Amnesia is partial or complete loss of memory that goes beyond mere forgetting. Often it is temporary and involves only part of a person's experience. Amnesia is often caused by an injury to the brain, for instance after a blow to the head, and sometimes by psychological trauma. Anterograde amnesia is a failure to remember new experiences that occur after damage to the brain; retrograde amnesia is the loss of memories of events that occurred before a trauma or injury. For a memory to become permanent (consolidated), there must be a persistent change in the strength of connections between particular neurons in the brain. Anterograde amnesia can occur because this consolidation process is disrupted; retrograde amnesia can result either from damage to the site of memory storage or from a disruption in the mechanisms by which memories can be retrieved from their stores. Many specific types of amnesia are recognized, including:

- Childhood amnesia is the normal inability to recall memories from the first three years of life. Sigmund Freud observed that not only do humans not remember anything from birth to three years, but they also have “spotty” recollection of anything occurring from three to seven years of age. There are various theories as to why this occurs: some believe that language development is important for efficient storage of long-term memories; others believe that early memories do not persist because the brain is still developing.

- A fugue state, formally dissociative fugue, is a rare condition precipitated by a stressful episode. It is characterized by episode(s) of traveling away from home and creating a new identity.

The form of amnesia that is linked with recovered memories is dissociative amnesia (formerly known as psychogenic amnesia). This results from a psychological cause, not by direct damage to the brain, and is a loss of memory of significant personal information, usually about traumatic or extremely stressful events. Usually this is seen as a gap or gaps in recall for aspects of someone's life history, but with severe acute trauma, such as during wartime, there can be a sudden acute onset of symptoms.

Repressed memories are memories that have been unconsciously blocked due to the memory being associated with a high level of stress or trauma. The theory postulates that even though the individual cannot recall the memory, it may still be affecting them consciously, and that these memories can emerge later into the consciousness. Ideas on repressed memory hiding trauma from awareness were an important part of Sigmund Freud's early work on psychoanalysis. He later took a different view.

The existence of repressed memories is an extremely controversial topic in psychology; although some studies have concluded that it can occur in a varying but generally small percentage of victims of trauma, many other studies dispute its existence entirely. Some psychologists support the theory of repressed memories and claim that repressed memories can be recovered through therapy, but most psychologists argue that this is in fact rather a process through which false memories are created by blending actual memories and outside influences. One study concluded that repressed memories were a cultural symptom due to the lack of written proof of their existence before the nineteenth century, but its results were disputed by some psychologists, and the lack of written proof was eventually partially disproven.

According to the American Psychological Association, it is not possible to distinguish repressed memories from false ones without corroborating evidence. The term repressed memory is sometimes compared to the term dissociative amnesia, which is defined in the DSM-V as an “inability to recall autobiographical information. This amnesia may be localized (i.e., an event or period of time), selective (i.e., a specific aspect of an event), or generalized (i.e., identity and life history).”

According to the Mayo Clinic, amnesia refers to any instance in which memories stored in the long-term memory are completely or partially forgotten, usually due to brain injury.

According to proponents of the existence of repressed memories, such memories can be recovered years or decades after the event, most often spontaneously, triggered by a particular smell, taste, or other identifier related to the lost memory, or via suggestion during psychotherapy.

Neurological cause of psychogenic amnesia is controversial. Even in cases of organic amnesia, where there is lesion or structural damage to the brain, caution must still be taken in defining causation, as only damage to areas of the brain crucial to memory processing is it possible to result in memory impairment. Organic causes of amnesia can be difficult to detect, and often both organic cause and psychological triggers can be entangled. Failure to find an organic cause may result in the diagnosis that the amnesia is psychological, however it is possible that some organic causes may fall below a threshold of detection, while other neurological ails are thought to be unequivocally organic (such as a migraine) even though no functional damage is evident. Possible malingering must also be taken into account. Some researchers have cautioned against psychogenic amnesia becoming a 'wastebasket' diagnosis when organic amnesia is not apparent. Other researchers have hastened to defend the notion of psychogenic amnesia and its right not to be dismissed as a clinical disorder. Diagnoses of psychogenic amnesia have dropped since agreement in the field of transient global amnesia, suggesting some over diagnosis at least. Speculation also exists about psychogenic amnesia due to its similarities with 'pure retrograde amnesia', as both share similar retrograde loss of memory. Also, although no functional damage or brain lesions are evident in the case of pure retrograde amnesia, unlike psychogenic amnesia it is not thought that purely psychological or 'psychogenic triggers' are relevant to pure retrograde amnesia. Psychological triggers such as emotional stress are common in everyday life, yet pure retrograde amnesia is considered very rare. Also the potential for organic damage to fall below threshold of being identified does not necessarily mean it is not present, and it is highly likely that both psychological factors and organic cause exist in pure retrograde amnesia.

