First, it's important to note that, barring a traumatic brain injury, illness or other extraordinary event, there are some things we don't forget. We don't forget how to walk or how to lift our arms, and that's because these abilities aren't coded in the parts of the brain that we traditionally associate with conscious memory. They're deeper in the brain, associated with the hippocampus, nervous system and other regions.
But when we talk about why we forget, we're really considering why some things enter our conscious memory and eventually seem to leave, even if we had retained that knowledge for some time. So why do we forget, say, the name of our third grade science teacher or what we ate for dinner last Wednesday?
With so-called "working," or short-term, memory -- e.g., where you placed your wallet when you got home after work -- your brain generally focuses on the information that it deems most important. This is particularly the case if there's a lot going on around you. Your brain doesn't want to be overloaded, so if you place your wallet on a dresser while having an important conversation with your husband or wife, it's likely that your brain never retained that information at all. The brain also has a tendency to remove or update memories. For example, you're more likely to remember where you put your wallet today but not last week, in part because your brain has deemed that previous memory unimportant.
Contrast short-term memory with long-term memory. The latter provides a much deeper reservoir of knowledge and experience but often with less detail than working memory. However, some studies have found that our long-term memory can retain a surprising amount of detail [source: Vogel and Drew]. However, our memory of that detail may only emerge when we're challenged to do so -- for example, by being shown two very similar objects and asked which one seems familiar. In that way, the problem may not always be remembering information but rather accessing it. We know from everyday experience that memories can emerge in surprising ways, sometimes in response to unexpected stimuli, so if you think you've forgotten something, it may still be there, waiting for the proper cue.
If you forget something, such as where you placed your car keys, it may be because you did not encode it properly. In other words, you haven't really "forgotten" where you put them; instead, you may have never really registered it into your memory to begin with. Distractions that happen while you try to remember something is the main reason memories don't get encoded properly. For instance, if you try to read the paper in a busy airport, you most likely won't encode the material effectively and won't be able to remember much of it later.
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