When people lose their ability to memorize data they have amnesia. Amnesia also refers to an inability to recall information that is stored in memory. In simple terms, amnesia is the loss of memory. Memory in this context refers either to stored memories or to the process of committing something to memory. The causes of amnesia have traditionally been divided into the "organic" or the "functional".

Organic causes include damage to the brain, through physical injury, neurological disease or the use of certain (generally sedative) drugs. Functional causes are psychological factors, such as mental disorder, post-traumatic stress or, in psychoanalytic terms, defense mechanisms. Amnesia may also appear as spontaneous episodes, in the case of transient global amnesia

People with amnesia also find it hard to imagine the future, because our constructions of future scenarios are closely linked to our recollections of past experiences.

Anterograde amnesia. This form of amnesia follows brain trauma and is characterized by the inability to remember new information. Recent experiences and short-term memory disappear, but victims can recall events prior to the trauma with clarity.

Retrograde amnesia. In some ways, this form of amnesia is the opposite of anterograde amnesia: the victim can recall events that occurred after a trauma, but cannot remember previously familiar information or the events preceding the trauma.

Transient global amnesia. A temporary loss of all memory, but it particularly affects the ability to form new memories (severe anterograde amnesia), with milder loss of past memories (retrograde amnesia) going back a few hours. It is rare and is most common among older persons with vascular disease.

Traumatic amnesia. Traumatic amnesia is caused by brain damage from a hard blow to the head, such as in a car accident. It can lead to anything from a brief loss of consciousness to coma. Traumatic amnesia is often transient; the duration of the amnesia is related to the degree of injury and may give an indication of the prognosis for recovery of other functions.

Wernike-Korsakoff's psychosis. Wernike-Korsakoff's psychosis is memory loss caused by extended alcohol abuse. This tends to be a progressive disorder and is usually accompanied by neurological problems, such as uncoordinated movements and loss of feeling in the fingers and toes.

Hysterical (fugue) amnesia. Hysterical (fugue) amnesia is usually triggered by a traumatic event that the person's mind is unable to properly handle. Usually, the memory slowly or suddenly returns a few days later, although memory of the trauma itself may remain incomplete.

Infantile/childhood amnesia. Infantile/childhood amnesia refers to a person's inability to recall events from early childhood. Some say this type of amnesia could be linked to language development or the fact that some areas of the brain linked to memory were not fully mature.

Posthypnotic amnesia is where events during hypnosis are forgotten, or where past memories are unable to be recalled.

Any disease or injury that affects the brain can interfere with the intricacies of memory. Memory function engages many different parts of the brain simultaneously. Damage to brain structures that form the limbic system, the hippocampus and thalamus, can lead to amnesia - the limbic system controls our emotions and memories. Scientists at the University of Liverpool discovered dramatic differences in the memory performance of patients with damage to the hippocampus.

Causes of neurological or organic amnesia
This refers to amnesia caused by brain injury or damage. Possible causes are:

• Stroke.
• Encephalitis - brain inflammation. This can be caused by a virus infection, such as herpes simplex (HSV), or an autoimmune reaction to cancer in another part of the body (paraneoplastic limbic encephalitis, PLE).
• Celiac disease - although no clear link has been completely agreed on. Researchers reported that the most common reasons for seeking medical help among patients with celiac disease were amnesia, confusion and personality changes.
• Oxygen deprivation - any illness or situation which undermines the supply of oxygen to the brain, such as a heart attack, respiratory distress, or carbon monoxide poisoning.
• Some medications - such as the sleeping drug, ambien. This interesting study explains why so many people report not remembering what they did after taking ambien (zolpidem).
• Subarachnoid hemorrhage - bleeding in the area between the skull and the brain.
• A brain tumor that lies in a memory-controlling part of the brain.
• Some seizure disorders.
• ECT (electroconvulsive therapy) - also known as electroshock therapy. This is a well established psychiatric treatment in which seizures are induced for therapeutic effect on anesthetized patients. It is sometimes used for patients with major depression whose illness has not responded to other treatment. ECT is also sometimes used for treating schizophrenia, bipolar disorder and catatonia. The memory loss is nearly always temporary.
• Head injuries - such as those that occur in car accidents, can lead to memory problems. In most cases the amnesia is not severe and is not long-lasting. Causes of functional or psychogenic amnesia

Also known as dissociative amnesia. This is caused by an emotional shock, such as: • Being the victim of a violent crime. • Sexual abuse. • Child abuse. • Being involved in combat (soldiers). • Being involved in a natural disaster. • Being present during a terrorist act. The list is endless - basically, any intolerable life situation which causes severe psychological stress and internal conflict.

• The ability to learn new information following the onset of amnesia is impaired. Simply put, the patient finds it hard to remember new stuff. (Anterograde amnesia).
• The ability to remember past events and previously familiar information is impaired. Simply put, the patient finds it hard to remember past stuff. (Retrograde amnesia).
• False memories - these may be either completely invented or made up of real memories misplaced in time. (Confabulation). An interesting French study, called "Do you remember what you did on March 13, 1985?, looked at how one specific patient responded to questions.
• Uncoordinated movements, sometimes tremors (Neurological problems).
• Confusion or disorientation.
• Problems with short-term memory.
• Partial loss of memory.
• Total loss of memory.
• Failure to recognize faces.
• Inability to recognize places.