Memory is the ability to remember and to recall previous sensations, ideas, experiences, or such information as has been consciously learned. Amnesia (am-NEE-zha) is the loss of memory of one or more past experiences that goes beyond normal forgetfulness in a person with a normal level of consciousness (that is, the person is not asleep or in a coma * ). Amnesia can be caused by a variety of physical and mental illnesses.
To understand human memory and how diseases, toxic substances, or physical injuries can interfere with it, one should have a basic understanding of the structures in the human brain involved in memory formation, storage, and retrieval. One important part of the brain is the temporal (TEMpor-al) lobes, located toward the lower front of the brain over the ears. In the 1930s, while performing brain surgery on patients with epilepsy * * .
Inside each temporal lobe is the hippocampus (hip-o-KAM-pus) (plural, hippocampi), a seahorse-shaped structure that stores short-term memories. It is one of the first portions of the brain destroyed by Alzheimer's (ALTS-hy-merz) disease * ; it can also be damaged by encephalitis * , oxygen deprivation, or temporal lobe epilepsy.
As an example, when surgeons removed the hippocampi from a patient known by the initials H.M., hoping to treat his epilepsy, they discovered that H.M.'s epilepsy improved but his short-term memory disappeared. H.M. could remember events that happened many years before, but not events of the previous day or the previous hour. H.M.'s doctors had to reintroduce themselves to him every single day. On his death on December 2, 2008, H.M. was identified as Henry Gustav Molaison.
Another important brain structure is the thalamus. The thalamus is a structure that lies between the cortex * of the brain and the brain stem * . It relays information from the senses to the cortex and also regulates a person's level of awareness. Damage to the thalamus impairs recent memory and the ability to form new memories.
The formation and recall of memory have an emotional as well as a cognitive * dimension. Experiences that make people feel happiness, sadness, intense fear, or some other strong emotion are more likely to be remembered. The structures in the brain that are most closely involved with processing emotion-laden memories are the hippocampus and the amygdala * , a small almond-shaped structure in the middle portion of the temporal lobe. The amygdala plays an important role in regulating the emotion of fear and of long-term memory consolidation. The amydal's activity level increases when a person is highly aroused emotionally; memories formed at that time are more likely to be retained than those associated with less exciting situations. The amygdala is activated by traumatic events and by recalling memories of trauma. Certain studies using various brain imaging techniques indicate that high levels of activity in the amygdala are found in patients with psychogenic amnesia.
Human memory is generally divided into two broad categories: shortterm and long-term.
Short-term memory is the form of memory that a person uses for such activities as remembering a new phone number after calling directory assistance. The person may repeat the number silently before dialing it and then promptly forget it. If the person is distracted before dialing and did not write down the number, the person may have to call directory assistance again. That is because short-term memory is fairly easily disrupted. When students must learn new material, they must go over it several times, perhaps even write it out, rather than just see it one time. They are converting the information from unstable short-term memory into more stable long-term memory by paying close attention to the information and rehearsing it.
People who experience a severe head injury demonstrate how easily the process of short-term memory can be interrupted. For example, a car accident victim may not recall what happened just before impact or even during the accident itself. Some athletes who are knocked unconscious during an especially rough play may not remember what happened minutes before they were hit. This type of amnesia occurs because there was not enough time for experiences to be converted from short-term to long-term memory.
He is a well-known Australian artist who was diagnosed in 1994 with Wernicke-Korsakoff (VER-ni-kee–KOR-sa-kof) syndrome, a disorder of memory most often caused by alcoholism. Once wealthy from the sales of his art, Charles Blackman (b. 1928) survives on money channeled into a trust fund. His memory comes and goes; when his amnesia returns, he resorts to confabulation * in order to carry on conversations with visitors. It is difficult for them to know how much of what he tells them really happened and how much is confabulated.
She was 16 years old when she collided with another player on the basketball court and hit her head sharply. The next day she felt nauseated, dizzy, and her vision was blurred. Then she had problems remembering what she was supposed to be learning in school; her memory was failing. Tracy had amnesia following a concussion * . She had three years of therapy in order to regain her memory well enough to complete high school.