A false memory is the psychological phenomenon where a person recalls something that did not happen. False memory is often considered in legal cases regarding childhood sexual abuse. This phenomenon was initially investigated by psychological pioneers Pierre Janet and Sigmund Freud. Freud wrote "The Aetiology of Hysteria", where he discussed repressed memories of childhood sexual trauma in their relation to hysteria. Elizabeth Loftus has, since her debuting research project in 1974, been a lead researcher in memory recovery and false memories. False memory syndrome recognizes false memory as a prevalent part of one's life in which it affects the person's mentality and day-to-day life. False memory syndrome differs from false memory in that the syndrome is heavily influential in the orientation of a person's life, while false memory can occur without this significant effect. The syndrome takes effect because the person believes the influential memory to be true. However, its research is controversial and the syndrome is excluded from identification as a mental disorder and, therefore, is also excluded from the "Diagnostic and Statistical Manual of Mental Disorders". False memory is an important part of psychological research because of the ties it has to a large number of mental disorders, such as PTSD.

Psychogenic amnesia is defined by the presence of retrograde amnesia (the inability to retrieve stored memories leading up to the onset of amnesia), and an absence of anterograde amnesia (the inability to form new long term memories). Access to episodic memory can be impeded, while the degree of impairment to short term memory, semantic memory and procedural memory is thought to vary among cases. If other memory processes are affected, they are usually much less severely affected than retrograde autobiographical memory, which is taken as the hallmark of psychogenic amnesia. However the wide variability of memory impairment among cases of psychogenic amnesia raises questions as to its true neuropsychological criteria, as despite intense study of a wide range of cases there is little consensus of which memory deficits are specific to psychogenic amnesia.

Past literature has suggested psychogenic amnesia can be 'situation-specific' or 'global-transient', the former referring to memory loss for a particular incident, and the latter relating to large retrograde amnesic gaps of up to many years in personal identity. The most commonly cited examples of global-transient psychogenic amnesia are 'fugue states', of which there is a sudden retrograde loss of autobiographical memory resulting in impairment of personal identity and usually accompanied by a period of wandering. Suspected cases of psychogenic amnesia have been heavily reported throughout the literature since 1935 where it was reported by Abeles and Schilder. There are many clinical anecdotes of psychogenic or dissociative amnesia attributed to stressor ranging from cases of child sexual abuse to soldiers returning from combat.

The main symptom of memory distrust syndrome is the lack of belief in one's own memory, however this comes with the side effect of using outside sources for information. The individual may have their own memory, but will readily change it depending on chosen outside sources. The memories that they have may be correct, but due to their distrust they will still alter their belief of what is true if contrary information is suggested.

For example, a person has a memory of a house and recalls it to be white. Then, a trusted family member begins talking with them and suggests that it was red instead. The afflicted individual will then believe the house was red despite their recollection of it being white. It is unknown if the person's memory of the house is permanently altered; however, they will say that the house was red regardless of the memory's condition.

Also, this does not necessarily allow for confabulatory memory fabrication. Currently it is not believed that an afflicted individual will readily believe an outside source on a memory of which the person is not involved, such as a randomly shared story. This further suggests that memory distrust syndrome solely alters the individual's currently retrievable memories, and not randomized information.

It is normal to have some level of memory distrust, or the lack of trusting in one's own memory. This may occur when speaking with your parents about your childhood, for example. However it seems that everyone has their own level of memory distrust, and memory distrust syndrome seems to be a severe case.

