Deleting old memories to make room for new ones...
I'm sure most of you out there will have run out of disk space on a computer and had to overwrite a few files. And it looks like the short-term memory of animals isn't all that far-removed.
Publishing in the journal Cell, neuroscientists reported that, in mice and rats, newly formed neurons seem to be deleting older connections. Kayoru Inokuchi and colleagues from the University of Toyama think that short term memory is updated by new neurons emerging in the hippocampus area of the brain, which is sort of in the bottom middle bit, and these new neurons essentially overwrite connections between the old ones.
The researchers looked at this by irradiating rat's brains, which considerably slows down the formation of new neurons in the hippocampus. And they placed the rats in a chamber which would give their feet an electrical shock! Once the rats had this experience in their short-term memories the researchers applied a bit of x-ray radiation and afterwards the rats continued to use their hippocampus to recall that fear memory. But in those rats without x-ray treatment the fear memories were eventually displaced to elsewhere in the brain.
The researchers knew this memory displacement was occurring because they also looked at mice which were born without the ability to make new hippocampal neurons and mice who received an infusion to block any neuron activity in the hippocampus. That way they could tell which animal was depending on its hippocampus for memory and which wasn't.
Again, mice which couldn't produce new hippocampal neurons seemed to rely on their old short-term memories when placed in the shock chamber. And the researchers said this is because there were no new neurons to displace the old ones and essentially push them into long-term storage, somewhere else in the brain.
They've known for a while that exercise can improve short term memory so they put some of these mice onto an exercise wheel and lo and behold, they did not use the same area of the brain for their fear memories. And the scientists think this is because the exercise made them generate new neurons in their hippocampi.
So the conclusion is that if you can't make new neurons then you could have problems because the brain's short-term memory is literally full. So perhaps this could lead to a better understanding of memory-related diseases such as dementia and alzheimer's.