How We Forget

(HealthScout) -- We all know it's possible to forget a grudge and even that first love we swore we'd remember forever -- and now new research shows us how and why.

Scientists in Israel illustrate it this way: Say you get so sick from something you eat that you swear you'll never eat it again. But time passes, and you break down and eat it again, only this time you don't get sick, so you stop worrying about eating that food.

The scientists call the process memory extinction, and it makes us forget old memories when they are no longer relevant.

Researchers found the process occurs on a cellular level within the brain.

"This study opens up a new chapter in the field of memory retrieval," says lead author Yadin Dudai, because memories could be manipulated if scientists understood how they were saved in the first place.

Each memory event causes a host of molecular changes, and researchers observing rats found the process of learning things for the first time is different from learning them the second or third time around.

"When we learn something for the first time, or when we learn anew, the brain distinguishes between those events on the molecular level," says Dudai, a professor of neurobiology at the Weizmann Institute of Science in Rehovot, Israel.

Relearning something and then forgetting it occur in very similar ways in the brain, says the study.

"If an association doesn't hold, we tend to forget the memory, and the biological mechanisms for forgetting in this way are very similar to learning something anew," says Dudai. "We don't really obliterate the memory, but we change the memories in our minds."

The Israeli scientists conditioned rats to avoid a particular food by coating it with a substance that made them feel nauseous. After being repeatedly offered the food for several days, they started to eat it again. This time they suffered no ill effects because the food wasn't treated.

The study appears in tomorrow's issue of Science.

"It's possible for animals to learn and unlearn conditioned taste aversion using overlapping, but different [biological] mechanisms," says Craig Kinsley, an associate professor of psychology at the University of Richmond in Virginia. "This enables the animal to override initial learning in favor of memory extinction."

Humans go through this process as well, Kinsley says.

"People develop taste aversion all the time, but you can also learn that even though the food made you sick once, it won't necessarily again," he says.

He says this type of mechanism could be the basis of some relationships; for example, when people keep going back to the same person, even though they know they shouldn't.

To learn more about how memories are made, check this MSNBC article, or this ABC News story that examines how even birds store memories in a way similar to humans.

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