Memory problems are among the most common complaints people make to their doctors. What is memory? How does it work? Why does it sometimes NOT work? What can I do about it?
Memory is known as the mental capacity of retaining and reviving information from previous experiences. However, there are different kinds of memory. There is working memory, recent or retentive memory and remote memory.
Working memory, also known as short-term memory, is in your immediate awareness and temporary storage. An example of working memory is remembering a phone number. It only lasts for a few seconds unless you store it! In your typical day, you are bombarded with over a million bits of information, but only a very small part of it is stored. Your frontal lobes (prefrontal cortex) tells you when you need to store parts of this incoming data.
The next type is recent memory or what I prefer to call “retentive memory” because this information is now retained in storage. For example, if the phone number you were provided with was a close friend or family members, you would be more likely to store this information. In contrast to recent memory, remote memory refers to memories of things that are not recent -- like the vacation you took years ago, or even childhood recollections. In many brain diseases, such as Alzheimer’s, remote memory is unaffected until the disease advances.
In contrast, brain diseases often attack recent memory first. They increasingly prevent you from storing new information. This is why people sometimes complain, “I can remember many details of my childhood but not what I had for breakfast this morning!” So why are remote memories so strong in the face of many types of brain damage? Because these memories were stored by a healthy brain with healthy memory circuits. Retentive memory is the most fragile kind of memory; the storage process is easily damaged.
Imagine the brain being like a file cabinet. When papers are filed properly, they are easy to access. But if the file clerk starts filing papers randomly and even drops pages on the floor, these papers will be inaccessible. The only accessible papers are the ones that were filed by an organized file clerk.
In some diseases, like Alzheimer’s, the problem is poor storage. Using our file cabinet analogy, almost all of the papers are accidentally dropped on the floor, so almost nothing gets filed. In other diseases, including traumatic brain injuries and Parkinson’s disease, the problem is disorganized storage. Then it is difficult to retrieve the stored information. The difference occurs because these conditions typically affect different areas of the brain. Alzheimer’s initially destroys neurons in a small area (about half the size of a dime) on each side of the brain called the hippocampus. This is an area that is critical to changing working memory into retentive memory. We learned this in a famous historical case of a 27-year old man (referred to as “HM”) with constant seizures who had his hippocampus on both sides surgically removed. The operation was successful in reducing his seizures, but prevented him from storing new memories. He couldn’t remember new conversations, new events, meeting with people, or doing countless psychology experiments. He could do the same crossword puzzle repeatedly with the enjoyment of a totally new challenge each time. “HM” was mentioned in nearly 12,000 scientific journal articles and this taught us the value of the hippocampus and a nearby structure (amygdala) for storing many new memories.
Did you know the human brain is largely composed of fat and water? How about that it has 100 billion neural cells that are amazingly complex? Memory, just like the brain, is also complex. So how does it really work?
The two hippocampi (remember this is the area that changes short term memory to retentive memory where it stores information) are only one part of the memory circuitry. Each hippocampus is connected to an amygdala (involved in fear reactions), to the thalamus and to the mammillary bodies. The hippocampus is also attached to the uncus, which is involved in our sense of smell. You might notice how certain fragrances and scents are powerfully ingrained in your memory. Hippocampi are initially attacked by Alzheimer’s disease, but other conditions (such as B1 deficiency in alcoholism) destroy parts of the thalamus and mammillary bodies, also causing severe problems with memory storage. These areas can also be damaged by stroke, tumor, infection, abscess and other types of brain lesions. The frontal part of the brain (prefrontal cortex) helps to store and retrieve stored memories. It is often damaged in severe head trauma. The result is new information cannot be stored in an organized way and it’s even harder to retrieve.
Regardless of what type of memory you may have problems with, if memory is a concern, it’s important to ask your primary care physician if you need further evaluation.
To learn more about Neuropsychology at Tallahassee Memorial HealthCare, please call 850-431-5001 or visit TMH.ORG/Neuro.