The direct cause is unknown; however, it is possibly a defense or coping mechanism to a preexisting condition that would alter one's memory. This could involve frontal lobe lesions, Alzheimer's disease, amnesia, dementia, or other conditions. Any condition that would alter either existing memories or the formation of new memories could cause a coping scheme such as memory distrust syndrome. Alternatively, an individual may have learned over time to not trust their own memory from conditioning, and as such the individual would develop a defense mechanism to remove themselves from potential embarrassment.

False memory syndrome (FMS) describes a condition in which a person's identity and relationships are affected by memories that are factually incorrect but that they strongly believe. Peter J. Freyd originated the term, which the False Memory Syndrome Foundation (FMSF) subsequently popularized. The term is not recognized as a psychiatric illness in any of the medical manuals, such as the ICD-10 or the DSM-5; however, the principle that memories can be altered by outside influences is overwhelmingly accepted by scientists.

False memories may be the result of recovered memory therapy, a term also defined by the FMSF in the early 1990s, which describes a range of therapy methods that are prone to creating confabulations. Some of the influential figures in the genesis of the theory are forensic psychologist Ralph Underwager, psychologist Elizabeth Loftus, and sociologist Richard Ofshe.

Fragmentation of memory is a type of memory disruption pertaining to the flaws or irregularities in sequences of memories, "coherence, and content” in the narrative or story of the event. During a traumatic experience, memories can be encoded irregularly which creates imperfections in the memory. It is also described as a memory that has been jumbled, confused, or repeated unnecessarily.

Fragmentation of memory is a memory disorder in when an individual is unable to associate the context of the memories to their autobiographical (episodic) memory. The explicit facts and details of the events may be known to the person (semantic memory). However, the facts of the events retrieve none of the effective and somatic elements of the experience. Therefore, the emotional and personal content of the memories can't be associated with the rest of the memory. Fragmentation of memory can occur for relatively recent events as well.

The impaired person usually suffers from physical damage to or underdevelopment of the hippocampus. This may be due to a genetic disorder or be the result of trauma, such as post-traumatic stress disorder. Brain dysfunction often has other related consequences, such as oversensitivity to some stimuli, impulsiveness, lack of direction in life, occasional aggressiveness, a distorted perception of oneself, and impaired ability to empathize with others, which is usually masked.

False memory syndrome is a condition in which a person's identity and interpersonal relationships center on a memory of a traumatic experience that is objectively false but that the person strongly believes occurred.

The FMS concept is controversial, and the Diagnostic and Statistical Manual of Mental Disorders does not include it. Paul R. McHugh, member of the FMSF, stated that the term was not adopted into the fourth version of the manual due to the pertinent committee being headed by believers in recovered memory.

RA has been found among alcohol-dependent patients who suffer from Korsakoff's syndrome. Korsakoff's syndrome patients suffer from retrograde amnesia due to a thiamine deficiency (lack of vitamin B1). Also, chronic alcohol use disorders are associated with a decrease in volume of the left and right hippocampus.

These patients' regular diet consists mostly of hard alcohol intake, which lacks the necessary nutrients for healthy development and maintenance. Therefore, after a prolonged time consuming primarily alcohol, these people undergo memory difficulties and ultimately suffer from RA. However, some of the drawback of using Korsakoff patients to study RA is the progressive nature of the illness and the unknown time of onset.

Alzheimer's disease (AD), which is known to be associated with frontal lobe dysfunction, is implicated as a cause of source amnesia. In laboratory conditions, one study found source monitoring to be so poor that the AD participants were correctly performing source memory attributions at approximately chance. This lack of ability to attribute the source of memories is likely related to AD patients' deficits in reality monitoring. Reality monitoring, the process of distinguishing whether information originated from an external or an internal source, relies on judgement processes to examine the qualitative characteristics of the information in order to determine if the information was real or imagined. It appears that it is this process that is experiencing the dysfunction, which causes mild confabulation in some AD patients, as well as being related to the source amnesia experienced in some individuals with AD.

RA can occur without any anatomical damage to the brain, lacking an observable neurobiological basis. Primarily referred to as psychogenic amnesia or psychogenic fugue, it often occurs due to a traumatic situation that individuals wish to consciously or unconsciously avoid through intrapsychic conflicts or unconscious repressions. The onset of psychogenic amnesia can be either global (i.e., individual forgets all history) or situation specific (i.e., individual is unable to retrieve memories of specific situations).

