How are memories made?

  Your memory of your last birthday began as a complex mix of sensory events that included a large variety of odors, sights, tastes, sounds, and emotions; these experiences were first processed by the specific areas of your brain that are responsible for processing each of these sensory inputs. Your sensory experiences then were funneled through a structure called the hippocampus, which is located within the temporal lobe of the brain. This part of your brain lies near your ears. The hippocampus is responsible for binding together the diverse sensory elements required to create vivid and coherent memories full of emotion.

  In addition, memories often get a specific time stamp associated with them. Let’s use the example of the memory you have of your last birthday. First, the hippocampus gathers all of the sensory aspects of the event and conducts some initial processing of these sensory elements into a neurological format that is not yet completely understood. After this initial phase is completed, the entire memory of your last birthday is distributed widely throughout various brain regions. Sensory memories initially are stored in regions of cortex responsible for processing the particular type of sensation, for example, sight, sound, or odors. Thus, the components of a memory of an event in your life are stored throughout your brain.

  Your memories are much more than just sensations. They also contain your feelings at the time (e.g., whether you were happy or sad). These are stored simultaneously in brain regions devoted to emotional memories (more on these brain regions in the next chapter). Ultimately, the complex sensory and emotional experience that was your birthday bash gets stored in many different brain regions; memories are not stored in just one place within the brain. By storing the information in many places your rich memory of your last birthday is far less likely to be lost due to injury or time. Due to the period of time that has passed since that birthday celebration, you might not be able to recall every aspect of your last birthday, but you always will be able to remember many of the most salient and emotional aspects of what happened.

  We use our hippocampus to remember past events, familiar objects, people, and places; we also use it to construct possible futures. After all, how different is anyone’s future from the past? For most of us, for most of our lives, tomorrow is often just a simple variation on yesterday and involves the same people and places. Some memories are stronger than others; indeed, some memories you might wish to forget. Strong memories usually have robust emotional components. On average, women retain more vivid emotional memories than men, particularly if those events have a strongly negative emotional component. Psychologists speculate that because women may tend to dwell on “memories of negative life experiences” more often than do men, women are diagnosed more often with depression. Memories that are consolidated in association with extremely unpleasant events can lead to post-traumatic stress disorder, a condition that is twice as common in women as in men.

  Do I need to pay attention?

  Memories form in a stepwise fashion in the brain. If you are paying attention to a particular sensory experience, usually because it has some importance to you at that moment, the collection of sensory inputs into your brain are temporarily stored as a short-term memory. Can you learn when you are not paying attention, or when you are bored or completely uninterested in what is happening around you? Yes, but not well, and certainly not efficiently, because these states produce interference with the memory consolidation process. The memory consolidation process usually occurs within the hippocampus and is rather unstable. It is easy for memories to be lost at this point by paying attention to other sensory inputs. We all have experienced the impact of distraction upon our ability to remember something. For example, most people find it difficult to concentrate or read a book in a noisy room. In contrast, if an experience is significant to you because it is associated with a strong emotion or has survival value, it is far more likely to be converted into a long-term memory that will last virtually indefinitely.

  Why do I forget some things but not others?

  Storing new memories, especially important memories, is easy, but the size of the brain is finite; it thus has a limited amount of storage space. You can easily grasp the problem: What to do with all of the information that flows into your brain every second of every day? The solution: delete the memories. Intentional forgetting, or deliberately erasing memories, plays just as vital a role in the brain as does remembering. The hippocampus automatically encodes all experience, yet the vast majority of our experiences are not remembered later. This forgetting is deliberate, but why does your brain intentionally forget things? Information bombards your senses every day, and your brain initially attempts to store this information just in case it is important. Subsequently, your brain wastes a considerable amount of storage space on useless information. For example, consider this scenario: Your roommate or spouse appears wearing a new shirt or blouse and you know immediately that it is new. How is this possible? Did you memorize his or her entire wardrobe? Apparently you did; otherwise you would not have noticed that the shirt or blouse is new. Clearly, we all waste a lot of brain space storing useless or unimportant information. As you will read later, our brain actively removes unwanted memories while we are sleeping; this process allows us to be smarter, and to learn information more critical to our survival, during the day.

  How are memories recalled?

  Your brain evolved to pay attention to everything that was novel because such knowledge might increase your chances of survival. Just because you have a specific memory stored somewhere in your brain, however, does not guarantee that you will be able to access it when required. Retrieving a memory is an active process that requires the brain to reproduce some component of the original memory trace in order to recall all of the essential pieces of the memory. Scientists speculate that the process of recalling a memory involves the restoration of the pattern of neuronal activations that was present during the encoding process. Recollection requires finding all of the component parts of a memory and then recombining all, or almost all, of the pieces together into a whole memory. This is a challenging task that your brain must accomplish in just a few seconds.

