1. What is Memory?

Memory is the ability to store and retrieve information. You use memory whenever you greet an old friend by name, discuss a book you’ve recently read, or dial the phone without first looking up the number. Memory allows us to recall who we are, where we are, and where we’ve come from.

Some people are naturally better than others at remembering, but how they got that way stems from two separate factors. Genetics accounts for about half of our memory—some people may be born with the ability to easily retain what they learn, while others may have to work harder. Environment shapes the other half . Diet, education, and the way we care for our health work synergistically to determine how well our brain functions. If you weren’t blessed with a strong memory, there is evidence that mental activity, social involvement, and physical exercise may help you improve your memory performance.

How Memory Works

Picture yourself driving along the highway, when you see a colorful hot-air balloon taking off from a field nearby. It’s such an unexpected site that you want to make sure to tell your grandchildren about it when you see them at dinner that evening.

In order to describe the event to your grandchildren, you have to create a memory first. Your eyes and ears have gathered information about the balloon, from the colors and size of the balloon to the sound of the burners filling it with hot air. The memory process is beginning.

Memory has three distinct phases: encoding, storage, and retrieval. As you drove past the balloon, your eyes and ears were transmitting information via neurons to the regions of your brain that are associated with sight and sound. The impulses sped from one nerve cell to another across tiny gaps called synapses. Each of the individual cues was combined into a single memory in a region of your brain called the hippocampus (see Box 1-1, “The Brain’s Memory Centers”). This process is known as encoding.

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Next came the storage phase of the process. Your hippocampus sent the memory to the cerebral cortex, a kind of permanent file cabinet.

Mechanisms for Memory

There are two distinct mechanisms to memory storage: short-term (working) memory and long-term memory. Short-term memory is the information your mind stores for immediate recall. Your short-term memory can hold small amounts of information for limited periods of time. If you drove home and called your grandchildren to tell them about the balloon lift-off, you would have been using your short-term memory.

Long-term memory involves retaining information for days, months, or years. When you recall the balloon incident at a high school reunion six months later, you are using your long-term memory. The brain can store almost limitless amounts of long-term memory, which is why we can learn so many new things and retain what we’ve learned for many years.

Long-term memories come in different forms. The name of your first pet, the sum of two plus two, and the words to “Happy Birthday” are semantic memories. They are general facts. Episodic memories capture the sights, sounds, and emotions of specific places and events: the excitement you felt as you embarked on your first camping trip, the smell of your mother’s favorite perfume, or the love you experienced when your first child was born. Procedural memories are the skills you’ve picked up throughout your life, such as writing in cursive, driving a car, and brushing your teeth.

Recalling a Memory

Imagine that it is dinnertime. With your family surrounding you at the table, you’re ready to recount the story of the balloon rising up near the highway. Now the retrieval process begins. When you retrieve information, you are literally pulling it from nerve pathways. You remember information as your brain reactivates the same pathway that was originally triggered when you stored the memory. This process can be fast or slow, depending on how familiar you are with the information, and how well you learned it in the first place.

Memories can be retrieved via one of two processes: recall or recognition. In this case, you’ll probably use recall to remember the balloon. Recall involves directly accessing the memory. You also use recall when you remember the name of a movie you saw a week ago. Recognition, on the other hand, uses cues to help you retrieve a memory. When you were in school, you used recognition every time you took a multiple-choice test.

Studying How Normal Memory Works

Through the use of new technologies such as specialized scanning equipment, scientists are better able to study how the brain works. Many scientists believe that a memory is formed when a brief pattern of electrical impulses moves through a network of neurons, strengthening connections between the affected brain cells. This leaves a “memory trace” in the brain, which is revived when the information is later recalled. The brain has many different areas, each of which specializes in different types of information.

Researchers now know that certain attributes of a memory are grouped with other, similar recollections; for example, the smell of roasted peanuts may be grouped together with memories of watching baseball games as a kid. When you recall information, your brain cross-references the many different attributes of that memory. Using techniques like functional magnetic resonance imaging (fMRI), researchers such as Randy Buckner, PhD, professor of psychology and neuroscience at the Massachusetts General Hospital Center for Brain Science, have been able to map the connectivity between different brain regions. Investigators have also been able to capture brain activity that indicates where certain memories are created, and what the likelihood is that someone will be able to recall that information.

How Memories Are Lost

An area of the brain called the perirhinal cortex is thought to help form memories based on detailed depictions of objects. The complexity of these depictions makes it possible to distinguish the memory of one object from the memory of another object with similar characteristics. Memory loss has long been thought to be associated with forgetting previously encountered experiences, people, or objects. The memories were believed to be lost or inaccessible.

But research by scientists at MGH suggests that in Alzheimer’s disease the memories may still be intact, but are becoming confused with other memories. These scientists believe that AD and other memory disorders may not destroy memories altogether, but that they may instead impair a person’s ability to distinguish one memory from another.

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