People experiencing psychogenic amnesia have impaired episodic memory, instances of wandering and traveling, and acceptance of a new identity as a result of inaccessible memories pertaining to their previous identity.

Recent research has begun to investigate the effects of stress and fear-inducing situations with the onset of RA. Long-term potentiation (LTP) is the process by which there is a signal transmission between neurons after the activation of a neuron, which has been known to play a strong role in the hippocampus in learning and memory. Common changes in the hippocampus have been found to be related to stress and induced LTP. The commonalities support the idea that variations of stress can play a role in producing new memories as well as the onset of RA for other memories. Also, the amygdala plays a crucial role in memory and can be affected by emotional stimuli, evoking RA.

Studies of specific cases, such as 'AMN', support evidence of traumatic experiences as a plausible cause of RA. AMN escaped a small fire in his house, did not inhale any smoke, and had no brain damage. Surprisingly, the next day, he was unable to recall autobiographical based knowledge. This case shows that RA can occur in the absence of structural brain damage.

After a traumatic head injury, emotional disturbances can occur at three different levels: neurological, reactionary, and long-term disturbances. Neurological disturbances can change emotional and motivational responses. Reactionary disturbances effect emotional and motivational responses as well, but they reflect failure to cope with environmental demands. Someone with this might withdraw from the environment that they are placed in because they no longer know how to handle the cognitive resources.

In psychiatry, confabulation (verb: confabulate) is a disturbance of memory, defined as the production of fabricated, distorted, or misinterpreted memories about oneself or the world, without the conscious intention to deceive. People who confabulate present incorrect memories ranging from "subtle alterations to bizarre fabrications", and are generally very confident about their recollections, despite contradictory evidence.

Schizophrenia is associated with episodic memory deficits often characterized by a confusion of internal stimuli and real events. It appears that individuals with schizophrenia often display failures in monitoring/remembering the source of information, especially for self-generated items – that is, they display source amnesia. This is a stable trait in this disease – one experiment found that over a two-year period, an individual's rate of source attributing errors was maintained, despite fluctuations in medication status and the individual's symptoms. This effect is possibly due to the malformation of associations among aspects of an episode needed for remembering its source; one neuroimaging study found that individuals with schizophrenia had lower activation of areas associated with source memory.

Individuals with schizophrenia who display source memory deficits often do so due to reality-monitoring dysfunction, which is a contributing factor towards the hallucinations that characterize the disorder. One study found that schizophrenia patients were not only slower, but also less accurate, at tasks involving reality-monitoring. The hallucinations that characterize schizophrenia are a result of deficit in reality monitoring – they exhibit an inability to differentiate between internally and externally derived information. Overall, there is evidence of a relationship between source monitoring errors and the disorganized thinking that characterizes those who have schizophrenia in that there is a strong tendency for those people with hallucinations to attribute their internally generated events (i.e.: hallucinations and delusions) to an external source (e.g., the experimenter). That is, schizophrenia is characterized by failing to encode themselves as the source of the idea, compounded by attributing these ideas/beliefs to an external source, all of which leads to those individuals with schizophrenia exhibiting behaviours typical of those with source amnesia; they misattribute the source of their knowledge, ideas, or beliefs.

Confabulation is distinguished from lying as there is no intent to deceive and the person is unaware the information is false. Although individuals can present blatantly false information, confabulation can also seem to be coherent, internally consistent, and relatively normal.

Most known cases of confabulation are symptomatic of brain damage or dementias, such as aneurysm, Alzheimer's disease, or Wernicke–Korsakoff syndrome (a common manifestation of thiamine deficiency caused by alcoholism). Additionally confabulation often occurs in people who are suffering from anticholinergic toxidrome when interrogated about bizarre or irrational behaviour.

Confabulated memories of all types most often occur in autobiographical memory and are indicative of a complicated and intricate process that can be led astray at any point during encoding, storage, or recall of a memory. This type of confabulation is commonly seen in Korsakoff's syndrome.

