Our mind often seems like a gigantic library, where memories are written on parchment and stored away on shelves. Once filed, they remain steadfast and inviolate over time, although some may eventually become dusty and forgotten.
Now, Reut Shema, Yadin Dudai and colleagues from the Weizmann Institute of Science have found evidence that challenges this analogy. According to their work, our memory is more like a dynamic machine - it requires a constant energy supply to work. Cut the power and memories are lost.
Shema found that the plug that powers our memories is an enzyme called PKMzeta. This molecule is vital for a process called long-term potentiation (LTP) where a the strength of a synapse - the connection between two nerve cells - is increased in the face of new experiences. This process, and thus PKMzeta, fuels the production and storage of new memories.
Shema demonstrated the importance of PKMzeta by inactivating it in the brains of rats. He trained the rats to avoid the taste of the artificial sweetener saccharin and then injected the part of their brains that control taste with a chemical called ZIP that stops PKMZeta from doing its thing.
The results were striking - ZIP erased the rodents' memories of what they'd learnt. It even killed off the relevant memories when the rats were injected a month after their training. In human terms, that's the equivalent of erasing memories that were several years old.
Even if the rats were given trials to reinforce their aversion to saccharin, they forgot all about it once ZIP was brought into play. Rehearsal didn't 'immunise' them against the loss of PKMzeta.
More surprising still, the process seemed to be irreversible, at least within the duration of the experiment. Twelve days after ZIP was administered, the rats still had no recollection of the supposed unpleasant taste of saccharin. Their memories had not just been clouded over, they seemed to have been truly erased.
ZIP has no effect on how well the rats created new memories - if they were injected before they were taught to avoid saccharin, they picked things up just as well. But it seems that PKMZeta is vital for the continuing existence of new memories.
If results can be generalised to other parts of the brain, and indeed, to humans, they suggest that memories are not simply writ once on our mental network and left to be retrieved. They exist because of ongoing processes in the brain. Long after they are created, memories are still incredibly vulnerable to loss, perhaps even irreversibly so.
This impermanence may actually be beneficial - it could render the entire network more flexible and make it easier for new experience to be added to the mix.
Reference: Shema, Sacktor & Dudai. 2007. Rapid erasure of long-term memory associations in cortex by an inhibitor of PKMzeta. Science 317: 951-953.
Brain & Behavior
Comments
This is scary because government or criminals could use this to purposely cover up crimes by perpetrators or memory of victims, believably due to honestly being unable to recall anything. Something similar I had read (can't remember where when online) was proposed to help trauma victims erase bad memories. As a TBI victim, severe accidents can completely wipe memory of the incident and surrounding time period of hours, days, weeks. It's not a concept as therapy or social manipulation that should allowed or even considered in human society.
Posted by: megan | July 12, 2009 2:47 PM
Cool.
Note though, that Reut is a female name.
Posted by: Assaf | July 12, 2009 4:14 PM
Yadi is a brilliant dude. Jerry Yin's group established a role for PKMz in memory in Drosophila about five years before Yadi's group.
Posted by: Comrade PhysioProf | July 12, 2009 6:00 PM
Why am I thinking of abuse potential?
Posted by: D. C. Sessions | July 12, 2009 6:14 PM
I fear the abuse potential, too. I'm thinking specifically of torturers from the US, Guatemala, or elsewhere erasing memories of what was done to their victims, or, worse, erasing personalities by erasing memories. *shudder*
Posted by: deang | July 12, 2009 7:30 PM
"Mindwipe" has been a recurring plot device in science fiction since before I was born -- and that's a while back.
Posted by: D. C. Sessions | July 12, 2009 7:48 PM
I imagine the rats also forgot all of their other favorite and hated foods as well. "ZIP" (although it has an appropriately Orwellian distopian name) is a long way from selective memory wiping. The injection was centered on a local part of the brain, in this case a fairly well-known and obvious one, as opposed to the one that controls our memories about, say, patriotism or ex-girlfriends (I'm being facetious here - such complicated memories are non-local). I imagine that a non-local application of ZIP would be extraordinarily disruptive to the identity of the person, perhaps even fatal, considering that the global effect of the drug has not been explored in this study. In any case, I am pleased to see ethical questions being raised about research - that is what I call advanced science.
Posted by: um | July 12, 2009 10:31 PM
Who said "selective?" Most SF references to "mindwipe" are much closer to tabula rasa -- effectively a death sentence that doesn't involve a corpse. So convenient for "rehabilitating" the poorly socialized, don't you know ...
And, yes, I'm aware that some things are not readily learned by adult brains. Like walking, language, ...
Posted by: D. C. Sessions | July 12, 2009 10:47 PM
when can i get my PKMzeta potentiator pills so I can have photographic memory?
Posted by: bd | July 13, 2009 5:01 PM
Rats, rats, (rodents), mice, rats, mice, and then mice. Have you been at the ZIP, Mr Yong?
Posted by: Michael Houghton | July 13, 2009 5:30 PM
Huh. How embarrassing. All of those should be rats. Have altered.
Posted by: Ed Yong | July 13, 2009 6:03 PM
The "saying mice when you mean rats" brain glitch was something I observed often and myself engaged in when I taught neuroscience using rodents. Maybe it is because the tiny patch of cortical real estate that encodes "lab rat" is right next door to the patch that encodes "lab mouse".
Posted by: cm | July 13, 2009 7:42 PM
You've just scared the hell out of me. I would be very surprised if this isn't abused.
Posted by: Marc Abian | July 14, 2009 8:15 AM
I think you may have found a cure for Alzheimers......
Posted by: Amber | July 23, 2009 1:47 AM