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Why Do We Forget?

Timothy Brady, a cognitive neuroscientist at the Massachusetts Institute of Technology, answers

When a person loses his sense of smell, does he also lose any memory associated with a smell?
—Ana Artega, via e-mail

David Smith, a professor of psychology and a researcher at the Center for Smell and Taste at the University of Florida, replies:

NORMALL PEOPLE CAN DETECT A CACOPHONY of odors using the 40 million olfactory receptor neurons that reside in the nasal cavity. When we encounter a new odor, these neurons send information about the whiff to a brain area called the olfactory cortex, leaving an imprint of the smell there. These memories accumulate over time to create a library of odors. Although we do not fully understand how the olfactory cortex encodes these
memories, we do know that olfactory memories seem to be particularly rich—perhaps because the olfactory cortex is closely connected to the brain regions important for recollection. These areas include the amygdala, which processes emotions, and the hippocampus, which encodes and stores memories.

Damage to the olfactory receptor neurons because of a respiratory infection, a head injury or a neurodegenerative disease can disrupt the brain’s ability to process different smells. When olfactory neurons stop working altogether, a person develops anosmia, or the inability to discern odors. According to a 2008 report from the National Institutes of Health, 1 to 2 percent of the U.S. population younger than 65 years old, and more than half older than 65, have almost completely lost their sense of smell.

Smell (or olfactory) memory refers to the ability to recognize different odors in your environment. Some olfactory memories are unconscious—for instance, you may recognize a rose’s fragrance without remembering when you first encountered one. Other olfactory memories are conscious: they revive specific scenarios or emotions from the past. For instance, the musky smell of a friend’s apartment may remind you of your husband’s cologne.

Losing one’s sense of smell may not mean forgetting what things smell like. Even without the ability to detect odors in the environment, you may be able to imagine the nutty aroma of coffee brewing in the morning. And your memories associated with coffee may not be lost, either: you will probably still be able to recall the first time you tasted the bitter brew.

According to a 2008 study from the Journal of the Alzheimer’s Association, however, it is sometimes possible to lose the memories associated with smells. Neurodegenerative diseases such as Alzheimer’s and Parkinson’s can decrease a person’s sensitivity to certain smells as well as diminish the memories associated with these odors.

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.

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