There are three generalized categories in which amnesia could be acquired by a person. The three categories are head trauma (example: head injuries), traumatic events (example: seeing something devastating to the mind), or physical deficiencies (example: atrophy of the hippocampus). The majority of amnesia and related memory issues derive from the first two categories as these are more common and the third could be considered a sub category of the first.

- Head trauma is a very broad range as it deals with any kind of injury or active action toward the brain which might cause amnesia. Retrograde and anterograde amnesia are more often seen from events like this, an exact example of a cause of the two would be electroshock therapy, which would cause both briefly for the receiving patient.

- Traumatic events are more subjective. What is traumatic is dependent on what the person finds to be traumatic. Regardless, a traumatic event is an event where something so distressing occurs that the mind chooses to forget rather than deal with the stress. A common example of amnesia that is caused by traumatic events is dissociative amnesia, which occurs when the person forgets an event that has deeply disturbed them. An example would be a person forgetting a fatal and graphic car accident involving their loved ones.

- Physical deficiencies are different from head trauma, because physical deficiencies lean more toward passive physical issues.

Amongst specific causes of amnesia are the following:

- Electroconvulsive therapy in which seizures are electrically induced in patients for therapeutic effect can have acute effects including both retrograde and anterograde amnesia.

- Alcohol can both cause blackouts and have deleterious effects on memory formation.

Childhood amnesia, also called infantile amnesia, is the inability of adults to retrieve episodic memories which are memories of specific events (times, places, associated emotions, and other contextual who, what, when, and where) before the age of 2–4 years, as well as the period before age 10 of which adults retain fewer memories than might otherwise be expected given the passage of time. The development of a cognitive self is also thought by some to have an effect on encoding and storing early memories. Some research has demonstrated that children can remember events from the age of 1, but that these memories may decline as children get older.

Most psychologists differ in defining the offset of childhood amnesia. Some define it as the age from which a first memory can be retrieved. This is usually at the age of 3 or 4, but it can range from 2 to 8 years. Changes in encoding, storage and retrieval of memories during early childhood are all important when considering childhood amnesia. Some other research shows differences between gender and culture, which is implicated in the development of language. Childhood amnesia is particularly important to consider in regard to false memories and the development of the brain in early years. Proposed explanations of childhood amnesia are Freud's trauma theory, neurological development, development of the cognitive self, emotion and language.

Anterograde amnesia can also be caused by alcohol intoxication, a phenomenon commonly known as a blackout. Studies show rapid rises in blood alcohol concentration over a short period of time severely impair or in some cases completely block the brain's ability to transfer short-term memories created during the period of intoxication to long-term memory for storage and later retrieval. Such rapid rises are caused by drinking large amounts of alcohol in short periods of time, especially on an empty stomach, as the dilution of alcohol by food slows the absorption of alcohol. Alcohol-related anterograde amnesia is directly related to the rate of consumption of alcohol (and is often associated with binge drinking), and not just the total amount of alcohol consumed in a drinking episode. Test subjects have been found not to experience amnesia when drinking slowly, despite being heavily intoxicated by the end of the experiment. When alcohol is consumed at a rapid rate, the point at which most healthy people's long-term memory creation starts to fail usually occurs at approximately 0.20% BAC, but can be reached as low as 0.14% BAC for inexperienced drinkers. The exact duration of these blackout periods is hard to determine, because most people fall asleep before they end. Upon reaching sobriety, usually after waking, long-term memory creation is completely restored.

Chronic alcoholism often leads to a thiamine (vitamin B) deficiency in the brain, causing Korsakoff's syndrome, a neurological disorder which is generally preceded by an acute neurological condition known as Wernicke's encephalopathy (WE).

The memory impairment that is pathognomonic to Korsakoff's syndrome predominantly affects the declarative memory, leaving non-declarative memory that is often procedural in nature relatively intact. The disproportionate severity in anterograde episodic memory processes in contrast to other cognitive processes is what differentiates Korsakoff syndrome from other conditions such as alcohol-related dementia. Evidence for the preservation of certain memory processes in the presence of severe anterograde episodic memory serve as experimental paradigm to investigate the components of human memory.

