Have you ever had any unpleasant memories? What would you say to having them permanently erased from your brain? Well, scientists did just that with rats. They trained rats to fear two different sounds by sending a mild electric shock through the rats' paws when they played the sounds. This Pavlovian technique resulted in the rats freezing up in nervous anticipation of the shocks every time they heard either sound. Scientists wanted to know whether they could erase one of the memories without erasing the other one, even though both sounds result in the same fear.

It turned out that they could.  You see, long-term memories aren't nearly as solid as we like to think. They're actually very fragile. Every time we recall a long-term memory, it becomes malleable. Just before playing one of the sounds, scientists injected half the rats with a chemical that interferes with memory reconsolidation. Thus, when they were in the process of remembering what accompanies the sound, the drug went to work.

When they played both sounds a day later, the rats that had been injected with the chemical did not freeze up when they heard the sound that had accompanied the drug. They froze as usual when they heard the second sound; they had forgotten to be afraid.

If the technique can be made to work on humans, such as people with post-traumatic stress disorder, selective memory erasure could prove very useful.