I’m sure you all know that you have both a short-term and a long-term memory. Many people think of those as separate things, and to us, it may seem that way. But in fact, the formation of short and long term memories in the brain is very intertwined, and a short-term memory can become a long-term one. What we don’t really know is HOW this happens. What makes the difference between remembering a phone number for a few minutes and remembering it for months? Turns out, it’s a simple tag.
Ballarini et al. “Behavioral tagging is a general mechanism of long-term memory formation.” PNAS, 2009.
Short term memory (STM) and long term memory (LTM) are actually not just different in terms of time, they are different in mechanism, in how they form. LTM relies on actual synaptic changes in the brain over a period of time, meaning that new proteins have to be synthesized, while right now we think that STM relies more on increases or decreases in firing rates of neurons, without permanent changes at the cellular level.
So if LTM formation means that synapses are changed, it’s going to require the synthesis of a lot of new proteins, specifically in the area of the hippocampus, a brain area closely associated with learning and memory. The hippocampus is a little curly bit near the bottom of your brain, inside the temporal lobes, and the word means “sea horse” in Greek, probably because it’s nice and curly.
What the scientists in this experiment wanted to do was to test what triggers a STM (changes in firing rate), to become LTM (actual changes in synapses). We know that, for LTM to form, plasticity-related proteins have to be synthesized. But what triggers the synthesis of these proteins? The scientists found that there was a behavioral “tag” that would cause formation of new protein and formation of LTM. And they had a beautiful way of doing it.
Take a bunch of rats. Give them weak training in a behavioral task, which will induce short-term memory (about 30 minutes), but not LTM (it was gone the next day). This task is known as the spatial object recognition task. Give the rat two things, say a blue block and a red ball, and put them in different areas of the cage. The rat will explore the new toys, and because they are both new, it will spend about 50% of its time at each toy. Take out the toys.
30 minutes later, put them back. But THIS time, put one of them (say, the red ball) in a different place in the cage, while the blue block remains in the same condition. If the rat has STM, it will spend far more time exploring the red ball in the new location, because it remembers that the blue block was there before.
Because the rats are only trained once on this, they are only good for a little while. But, if you shake them up a bit, something different happens. If you put them in an open field (an open box that is different from their cage) for 5 minutes an hour before or an hour after exposure to the toys for the first time, the rats developed LTM for the position of the toys, lasting well into the next day. Why? Well, previous studies had shown that exposing rats to something new before training makes them more likely to form LTM, and more likely to have the changes in protein expression that go along with LTM (whether this is due to stress, vigilance, or simply being more awake, Sci isn’t sure, she’ll have to read up on that and get back to you).
So then, the scientists did something interesting. They took these rats and went to train them with LTM, but before they did, they infused a drug into the brain (anisomycin), which blocks the transation of RNA into new protein. Thus, the rats would be unable to produce new proteins during training. And when they were unable to produce new proteins, the rats could not form the LTM during the training task. They tried this with two separate hippocampus-dependent tasks (spatial object, conditioned freezing, and one task that is hippocampus-independant (conditioned taste aversion), and all of them had the same result. Translation of new protein is clearly required for the formation of LTM.
Thus, is appears that a novel environment before training is a behavioral “tag” that induces protein synthesis, causing memories formed either immediately after or immediately before to become LTMs. A behavioral “tag” induces physical changes in the brain. And it turns out that this is not just hippocampus-dependant. The authors got similar results when they inactivated the hippocampus during training, meaning that the cortex or other areas may also be involved.
What I like about this study is that it’s a study done entirely in a behavioral model, with infusions to changes specific brain areas affecting the behavior. That way, it narrows the development of the behavior down to specific brain region in the rat (and presumably, in the human), showing a really nice link between protein synthesis in the hippocampus and cortex and development of LTM.
But what does this mean? It means that, if you want to store something in your LTM (anybody got a test coming up?), it might help to associate your learning it with something novel. Of course, you can’t go out and ride a roller coaster for each new fact, but sometimes writing a haiku or something silly about what you’re learning could be novel enough to help form that memory.
attempt some poetry for
There once was a sad undergrad
and her memory, oh it was BAD!
But she read this science note
and science poetry she wrote
and her orgo chem grades now are rad!
Ballarini, F., Moncada, D., Martinez, M., Alen, N., & Viola, H. (2009). Behavioral tagging is a general mechanism of long-term memory formation Proceedings of the National Academy of Sciences, 106 (34), 14599-14604 DOI: 10.1073/pnas.0907078106