  The act of retrieving a memory appears to render the memory highly susceptible to modification. This is a critical feature of our brain: memories are not video/audio recordings of actual events. Memories are made of the pieces of the event or experience that we were paying attention to at the time. Thus, when you retrieve a memory, you are recalling all of its parts into consciousness. The process of recalling a memory leads to distortions resulting from the incorporation of misinformation into the memory. Then, when you are done with the memory, your brain stores it away again. What this means is that familiar memories are recalled, retold, embellished (intentionally or not), and then re-stored as the newly edited storyline. Usually, you are not aware that this distortion occurred! Each time a memory is recalled it is vulnerable to alteration.

  Do memories last forever?

  Unfortunately, no, they do not last forever, at least not perfectly. The manner in which memories are recorded and stored makes certain aspects of them quite unstable over time. Lawyers often take advantage of this susceptibility of memories to alteration and distortion; it is easy for them to catch a witness in an apparent lie by comparing the witness’s memories recorded immediately after an event with his or her recollection many months or years later. Test this out with your friends; ask them to remember where they were, what they were doing, or who they were with when they saw the Twin Towers in New York City collapse, or when they witnessed the space shuttle Challenger explode, or even (for you older readers) what they were doing when they learned that President Kennedy had been assassinated. Then compare their recollections with those of their friends who were (supposedly) with them during these tragic, emotionally charged events. You will quickly discover that they got their so-called facts wrong. They have retold and remembered the events surrounding these tragic days so many times that the or
iginal memory has been completely corrupted and altered. Our brains are not accurate recording devices; never depend upon them as such. Brains evolved to help you survive and procreate, not to record events with great detail. There was simply never any evolutionary pressure for our brains to record every detail of an event. The instability of making and recalling memories has numerous social and legal ramifications, such as the questionable value of eyewitness accounts, particularly for events that occurred in the distant past. Scientists recently have succeeded in implanting false memories in mice; that is, the mice behave as though they know something to be true even though the event never occurred. Social psychologists have demonstrated that humans are quite vulnerable to the implantation of false memories that are later recalled as “repressed memories” for experiences that never occurred. Your brain can very easily believe that specific events occurred, and discount all evidence to the contrary, and then fail to recall memories of events that you actually experienced. This is called amnesia.

  What is amnesia?

  Sometimes, old memories are not accessible for recall; this is called retrograde amnesia. Retrograde amnesia is memory loss for events that occurred before some kind of trauma or in response to degeneration of specific parts of the brain. Usually the amnesia is not comprehensive but only extends to events that occurred during the weeks or months before the incident. Because aspects of memories are stored in different brain regions, people with retrograde amnesia usually do not lose their entire memory store. Some aspects of the original memory are usually intact in these patients. For example, patients who have suffered a brain trauma that produced amnesia still remember many skills, such as how to walk, talk, and write, as well as many facts about the world, such as what a red light means or how to play with a yo-yo.

  The inability to form new memories is called anterograde amnesia and is often due to brain trauma or to a neurodegenerative disease such as Alzheimer’s disease. The symptoms can range from slowed learning to a complete inability to learn new things. Trauma or neurodegeneration usually results in the presence of both retrograde and anterograde amnesia with varying degrees of severity. For example, alcoholics usually spend all of their money on their addiction and, therefore, cannot afford to maintain a diet that contains all of the nutrients the brain needs to function; thus, a poor diet can lead to the degeneration of vulnerable brain regions. Ultimately, due to the degeneration of vulnerable regions in the temporal lobe and other nearby brain regions, alcoholics demonstrate both anterograde and retrograde amnesia in addition to mental confusion and personality changes. By studying how amnesia occurs, scientists have learned much about the biological mechanisms the brain uses to learn and remember.

  How does the brain create a memory?

  In order to understand how your brain makes a memory, you first need to learn about brain chemistry and the role specific chemicals play in the creation of a memory. First, you need to know about a chemical in the brain called acetylcholine. Acetylcholine is a neurotransmitter. A neurotransmitter is a chemical substance produced within neurons from components of the diet. There are many different neurotransmitters in the brain. They are released by neurons to diffuse into the extracellular environment in order to influence the behavior of nearby neurons. Acetylcholine exists almost everywhere in nature; it is not unique to your brain. Acetylcholine has been found in multicellular organisms as well as in blue-green algae, where it may be involved with photosynthesis. Acetylcholine stimulates silk production in spiders and limb regeneration in salamanders. In humans, acetylcholine enables movement by stimulating our muscles to contract; it also plays an important role in the actions of the autonomic nervous system.

