All Your Memories Am Belonging to Us

While in general, the most painful memories are the hardest to forget, scientists at the Weizman Institute and SUNY Downstate have discovered how to use an inhibitor of Protein Kinase M Zeta (PKMzeta) to erase some memories.

...a study appearing in today's issue of Science suggests that memories remain a transient chemical property of the brain even weeks after they're formed.

A lot of work had shown that a protein called protein kinase M zeta (PKMζ) was involved in the consolidation of a subset of memories. The authors of the new article decided to see if memories of aversive tastes were among them. They introduced rats to a new taste (saccharine) and then injected them with a nausea-inducing salt. Normally, the rats remember the taste and avoid anything with saccharine in it. Injections of a PKMζ inhibitor called ZIP into the brain's cortex a few days after, while memory consolidation might still be in progress, blocked the formation of long-term memories. The rats would happily drink saccharine-laced water, despite the earlier bout of nausea.

This was all pretty consistent with past results. But then the authors performed an experiment that I can only assume was expected to act as a control: they injected ZIP at one week and at 25 days. The surprising result was that these later injections worked just as well as the earlier one had. By 25 days, a memory is generally considered as permanent as they get, yet a dose of ZIP in the cortex erased the averse association from the rats' minds. Testing for several weeks afterwards suggested that, once gone, these memories never come back.

Source

This study was done with rats, so the effect such PKMzeta inhibition might have on humans is difficult to predict. Not all memories in rats were susceptible to the ZIP treatment, so one should probably not make too much of these results until confirmatory and clarification studies are done.

The mechanisms of human memory no doubt share much with rat memory mechanisms, but it is likely that with human brains utilising such a large neocortex, human memory is far more sophisticated--probably with redundancies and backups that rats lack.