Memory – Let’s Not Forget About It

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?

What is memory? There are different kinds of memory. There is working memory, which is in your immediate awareness and temporary storage as when someone says something to you or tells you a phone number. This is what psychologists mean by short-term memory. This memory can only contain a little information – 7 plus or minus two bits of information. Also, it only lasts for 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. Then it becomes recent memory, which is what psychologists refer to as “long-term memory” or what I prefer to call retentive memory, because the information is now retained in storage. Later we will discuss certain strategies you can use to place working memory data into storage so that it doesn’t vanish in seconds. In contrast to recent memory, remote memory refers to memories of things not recent – like the vacation we took 20 years ago, or childhood recollections. In many brain diseases, such as Alzheimer’s, remote memory is unaffected until the disease advances. In contrast, brain diseases usually attack recent memory first. They increasingly prevent you from storing new information. This is why people sometimes complain, “I can remember so many details of my childhood but not what I had for breakfast this morning!”

But why are remote memories so strong in the face of many types of brain damage? The answer is because these memories were stored by a healthy brain with healthy memory circuits. Retentive (recent) memory is the most fragile kind of memory because the storage process is easily damaged.

Imagine the brain’s memory system 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 can become inaccessible. The most easily accessible papers are the ones that were filed by an organized file clerk. The least accessible papers are the ones that never reached the file cabinet. Randomly filed papers might be difficult to access even though they are stored.

In some diseases, like Alzheimer’s, the problem is poor storage. To use our file cabinet analogy, almost all of the papers are accidentally dropped on the floor, so few of them are filed. In other diseases, including traumatic brain injury (TBI) and Parkinson’s disease, the problem is disorganized storage. The papers are stored randomly inside the cabinet. It is then difficult to retrieve the stored information. The difference occurs because these conditions typically affect different brain areas. Alzheimer’s disease 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 named Henry Molaison (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 it prevented him from storing new information in long-term memory. He couldn’t remember new conversations, new events, meeting with people, or succeeding with countless memory 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. We learned from these studies the difference between short- and long-term memory and the critical role of the hippocampus and a nearby structure (amygdala) for storing many new memories.

The human brain is largely composed of fat and water, but it has about 100 billion neural cells and is amazingly complex. Memory is also complex. The two hippocampi are only one part of the memory circuitry. There is much more! Each hippocampus is connected to an amygdala (involved in emotional 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 – thiamine 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) is involved in attentional focus and helps to store and retrieve new information. It is often damaged in severe head trauma causing a TBI). The result is that new information is not stored in an organized way, making it even harder to retrieve later.

So far we’ve examined working memory, retentive memory, and remote memory, but we haven’t discussed other important kinds of memory and normal age-related forgetfulness. Why do normal people have such vivid recall of early life events, but sometimes forget where they just put their car keys, or why they just entered a room? I also haven’t addressed steps we can take to resolve these problems. No, I haven’t forgotten! Next time.