  The autonomic nervous system maintains homeostasis, or equilibrium, for your entire body. Among other functions, it controls the rate at which your heart beats, how fast you breathe, how much saliva your mouth produces, the rate of movement of material in your gut, your ability to initiate urination, how much you are perspiring, the size of your pupils, and the degree of visible sexual excitation you might experience. The actions of acetylcholine within your autonomic nervous system indirectly influence how you feel when memories are being recorded by your brain.

  The human brain’s numerous acetylcholine pathways influence the function of the cortex, hippocampus, and many other regions. Within these various regions, the actions of acetylcholine enable you to learn and remember, to regulate your attention and mood, and to control how well you move. Thus, anything that affects the function of acetylcholine has the potential to affect all of these brain and body functions. That “anything” could be a drug or disease.

  Once released into the synapse, a small space where two neurons almost touch each other, the neurotransmitter acetylcholine can act on two quite different protein receptors: one receptor is named for muscarine and the other is named for nicotine. Most of the acetylcholine receptors in the brain respond to muscarine. Scientists know quite a lot about the role of these muscarine receptors because many plants contain chemicals that can block their normal function in the brain. Chemicals found in plants, such as henbane, Jimson weed, mandrake, and the Deadly Nightshade, selectively block the muscarine receptors; as a result, if you ingest these plants, you would quickly lose your ability to form new memories or pay attention to someone talking to you. All of these plants are in the Solanaceae family; another member of this family is the tobacco plant, which contains nicotine. Less than 10% of the acetylcholine receptors in your brain respond to nicotine. However, if those few nicotinic receptors did not exist in the brain, no one would bother smoking because the nicotine would not be psychoactive. I will discuss the impact of nicotine on the brain at the end of this chapter.

  Acetylcholine does not act alone to make memories; it requires assistance from a very simple amino acid that is also an important neurotransmitter—glutamate. Glutamate makes and breaks connections between neurons and thus makes and breaks memories. It does this by allowing the passage of sodium or calcium ions into neurons. Following the entry of calcium ions, some truly interesting things begin to happen inside the neuron that leads to the production of a memory. Calcium ions activate a complex cascade of biochemical changes that ultimately involve the genes of the neuron and that may actually change how the neuron behaves for the rest of your life.

  These biochemical changes also may alter how one neuron communicates with hundreds of other neurons throughout your brain. Think of this neural process as a symphony of musicians playing together for the first time. Initially, everyone is playing his or her own song. Finally, the conductor, that is, glutamate, arrives and hands out a musical score; all of the musicians begin to play a complex pattern of musical rhythms. In the same way that a pattern of sounds produced by a symphony conveys feeling, the rhythms of activity of the neurons in your brain convey information. Glutamate initiates the process of forming an ensemble of rhythms that is the basis of a memory. Your neurons, the individual cells that process thoughts and feelings, are the musicians and they become linked to one another according to a common pattern of activity. Scientists call this background rhythm that gets your neurons singing together the slow gamma rhythm. Once this linking occurs the neurons form a stable collaborative group of neural musicians that plays a particular song, or memory, which can recur only when that particular ensemble of neurons plays the same pattern of music together. In this analogy, memories can be seen as a unique song stored as a stable pattern of neural activity within your brain. Just as we enjoy playing the same tunes over and over again, we also enjoy replaying pleasant memories. You may know this song of neural activity as daydreaming. More on the importance of daydreaming later.

  Glutamate often demonstrates quite different roles in your brain depending upon your age. When glutamate is functioning correctly, memories can be formed. When you are much older, or if you have Alzheimer’s disease or have experienced a stroke, glutamate’s behavior becomes destructive. When too much glutamate is present in the synapse, neurons m
ay die and memories may be destroyed forever. Thus, maintaining a good balance of glutamate function is a challenging but critical requirement for neurons.

  Glutamate also has a unique role in brain development. When you were an infant, the neurons in your brain developed many connections, or synapses, with other neurons to optimize your ability to learn a great amount of information quickly, such as how to move your hands and feet, the sound of your mother’s voice, or what the color red looks like. But as you grew older (during early adolescence), your brain became a bit like an overwired computer—for it to work better and faster it became beneficial for it to remove unnecessary “wires,” or connections. This is where glutamate’s unique dual abilities come into play. Your brain used glutamate to break connections between neurons that had become unnecessary, which, in turn, allowed the remaining neural circuits to function more efficiently. Now, as an adult, glutamate allows your brain to be “plastic,” to adjust your behavior to your environment in order to increase your chances of survival. Overall, the actions of glutamate are age-dependent: when you were young, it helped make thinking more efficient; as an adult, it is responsible for making memories last a lifetime. Sometimes, due to disease or degeneration, such as that associated with Alzheimer’s disease, the brain has trouble making memories and may even lose the ones it has stored.