’t matter what direction the rat was moving, if it had a goal in mind, or if there was anything in this region like food or an interesting object.  The rat went to that place, and the cell fired. 

This was totally cool.

The red and blue circles represent the location
the rat is in when a certain cell starts firing.

Think about this for a second.  I’m saying that rat brains are capable of building a representation of the rat’s world such that individual neurons can ‘tell’, for lack of a better word, when a rat is in a certain place.  And it has nothing to do with motivation or direction or other stimuli (blind rats also have working place cells, by the way).  And this representation builds and stabilizes within minutes of exploring a totally novel space.

Let me put it another way.  Let’s say you let a rat run around an arena and you record from a whole bunch of place cells, each with their own unique spatial fields (called ‘place fields’; see the picture below).  If you know what order the cells fired in, you can reconstruct where that rat walked during its time in the arena.

Even if you’d never seen this rat in your life and you came upon these data
years after this experiment finished, you could guess where this rat ran in this arena.

Okay, I need you to contain your excitement for a moment.  You have just found out that you, the researcher, can tell exactly how the rat ran through that arena by looking at the order its cells fired in, and I think that’s got you a little overstimulated.

But I know what you’re thinking: “If I can use this pattern of firing as a record of where that rat went… could the rat use it too?  I mean... could this be the way a rat later ‘remembers’ where it was?”

Well.  Let’s say you let the rat rest.  You just keep recording while he hangs out for a bit, curls up and takes a quick nap.  Here comes the craziest part, are you ready?

While he’s sleeping, you’re recording from those same place cells – and they’re firing again!  And they’re firing in the same order they fired while he was running around just a few minutes ago!

That rat is essentially replaying his waking path while he sleeps.  He may replay it again while he’s awake, too.  (I should say he’s almost certainly not conscious of this going on.  His hippocampus is reactivating itself while he goes about his business.)  His place cell representation of that space will stay intact even if you take him out and bring him back there days later.  For lack of a richer vocabulary, his hippocampus remembers where he was!

Many, many studies have shown that rats whose place cells aren’t stable also have poor memories.  I won’t go into all the details here, but suffice it to say that rats need their hippocampi – and the place cells in them – to effectively remember events that have happened to them. 

Did you ever think cells like this existed in your brain?  I didn’t.  These cells with their awesome properties might have arisen as an efficient method of sorting out navigation, but you can see how they’d help you form and maintain the context of your memories as well.  They’re part of what the hippocampus is built on, and that’s another reason you need to know about the hippocampus.

No comments:

Post a Comment