Why do we forget?
—Brian Qiu, Plainsboro, N.J.
Timothy Brady, a cognitive neuroscientist at the Massachusetts Institute of Technology, answers:
although the human brain has an impressive amount of storage space for memories, it does not keep each one indefinitely. We tend to forget memories that are similar to one another—remembering instead more novel events or information. In fact, forgetting is important because it makes it easier to recall new memories.
In a recent study my colleagues and I showed people 2,800 pictures of common objects, such as backpacks and toasters, for three seconds apiece. Later, we showed them hundreds of pairs of images and asked which of the pair they had seen already. We were testing their memory for details; for instance, asking if they had seen a picture of bread topped with sesame or poppy seeds. The volunteers remembered the correct picture 78 percent of the time when they had seen only one item of that type (for example, one kind of bread). When they saw many similar objects, however—say, 16 hats—they were more likely to forget the identifying details, remembering the correct item in the pair only 64 percent of the time.
Although forgetting can be annoying, it sometimes helps us learn. In 2007 researchers at Columbia University showed that genetically modified mice that cannot generate new neurons in the hippocampus—a brain area involved in storing memories—do better on memory tasks than mice that create new neurons as usual. Learning new information does not require new neurons; it simply requires that existing neurons connect in new ways.
Yet storing a memory does require the ability to sprout new neurons. Thus, the genetically modified mice could still learn new information, like the most recent location of food in the maze, but had no old memories of where food was hidden interfering with their most recent one. Forgetting, then, helps us remember.