Defects of the memory can be divided into two broad types, organic and psychogenic amnesia. Psychogenic amnesia is a defensive process that operates continuously to cope with emotionally disturbing experiences. Hypnotic amnesia is induced by suggestion under hypnosis. Hysterical amnesia is of two types: the failure to recall specific past events or a period of one's life and the failure to register and therefore the inability to recall events in one's ongoing life.
Memory defects of an organic nature may be transitory or enduring and usually result following brain surgery, encephalitis (brain inflammation), electroconvulsive therapy, senility, or physical injury. There is evidence that stress and the associated release of the hormone cortisol causes loss of memory. In all cases, the person recognizes the memory difficulty. They include: transitory global amnesia, which is an abrupt loss of memory lasting a few seconds to several hours without the loss of consciousness and is usually the result of a transient blockage of the blood supply to the brain; traumatic amnesia, which lasts from a few minutes to weeks and is usually the result of a blow to the head; post traumatic automatism, which results from a light blow to the head and is the loss of memory of the period following the blow without loss of consciousness and without a change of behaviour. Brain surgery to the temporal lobes causes a degree of memory loss. Diffuse diseases of the brain such as senility and cerebral arteriosclerosis may cause impairment of the memory. Frequent electro-convulsion sometime leads to exaggerated forgetfulness for day-to-day events.
Korsakoff syndrome may be due to chronic alcoholism (alcoholic psychosis), a variety of toxic and infectious brain illnesses, nutritional disorders such as deficiency of the B vitamins and cerebral tumours. The main psychological feature is the gross defect in recent memory, sometimes producing moment to moment consciousness where people store new information for a few seconds and report no continuity between one experience and the next. In addition the person experiences retrograde amnesia, remember experiences they never had, which is called confabulation, and deny their memory is in any way affected. Encephalitis may cause memory defects closely resembling Korsakoff's syndrome except the person has good insights into his memory loss and generally does not confabulation.
Another form of memory abnormality is hyperamnesia, the enhancement of the memory function.
Analogous phenomena may occur in the case of collective memory as a result of physical destruction of portions of a manual or electronic information system, uncontrolled hyper-development of such a system, or the subjection of such a system to frequent overload beyond the breakdown threshold.
Memory formation appears to correspond to the formation of new synapses in the brain. Other synapses are strengthened as a part of a more complicated, multi-faceted process. To accommodate this, the architecture of the brain is rearranged to accommodate additional synapses. Sleep is crucial; during sleep, the newly rearranging parts of the brain are activated while the rest shut down, suggesting that sleep is a vital time for the changes to be solidified and make the memories last.