He is a man in his 40s from Bellingham, Washington, who found himself in Denver, Colorado, in September 2006 with no idea of how he got there or who he is. He appeared on a Denver news show after three weeks in a local hospital asking the public for help in identifying him. His family had filed a missing person report; they were contacted by the Denver police. The man had been on his way to Canada to visit a dying friend when his emotional stress triggered a rare form of amnesia known as a fugue * state. According to the man's mother, he does not remember his entire past life, let alone the trip to Denver.
He had served as the 33rd governor of California and the 40th president of the United States. Starting about four years after Ronald Reagan left office, his family and friends started to notice that his memory was failing. In the fall of 1992, Reagan asked one of his personal physicians what he should do next after making a speech. In February 1994, the former president made his last trip to Washington; he failed to recognize one of his long-term Secret Service agents and told his wife when they returned to their hotel room that he was not sure where he was. In November 1994, Reagan told the American public that he had been diagnosed with Alzheimer's disease, the disease that killed him in 2004.
He is a middle-aged Englishman living in a mobile home who was found by his friends in a confused and disoriented state in late 2007. He had no memory of what had happened. His doctors performed a range of diagnostic tests that included brain imaging, a muscle biopsy * , and a test of the patient's spinal fluid. The tests did not yield any useful results. Several weeks later, the patient's amnesia suddenly cleared up. His most recent memory was turning on his gas heater for the first time when the weather turned cold. A repairman went to the patient's home and found that the heater had been releasing carbon monoxide into the air. This colorless and odorless but toxic gas was the cause of the patient's amnesia as well as of his physical symptoms.
She is a survivor of the Cocoanut Grove nightclub fire of 1942, the second-worst single-building fire in U.S. history. She has no conscious memory of the disaster. Yet every year on November 28, the anniversary of the tragedy, she suddenly begins to ask those around her how many women they rescued from the fire. Her anniversary reaction is a symptom of dissociative amnesia.
As these case studies indicate, amnesia is a loss of memory in a person with a normal level of consciousness that can arise from a bewildering variety of conditions or accidents. It is not a disease or disorder in its own right but rather a symptom of an underlying physical or psychological condition. Amnesia may last from a few hours to a lifetime; it may be reversible or permanent; it may improve, stay the same, or get worse over time.
There are three distinct phases in human memory: registering the event in the brain; storing the information; and retrieving the information that has been stored. Disorders or injuries affecting any of these phases may cause amnesia. Amnesia can be classified in several different ways:
Amnesia can be caused by a range of physical injuries, drug reactions, toxic substances, seizure disorders, infectious diseases, or emotional traumas. Amnesias resulting from a physical disorder are classified as organic, or body-related, amnesias, whereas those resulting from an emotional shock are called psychogenic * amnesias. As a rule, memory loss in organic amnesias is gradual and incomplete rather than sudden or total, which is more likely in psychogenic amnesia.
Specific organic causes of amnesia include the following:
Transient global amnesia, or TGA, is a form of memory loss that appears suddenly and causes confusion, disorientation, and forgetfulness for 30 minutes to 24 hours. This type of memory loss typically clears up on its own. The causes of TGA are not completely understood, but many doctors think that the disorder is an organic amnesia. Various explanations include an unusual form of migraine headache; a seizure in the temporal lobe of the brain; or a transient ischemic attack * or TIA, sometimes called a mini-stroke, which affects the thalamus or hippocampus.
Functional or psychogenic amnesia refers to amnesia triggered by psychological stress rather than a known physical cause. It may be localized (covering only specific events or a period of time in the person's life) or global. Or it may be systematized; that is, the person may have amnesia only for certain types of information or for specific people and have normal memory for everything else. The basic mechanism of psychogenic amnesia is dissociation, a process in which the formation of memories is interrupted or altered by extreme fear in response to trauma. Memories of the frightening event or time period remain in the mind but are not accessible to explicit memory. There is evidence from brain imaging studies that the hippocampus is damaged by exposure to high levels of psychological stress and that the amygdala becomes more easily activated after a traumatic experience.
Traumatic memories may reemerge in the form of nightmares or flashbacks *
Specific disorders characterized by psychogenic amnesia include:
Amnesia is not an unusual symptom by itself, but the frequency of different types of amnesia in the United States varies considerably:
People may or may not know that they have amnesia. As President Reagan's case indicates, those in the early stages of Alzheimer's disease are often aware that their memory is starting to fail. Many try to cope by writing notes to themselves or asking family members to remind them of appointments or such household chores as paying bills. Patients in the later stages of the disease gradually lose their awareness of their memory loss.
