You can picture the long-ago scene perfectly: The waiter opens your bottle of champagne with the familiar — yet always startling — pop. The bubbles tickle your nose as you sniff the effervescent liquid. You raise your glass as you look into the eyes of your spouse. You see pupils dilate as those eyes look at you in return. "Happy anniversary," you say, "to the love of my life." This is episodic memory in action.
Episodic memory allows you to mentally time-travel back to an episode of your life and relive it in vivid detail. You also use episodic memory to remember the name of someone you recently met at a party. It enables you to remember to take a detour because there is construction along your usual route. In fact, most of the time when you speak about "memory," you are referring to episodic memory, which involves several parts of the brain.
The hippocampus is crucial for episodic memory
If you drew a line between your ears you would pass through the most critical structure for episodic memory. The hippocampus looks somewhat like a seahorse with a head, body, and tail. It is always turned on, recording thoughts, feelings, perceptions, and sensations that arise from other regions of the brain. One part of the hippocampus binds these disparate aspects of experience into a coherent whole. Another part tags it with an index that will allow the memory to be retrieved minutes, hours, days, or years later.
Learning and retrieving information: the frontal lobes
What you pay attention to determines what you will remember. If you are watching your favorite television show and your spouse walks in and gives you a verbal to-do list, you will have difficulty remembering the list if your attention was focused on the television. You can, however, use your frontal lobes to focus your attention. Located behind your forehead, the frontal lobes also enable us to voluntarily retrieve memories. In fact, when you are searching for a specific memory, it is your frontal lobes that are doing the searching.
Trying to remember whether you learned that medical information from a Harvard Health Blog post or a supermarket tabloid? The frontal lobes also help you remember the source and context of information that you learn.
Providing context: the parietal lobes
Have you had the "aha!" experience where you suddenly recall the information you're looking for — such as the name of a friend who is walking toward you? The conscious recollection of episodic memory comes from the parietal lobes, located in the top, back part of the brain.
Episodic memory: left brain versus right brain
You have two hippocampi, frontal lobes, and parietal lobes, one on each side. The left-brain system is specialized for words and language. The right-brain system is particularly good at remembering non-linguistic information including images, body language, and tone of voice. So, when you recall a conversation with your friend, your left hippocampus remembers the words that were spoken, and your right hippocampus remembers how they were spoken, your friend's face, and the emotion conveyed.
Aspects of episodic memory decline in normal aging
One reason it is useful to know about the different parts of the episodic memory system is that frontal lobe functions — such as learning, searching, and ability to recall source — tend to decline in normal aging. For this reason, it's normal for people to notice three changes in episodic memory as they age:
- Because learning diminishes, information may need to be repeated a couple of times in order to get it into the hippocampus so it can be remembered.
- Because the search process slows, it may take more time or a hint or a cue to retrieve a memory.
- Because the ability to judge source declines, it may be more common to experience trouble recalling where we learned information.
In normal aging, however, once information is learned, a person should be able to retrieve it — even if it takes a bit of time, or a hint or cue. By contrast, if a person cannot retrieve learned information, this suggests some problem in addition to normal aging is present. In future blogs I will discuss what happens to episodic memory in disorders of aging, such as Alzheimer's disease.