Loss of memory is due to brain insult, injury, or disease. Loss of memory is called amnesia. Whenever one hears about memory loss the first thing comes to one’s mind is Alzheimer’s disease. It is a kind of dementia, not amnesia. Let us deal with dementia first.
Difference between amnesia and dementia
Dementia refers to a disease process marked by progressive impairment of all the mental domains like language, behaviour, learning, reasoning, memory and executive functioning differentiating it from simple amnesic disorder, which only involves impairment in memory. The word dementia derives from the Latin word dementatus meaning out of one’s mind. Dementia points to a decline from a previous level of functioning. It starts from forgetfulness and inability to remember names of common objects. For example if you show a pen to a person with onset of dementia he would grasp the object from you, examine it and describe that the object as a writing instrument. He may not remember the name of the object. The condition gradually progress and the individual forgets everything including his own past. The individual comes to a vegetating state of life. Dementia does not refer to low intellectual functioning or mental retardation, which are developmental and static conditions.
Anterograde and retrograde amnesia
Amnesia is of two types, viz. anterograde and retrograde. Anterograde amnesia refers to difficulty in remembering events that occur after the onset of amnesia. It appears to reflect a disruption in the transfer of events to long-term memory during leaning. Retrograde amnesia, on the other hand, is forgetting events that preceded the trauma and reflects forgetting from the long-term memory storage. In dementia retrograde amnesia gradually progresses along with the anterograde amnesia. In cases of brain injuries and strokes the memory loss is anterograde. In such cases there would be no retrograde amnesia.
The case of H.M.
In a famous case, a patient known by his initials “H.M.”, suffered from untreatable epilepsy. He finally found relief from violent seizures following the bilateral surgical removal of the front portions of the inner aspect of the temporal lobes of his brain. Although the operation was a success in treating the epilepsy, H.M suffered severe anterograde amnesia as a consequence. He could no longer recognize the hospital staff, apart from Dr. Scoville himself, whom he had known for many years. He did not remember and could not relearn the way to the bathroom, and he seemed to retain nothing of the day-to-day happenings in the hospital. A year later, H.M. had not yet learned the new address, nor could he be trusted to find his home. He is unable learn where objects are usually kept. But he retained all of his past memories prior to the operation. From the H.M.’s case the researchers arrived at a conclusion the part of the brain known as hippocampus plays an important role in consolidating memory and transferring them to the long-term memory storage. The word hippocampus means sea horse. (See the pictures)
To read more about HM http://brainconnection.positscience.com/topics/?main=fa/hm-memory
Lost memories of infancy
What can you remember about your life when you were a baby? Although long-term memory storage can retain information for decades, our earliest experiences in life are virtually always forgotten. The inability to recall events from the first two or three years of life is called infantile amnesia. The reasons for such amnesia are till unclear. One view is that the events of infancy are permanently stored but irretrievable. An alternative view is that these events were never encoded and stored adequately in the firs place because brain has not developed to that extent. Sigmund Freud who originated psychoanalysis championed the first view.
According to the psychological consultant John Yuille the impact of trauma on memory may well be complex, affecting different individuals in different ways. In some extreme cases, there may be a dissociation of consciousness, producing “trauma-induced amnesia”. This could cause the individual to experience the trauma as if it were happening to someone else or to travel mentally to different place and time to psychologically avoid the trauma altogether. Dissociation may also affect the storage and retrieval phase by causing amnesia for events that were at one time remembered. Trauma-induced amnesia is rare.
Upon termination of hypnosis, some subjects find themselves unable to remember the events and experiences which transpired while they hypnotized. This post-hypnotic amnesia does not occur unless it has been specially suggested to the subject while in hypnosis. Upon administration of a prearranged cue the subject remembers everything happened during hypnosis. Reversibility marks post-hypnotic amnesia as a disruption of memory retrieval, as opposed to encoding or storage which occurs in cases of brain insult, injury or disease.