People with dissociative identity disorder often become aware that something is wrong when they find items in their closet that they do not remember purchasing or when a person they do not recognize calls them by an unfamiliar name.
People in a fugue state may not realize that they are suffering from amnesia until they are asked for personal information, and then they become aware that they do not know who they are. In some cases, the dissociative fugue clears up by itself with dramatic speed, and individuals are bewildered by finding themselves in an unfamiliar setting.
People with dissociative amnesia may or may not be aware that they have gaps in their memory. Some who are aware that they have periods of “lost time” in their past may recover their memories through psychotherapy * ; but others do not succeed.
People who have blackouts as a result of heavy drinking or glue sniffing usually recognize afterward that they had amnesia for the period of intoxication.
People with some forms of organic amnesia, such as late-stage syphilis or Wernicke-Korsakoff syndrome, are unaware that they have amnesia.
The physical examination of the patient includes neurological tests. These include tests of the patient's level of consciousness, sight, sense of touch, and hearing, and tests of the patient's reflexes, sense of balance, muscle strength, and ability to walk normally.
Patients whose chief symptom is memory loss are interviewed by a psychiatrist so that possible emotional or progressive causes of the patient's amnesia can be evaluated. The psychiatrist gives these individuals a mental status examination or MSE. The doctor asks questions to determine their level of alertness; ability to think, pay attention, and carry out simple instructions; ability to speak and tone of voice; and emotional condition. Short-term memory is tested by asking the patients to repeat the names of three objects listed by the doctor after a one-minute delay. Long-term memory is tested by asking patients a question about their past, such as “Where were you born?” or “What kind of car did you use when learning to drive?” The MSE is useful in distinguishing between psychogenic amnesia and memory loss caused by Alzheimer's disease or a mood disorder * such as depression.
It is also important for the doctor to distinguish between a patient with genuine psychogenic amnesia and one who is malingering * , or faking their amnesia. There are cases of people faking amnesia caused by AIDS or PTSD in order to qualify for disability benefits. There are several diagnostic sets of questions that psychiatrists can use to distinguish real psychogenic amnesia from faking.
Treatment of amnesia depends on the cause. Amnesia that has an organic cause is treated by identifying and treating or removing the underlying cause. Infectious diseases, Alzheimer's disease, stroke, seizure disorder, Wernicke-Korsakoff syndrome, and substance abuse can be treated, but the amnesia may or may not improve. There is no specific treatment for memory loss.
Amnesia caused by prescription medications usually clears up when the drug is discontinued. Transient global amnesia and dissociative fugue often clear up spontaneously, sometimes within a few hours.
Amnesia related to PTSD, dissociative amnesia, or dissociative identity disorder usually require psychotherapy, often combined with hypnosis * * to help the patient cope with the emotions stirred up by traumatic memories.
Some of the organic causes of amnesia are at least potentially preventable by attention to safety measures. People can lower their risk of head injuries by wearing protective equipment during sports competition; by driving or riding a bicycle safely; and by checking their households for hazards that put the elderly at risk of falls. Installing and maintaining carbon monoxide detectors and having gas-powered appliances checked regularly can reduce the risk of carbon monoxide poisoning.
Other organic causes of amnesia are potentially preventable by lifestyle changes. Eating a healthful diet, keeping one's weight at the right level, exercising regularly, and quitting smoking can lower the risk of stroke. Practicing safe sex can reduce the risk of syphilis, AIDS, and other sexually transmitted diseases. Avoiding heavy drinking, illegal drugs, and glue sniffing lowers the risk of drug-related amnesia and Wernicke-Korsakoff syndrome.
Some organic amnesias cannot be prevented because doctors do not yet fully understand the causes of the disorders associated with the memory loss. These disorders include Alzheimer's disease, some types of brain tumors, and seizure disorder.
Psychogenic amnesia is not preventable because doctors do not know why some people react to abuse or other traumatic experiences by dissociating and others do not. And, as of 2016, researchers did not know why some people develop dissociative disorders without a history of trauma.
See also Alcoholism • Alzheimer's Disease • Brain Cancer • Brain Injuries • Dementia • Dissociative Identity Disorder • Encephalitis • Meningitis • Post-Traumatic Stress Disorder (PTSD) • Sports Injuries: Overview • Stroke • Substance Abuse
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Mech, Su, and Daniel de Visé. I Forgot to Remember: A Memoir of Amnesia. New York: Simon & Schuster, 2015.
