How Your Brain Remembersand Forgets

Memory storage in the brain is not like a videotape that we can wind or unwind at will. New learning, old information, and the links between them are constantly formed and destroyed in a dynamic process.

Implicit versus Explicit Memory

Memory can be classified into implicit and explicit categories. When you open your car door, turn on the ignition, and start driving, do you actually make a conscious effort to remember how to perform this sequence of actions? Of course not. The memory of how to drive a car is already hardwired and automatic, and you usually don't need to pay any attention to it. The memory of how to drive a car required conscious "explicit" mental effort when you first took driving lessons, but it is now "implicit" or automatic. This "macro" memory with many hardwired components has room for flexibility—for example, when you drive a rental car or a friend's automobile. It takes you a couple of moments to adjust to the new vehicle, to identify the positioning of the dashboard and driving controls, but soon you get the hang of it and you're off without a care in the world. But your macro memory of how to drive a car cannot make huge shifts, as any automobile driver who tries to ride a motorbike for the first time can testify. So the nerve cells that store this information are not made of concrete or steel, but neither are they like a bowl of jelly—maybe more like a hard lump of Plasticine that changes its shape only with considerable force.

Skills and habits come under implicit memory. Classical conditioning and other types of memory, which also fall into this implicit category, are related to simple reflex reactions, for example, jumping away when touching a hot object, that we execute automatically in our everyday lives. But when you think of "memory, " you probably think of something else altogether: discrete events, like recalling someone's birthday or where you went on vacation a few years ago. This type of memory is called "episodic" or "event-related" or "explicit" memory. You have to make a conscious effort to retrieve the explicit memory of a fact or event, unlike the implicit memory of knowing how to drive a car. In this book, I generally use the word memory as it is commonly understood: explicit memory of both short-term and long-term specific events.

For explicit memory, there are three elements to the sequence of remembering:

1. Acquiring information. Attention and concentration are key.

2. Storing the event or episode as a memory. Importance, meaning, and emotional impact of the event determine if the brain will store it as a memory

3. Retrieval. This is the active process of bringing the memory into the forefront of consciousness.

Facts about the Human Brain

It makes up 2 percent of body weight.

It consumes 25 percent of the body's glucose and oxygen for its energy needs.

It contains around 100 billion neurons, also called nerve cells.

Each neuron communicates via chemical messengers with hundreds of other nerve cells.

The brain may contain up to 60 trillion pieces of memory.

Mainly short-term, and some long-term, memories are located in the hippocampal and other parts of the temporal lobe.

Many long-term memories have migrated from the hippocampus to reside in the frontal lobe.

Loss of nerve cells in the temporal lobe, and in parts of the frontal lobe, leads to memory loss.

Short-Term versus Long-Term Memory

Explicit memory can be short-term (seconds to hours) or long-term (days to years). short-term memory has limited capacity, but long-term memory has a lot of available disk space to store data. A simple example illustrates this point: you may find it difficult to repeat more than two new telephone numbers recited consecutively to you, but you can easily recall several telephone numbers that are in long-term storage in your brain. This simple fact tells you that the mechanisms by which the brain stores short-term and long-term memories must be different.

Memory Modulation Human Brain

Figure 1. The human brain seen through the midline (as if sliced through the middle of the face to the middle of the back of the head).

Figure 1. The human brain seen through the midline (as if sliced through the middle of the face to the middle of the back of the head).

Hippocampus: Grand Central Station of Memory

The hippocampus is a wing-shaped, inch-long structure that makes up the inner part of the temporal lobe. The temporal lobe is a bigger structure, the size of a large kiwi fruit or oblong plum, that projects from the lower front part of the brain and lies just beneath the side, or temple, of the forehead. The brain is divided into two big halves, so the right hippocampus is part of the right temporal lobe, and the left hippocampus is part of the left temporal lobe.

Nerve impulses from our senses first pass through a filter that screens the information and ignores what is unimportant. if the information survives this first gauntlet, it is sent via nerve cells to the hippocampus and surrounding regions. Each specialized neuron in the hippocampus records an element of the fact or event, and these nerve cells link all the components together to form a composite memory trace. This memory trace is housed in thousands of nerve cells, probably in proteins and ribonucleic acids (RNA).

