Facts about human memory

Memory can be a playful thing. It collects minute details from childhood events, yet leaves us wondering where we left our keys.

There are several types of memories, and the brain has a unique way of forgetting each kind. Psychologists have classified various ways by which we forget, and biologists have studied forgetting mechanisms at the cellular level.

They've found that forgetting is normal, and actually vital to how the brain works. Here is a look at the strange facts about how people forget things.

How doorways destroy memory

In one common but mysterious short-term memory failure, people find themselves in a room, without remembering why they ended up there. Researchers say, in these circumstances, the doorway may be to blame. The very act of walking through a doorway may hint to the brain that a new scene has started and it should store prior memories away, thereby causing strange memory lapses. [Top 10 Mysteries of the Mind]

"Entering or exiting through a doorway serves as an 'event boundary' in the mind, which separates episodes of activity and files them away, " Gabriel Radvansky, a psychologist at the University of Notre Dame, told Live Science in a 2011 interview. "Recalling the decision or activity that was made in a different room is difficult because it has been compartmentalized."

Mind-erasing activities

Although rare, certain activities can result in a temporary memory loss and brain fog, called transient global amnesia. For example, sex has been reported to cause this memory problem, with patients forgetting the past day or so, and having difficulty forming new memories.

People with transient global amnesia suffer no serious side effects, and the memory problems usually disappear in a few hours. But it's not clear how this happens, and brain scans of patients who have had this type of amnesia show no signs of damage to the brain, or signs of stroke.

Memories can live on, even if we can't access them

Could forgotten songs continue to live on inside our heads, without us knowing?

In a 2013 report of a strange case in the journal Frontiers in Neurology, researchers described a woman who had musical hallucinations of song that she didn't recognize, but others did.

"To our knowledge, this is the first report of musical hallucinations of non-recognizable songs that were recognized by others in the patient's environment, " the researchers wrote.

The scientists said the woman had likely known the song at some point, but forgot it. The case raises the question of what happens to forgotten memories, they said, and suggests that memories can be stored in some form in the brain that renders them accessible, and yet unrecognizable.

It is possible that the woman had fragmented preservation of musical memories, with key portions of those memories lost. As a result, she couldn't recognize those memories, the researchers said.

Brains may be programmed to forget infancy

Our earliest childhood memories fade, and there's likely a reason for that, researchers say. Most often, people don't recall any memories from their earliest years of life, usually before age 3 or 4. This is called infantile amnesia.