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* coma (KO-ma) is an unconscious state like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.
* epilepsy (EP-i-lep-see) is a condition of the nervous system characterized by recurrent seizures that temporarily affect a person's awareness, movements, or sensations. Seizures occur when powerful, rapid bursts of electrical energy interrupt the normal electrical patterns of the brain.
* stroke is a brain-damaging event usually caused by interference with blood flow to the brain. A stroke may occur when a blood vessel supplying the brain becomes clogged or bursts, depriving brain tissue of oxygen. As a result, nerve cells in the affected area of the brain and the specific body parts they control do not function properly.
* Alzheimer's disease (ALTS-hymerz) is a condition that leads to gradually worsening loss of mental abilities, including memory, judgment, and abstract thinking, as well as changes in personality.
* encephalitis (en-seh-fuh-LYEtis) is an inflammation of the brain, usually caused by a viral infection.
* cortex (KOR-teks) is the outermost layer of the brain. It controls the brain's higher functions, such as thinking, learning, and personality.
* brain stem is the part of the brain connected to the spinal cord. The brain stem controls the basic functions of life, such as breathing, heart rate, and blood pressure.
* cognitive means associated with thinking, learning, perception, awareness, and judgment.
* amygdala (a-MIG-da-la) is a small almond-shaped structure in the brain that plays a part in processing emotions.
* confabulation refers to filling in gaps in memory by making up or fabricating facts. The gaps occur because the person's memory function is impaired.
* concussion (kon-KUH-shun) is an injury to the brain produced by a blow to the head or violent shaking.
* fugue (FYOOG) is a psychiatric condition in which people wander or travel and may appear to be functioning normally, but they are unable to remember their identity or details about their past.
* biopsy (BI-op-see) is a test in which a small sample of skin or other body tissue is removed and examined for signs of disease.
* psychogenic (SIGH-ko-JEN-ik) means originating in the mind as a result of emotional conflict.
* meninges (meh-NIN-jeez) are the membranes that enclose and protect the brain and the spinal cord.
* Epstein-Barr virus (EP-stine-BAHR VI-rus) is a common virus that causes infectious mononucleosis.
* AIDS (or Acquired Immunodeficiency [ih-myoo-no-dihFIH-shen-see] Syndrome) is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).
* syphilis (SIH-fih-lis) is a sexually transmitted disease that can lead if untreated to serious life long problems throughout the body, including blindness and paralysis.
* diuretics (dye-yoor-EH-tiks) are medications that increase the body's output of urine.
* transient ischemic attack (TRAN-shent iss-KEE-mik), also called a TIA, is a temporary loss of blood supply to a particular area of the brain.
* flashbacks are intensely vivid recurring mental images of a past traumatic event. People having a flashback may feel or act as if they were reliving the experience.
* metastatic (MET-uh-STAT-ic) refers to a cancer that has spread from the part of the body where it first started to other organs or parts of the body.
* psychotherapy (sy-ko-THER-apea) is the treatment of mental and behavioral disorders by support and insight to encourage healthy behavior patterns and personality growth.
* spinal tap, also called a lumbar puncture, is a medical procedure in which a needle is used to withdraw a sample of the fluid surrounding the spinal cord and brain. The fluid is then tested, usually to detect signs of infection or other diseases.
* meningitis (meh-nin-JY-tis) is an inflammation of the meninges, the membranes that surround the brain and the spinal cord. Meningitis is most often caused by infection with a virus or a bacterium.
* encephalitis (en-seh-fuh-LYEtis) is an inflammation of the brain, usually caused by a viral infection.
* electroencephalogram or EEG is an instrument that records the electrical activity of the brain.
* mood disorder is a mental disorder that involves a disturbance in the person's internal emotional state. Depressive disorders, bipolar disorders, and mood disorders are associated with the use of drugs or medical illnesses.
* malingering (ma-LING-er-ing) means intentionally pretending to be sick or injured to avoid work or responsibility.
* hypnosis refers to a trancelike state, usually induced by another person. The person under hypnosis may recall forgotten or suppressed memories and be unusually responsive to suggestions.
* antidepressant medications are used for the treatment of depression.