How Short-Term Converts to Long-Term Memory if the memory is important enough, or if the same event repeats many times over a long period, the short-term memory trace residing within these hippocampal nerve cells is eventually moved into permanent, long-term storage. The hippocampus has broad-band connections—fiber optic rather than regular copper wire—to the frontal lobes, where many long-term memories are stored (some long-term memories remain in the hippocampus).

The Web of Memory

Each memory is a complex web of material that mixes facts, sensations, and emotions. When a strong emotion accompanies an event, you release more of the chemical transmitters that communicate among nerve cells to help form memories. Emotional states represent an important "internal'' environmental cue for memory. Think of the emotion-laden memories that flooded through your mind at your graduation, your wedding, when you had major conflicts with family members, or when you lost someone close to you. These memories stay hardwired forever in your brain, ready to be recalled whenever the occasion arises. On the other hand, you remember only fragments of less important and less emotional events, such as the details of a boring business trip or meeting; the threads of the spider's web have broken because of lack of interest and disuse.

Over time, long-term memory tends to get pushed from consciousness into the subconscious. Then a simple cue, an odd association, a chance meeting, can activate the sleeping spider's web and fire the neuronal circuits, resurrecting the long-term memory that had seemingly evaporated from your mind.

Your Brain Is Plastic

If your skin is cut superficially, it heals within a few days. Many other organs in the body can also repair themselves: new cells are generated by cells that divide and reproduce in response to injury. Unfortunately, while brain cells do grow and specialize during infancy and childhood, by the time we become adults nearly all of them lose the capacity to divide and reproduce. And yet we know that our brains are constantly changing: we learn throughout our lives, we have a range of reactions that we can modulate in response to other people, places, and even time itself. So how do we explain this contradiction: the brain creates no new nerve cells but has great flexibility? The answer lies in the revolutionary new finding of brain plasticity.

Dr. Eric Kandel, a Nobel Prize winner, works a few floors above my office in the same research institute. For several decades, Kandel has studied a species of snail called aplysia, which looks like a small black blob with ears. Aplysia has large nerve cells that lend themselves to experiments. Kandel's groundbreaking studies have shown that many nerve cells in aplysia, and in more complex species, retain the property of plasticity, which means that they can change their structure or function over time. The nerve cells do this by sprouting new branches called dendrites and forming contacts with other nerve cells to compensate for those that have been lost. Using a different approach, Bruce McEwen's laboratory at Rockefeller University demonstrated what was once thought to be impossible: plasticity and regeneration of nerve cells in the hippocampus in animal studies.

As an analogy, we know that people who are born blind develop an exquisite sense of touch and hearing. For those who constantly use Braille and become expert at it, the brain region responsible for controlling the one finger used for reading physically grows in size. This type of compensation may also occur following memory loss, depending on the cause.

Are There Limits to Your Memory?

is there a limit to how much you can remember? Off hand, we all know that we can teach an old dog new tricks. Millions of people who are past their so-called prime are able to take college courses and graduate with advanced degrees. But if there are distinct brain regions for each subtype of memory— proper nouns versus other nouns, for example—don't these bookshelves get saturated over a lifetime of exposure to thousands of pieces of information that make up our brain libraries? And if these brain regions get saturated, how can a middle-aged or older person still have the capacity to learn whole new languages and technologies?

The answer is simple: memory is a dynamic, not a static, process. There are several ways by which your memory storage keeps expanding:

1. As you learn more, and learn more efficiently, the nerve cells responsible for memory develop new tricks and become more expert at importing new knowledge into the available nerve cells.

2. Few of us have taxed our memories to such an extent that all the memory nerve cells are clogged up and overflowing with knowledge, though if you're a quiz or game show expert you may come fairly close.

3. Finally, there is an obvious solution when you need more memory: drag the useless stuff to the trash, choose "empty trash" from the pop-up menu, and a few megabytes of memory storage immediately open up in your brain.


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