What would we be without our memories? If you didn’t remember where you’d been, or those you cared about, would you still be who you are now? What if you forgot how you normally respond to others, everything you’ve learned in school or what you’ve learned to do throughout your life?
Our ability to remember and learn is among the most fundamental and significant of our brain’s abilities. Not only does the brain permit us to experience everything around us, but it also allows us to re-experience our past. Moreover, it does this in several ways, using distinct types of memory.
What was the best thing that happened to you today? That’s an example of autobiographical or episodic memory, when we remember something in context, like remembering the morning’s coffee. It is distinguished from eidetic memory, a memory of facts disconnected from experience, such as the knowledge that Paris is the capital of France. Your ability to read this is based on another type of memory called procedural memory—the sort of memory that makes remembering how to do something “like riding a bike.”
Memory can be even further subdivided—for example, working memory allows you to hold something in mind for a few seconds at a time, then release, like a phone number you need to dial immediately and never again. Short-term memory lasts longer, perhaps an hour or so, and long-term memory can last a lifetime.
The divisions of these memories often blur in actuality but provide a framework for understanding how the brain remembers.
The Hippocampal Formation and Limbic System
A famous 1950s surgical misadventure spurred much of our knowledge about memory formation. H.M. was a young man with seizures coming from his medial temporal lobes that led doctors to remove them both. The result was something much like the movie Memento, in which the protagonist can only recall a few minutes at a time. H.M.’s memories before the surgery remained intact until his death, even though the physicians with whom he worked after the accident necessarily re-introduced themselves hundreds of times.
The medial temporal lobes contain the hippocampus, a brain structure with an elaborate S-shaped curve that inspired imaginative pathologists to name it after the Greek for “sea horse.” Within the curves of the hippocampus are different neurons folded over on each other, working together to cement the foundations of new memories.
While the role of the hippocampus in memory is well known, it is only part of a network that extends over practically the entire brain. Both very-long and very short-term memories can exist quite well without the hippocampus and nearby structures, as evidenced by some of H.M.’s retained abilities. Without the hippocampus and related structures, however, most new memories cannot last.
The hippocampus does not work alone, but as part of a neural network, well studied by medical students, called the Papez circuit. This includes the hippocampus, mammillary bodies (two small structures near the brainstem), parts of the thalamus, and the cingulate cortex. Other parts of the brain, such as the basal forebrain, play a role in memory. The basal forebrain sends acetylcholine to the cerebral cortex. These projections are damaged in Alzheimer’s disease—medications such as Aricept work by increasing acetylcholine levels.
The Cerebral Cortex
While the hippocampus and limbic system are critical in memory formation, those memories are ultimately stored throughout the cortex. Furthermore, the rest of the brain is involved with strategies for learning and recall, as well as attention, all of which are critical for effective learning and memorization.
Working memory is a form of memory that holds information just long enough to either use it or store it for later. This has been shown to depend on circuitry involving the frontal and parietal lobes. Injury to these regions can lead to difficulty keeping something in mind long enough to begin the initial stage of memorization, known as encoding. Encoding involves working with the hippocampus to organize and select which information should be stored more permanently.
In addition to encoding, the cortex can be involved with pulling memories out of storage in a process called retrieval. It is possible for someone to have problems with memory retrieval even if encoding was done properly. For example, most of us have had the experience of struggling to remember something only to have it pop into our minds later. Sometimes the wrong information may be retrieved, as in confabulation, where it seems that someone is lying about their past, though they earnestly believe the false memory.
Disorders of Memory
Different disorders of memory impact different regions of the brain. Alzheimer’s disease, for example, classically damages the hippocampus, resulting in difficulty forming new memories but no initial problem with memories that have already been stored. Frontal traumatic brain injury may lead to difficulty with working memory, meaning that it is difficult to keep information in mind long enough for it to be encoded. Once remembered, however, the information is more likely to stay, though some difficulty with retrieval may also be present.