Dreams help cement emotional memories, rat study suggests

New research published in Nature Neuroscience looks at how rats' sleep is affected by an unpleasant experience. This research is helping us understand the link between dreaming and our emotions. Science columnist Torah Kachur explains.

Do rats dream?

No, not in the way we can say that they have a subjective experience during their sleep. They can't exactly wake up and tell us about their dream of falling into the abyss or their teeth falling out — two very common human dreams.

But, they do share brain structures with humans that are active during sleep in both species — the amygdala and hippocampus. The amygdala is involved in processing emotions and the hippocampus is involved in memory consolidation, especially a type of memory called episodic memory, where you remember all the details of one event. Rats have those structures in the brain. We don't know if they are working the same way, but we now know that they are both active during sleep, similar, we think, to the way humans dream.

How was this discovered?

Researchers from New York University in the Neuroscience department studied trained rats by looking at their brain waves during sleep before and after a training session. Gabrielle Girardeau is the post-doctoral researcher who was the lead author on this new study.

"The rats are training to run back and forth on a linear track," says Girardeau.  

"They're getting water rewards… at both ends of the track. And when we start the experiment what we do is we introduce an aversive stimulus. It's not painful, it's just unpleasant... it is an air puff. And they learn to fear these precise locations because of the air puff… After we train them on that, we record a bunch of sleep."

That sleep the researchers recorded revealed that the amygdala and hippocampus were firing together in a more coordinated fashion than if the rats didn't experience the air puff.

Why is the fact that the amygdala and hippocampus were firing together important?

This is because it indicates that the rats are processing emotional information, this negative experience, in a way that mimics how humans consolidate memories — using the hippocampus. It's as if they are replaying the aversive experience as they sleep and learning or remembering it.

"What we didn't know… is what happened in the amygdala together with the hippocampus during sleep, and does it have anything to do with memory consolidation," says Girardeau.

"Memory has many components in it… you remember what, when, where, but you also remember your internal state."

Again, there is no way of knowing what the rats are thinking about as they sleep but we can infer certain ideas of what their brains are doing based on previous knowledge and similarities between us and rats. And we can say that the memory consolidation pathway was engaged. Were they really thinking and dreaming and visualizing the aversive experience? Let's just say probably not as consciously as when we dream. But their brains were doing something, and that something is interesting to us.

What does this study add to our understanding of the importance of sleep and dreaming?

It gives us insight that we never had before about emotional memory.  

"We know by instinct that sleep is important," says Girardeau. "What was striking to me is that we know a lot about sleep processes that occur in the hippocampus for a certain type of memory consolidation: episodic memory. But we know much less about, say, emotional memory. What we know about emotional memory is mostly focused on the amygdala and focused on the awake state."

Now we have evidence, at least in rats, that sleep is also important for consolidating, or cementing, memories about our emotional state. These experiments can't be done on humans yet because of ethical constraints but as technology advances and brain-reading hardware gets more and more sophisticated, I would expect that our insights into the importance of sleep for not only our memories but our emotional processing will come into light.