Symptoms of a dissociative fugue include mild confusion, and once the fugue ends, possible depression, grief, shame and discomfort. People have also experienced a post-fugue anger.

Amnesia is a deficit in memory caused by brain damage, disease, or psychological trauma. Amnesia can also be caused temporarily by the use of various sedatives and hypnotic drugs. The memory can be either wholly or partially lost due to the extent of damage that was caused. There are two main types of amnesia: retrograde amnesia and anterograde amnesia. Retrograde amnesia is the inability to retrieve information that was acquired before a particular date, usually the date of an accident or operation. In some cases the memory loss can extend back decades, while in others the person may lose only a few months of memory. Anterograde amnesia is the inability to transfer new information from the short-term store into the long-term store. People with this type of amnesia cannot remember things for long periods of time. These two types are not mutually exclusive; both can occur simultaneously.

Case studies also show that amnesia is typically associated with damage to the medial temporal lobe. In addition, specific areas of the hippocampus (the CA1 region) are involved with memory. Research has also shown that when areas of the diencephalon are damaged, amnesia can occur. Recent studies have shown a correlation between deficiency of RbAp48 protein and memory loss. Scientists were able to find that mice with damaged memory have a lower level of RbAp48 protein compared to normal, healthy mice. In people suffering with amnesia, the ability to recall "immediate information" is still retained, and they may still be able to form new memories. However, a severe reduction in the ability to learn new material and retrieve old information can be observed. Patients can learn new procedural knowledge. In addition, priming (both perceptual and conceptual) can assist amnesiacs in the learning of fresh non-declarative knowledge. Amnesic patients also retain substantial intellectual, linguistic, and social skill despite profound impairments in the ability to recall specific information encountered in prior learning episodes. The term is ; .

If a child experienced abuse, it is not typical for them to disclose the details of the event when confronted in an open-ended manner. Trying to indirectly prompt a memory recall can lead to the conflict of source attribution, as if repeatedly questioned the child may try to recall a memory to satisfy a question. The stress being put on the child can make recovering an accurate memory more difficult. Some people hypothesise that as the child continuously attempts to remember a memory, they are building a larger file of sources that the memory could be derived from, potentially including sources other than genuine memories. Children that have never been abused that undergo similar response-eliciting techniques can disclose events that never occurred. If one concludes that the child's recalled memory is false, it is a type I error. Assuming the child did not recall an existing memory, it is a type II error.

One of children's most notable setbacks in memory recall is source misattribution. Source misattribution is the flaw in deciphering between potential origins of a memory. The source could come from an actual occurring perception, or it can come from an induced and imagined event. Younger children, preschoolers in particular, find it more difficult to discriminate between the two. Lindsay & Johnson (1987) concluded that even children approaching adolescence struggle with this, as well as recalling an existent memory as a witness. Children are significantly more likely to confuse a source between being invented or existent.

Anterograde amnesia is a loss of the ability to create new memories after the event that caused the amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact. This is in contrast to retrograde amnesia, where memories created prior to the event are lost while new memories can still be created. Both can occur together in the same patient. To a large degree, anterograde amnesia remains a mysterious ailment because the precise mechanism of storing memories is not yet well understood, although it is known that the regions involved are certain sites in the temporal cortex, especially in the hippocampus and nearby subcortical regions.

Dissociative fugue, formerly fugue state or psychogenic fugue, is a dissociative disorder. It is a rare psychiatric disorder characterized by reversible amnesia for personal identity, including the memories, personality, and other identifying characteristics of individuality. The state can last days, months or longer. Dissociative fugue usually involves unplanned travel or wandering, and is sometimes accompanied by the establishment of a new identity. It is a facet of dissociative amnesia, according to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).

After recovery from fugue, previous memories usually return intact. Because of this, there is not normally any treatment necessary for people who have been in fugue states. Additionally, an episode of fugue is not characterized as attributable to a psychiatric disorder if it can be related to the ingestion of psychotropic substances, to physical trauma, to a general medical condition, or to dissociative identity disorder, delirium, or dementia. Fugues are precipitated by a series of long-term traumatic episodes. It is most commonly associated with childhood victims of sexual abuse who learn over time to dissociate memory of the abuse (dissociative amnesia).