What if you could choose which memories and associations to retain and which to discard? Using that capability rationally (whatever that word means to you) would be a significant challenge -- and that challenge has just come one step closer to being a reality.

Dr. Fenton had already devised a clever way to teach animals strong memories for where things are located. He teaches them to move around a small chamber to avoid a mild electric shock to their feet. Once the animals learn, they do not forget. Placed back in the chamber a day later, even a month later, they quickly remember how to avoid the shock and do so.

But when injected — directly into their brain — with a drug called ZIP that interferes with PKMzeta, they are back to square one, almost immediately. “When we first saw this happen, I had grad students throwing their hands up in the air, yelling,” Dr. Fenton said. “Well, we needed a lot more than that” one study.

They now have it. Dr. Fenton’s lab repeated the experiment, in various ways...