How do we lose our memory?

    Some people lose their memory after some accident or on hearing some tragic news.Such people forget their past and even fail to recognise their close friends and relatives.So much so they even forget their names.In psycholgy this disease is called 'amnesia'.
    Amnesia is caused by many factors.Head injury, metal shock, extreme tiredness, ill-effects of medicines, surgery of brain, phychological processes, old age
and after effects of drinking are some of the reasons which reduce or completrly destory memory.What ever may be the redsons causing amnesia,its effects on brain is almost the same in each case.Memory is said to be the brain as a"memory trace".What makes up this trace is not known. According to one theory each experience sets up an oscillating pattern or wave of electrical excitation in a group of cells. Each learning experience generates its own pattern of excitation. A given neuron may participate in thousands of separate memories but its removal will not diminish any of them.
    Memory is considered a three-part system. Sensory information store (SIS), short-term memory (STM) and long term memory. The SIS forms an instant, but very temporary, storage of every piece of information that come in. Information can last for only about three tenths of a second in the SIS. If it has not been selected and transferred to short-term memory within this time, it fades away.
    Short-term memory is used for carrying information a person needs for a few seconds, but can afford to forget later. Two characteristics of short term memory prevent its use as a permanent information store. First concentration is required to maintain a particular piece of information in it. Second, it is able to store only six or seven items such as a seven digit telephone number.
    For any information to be permanently stored, it has to be passed from short-term to long-term memory by the mechanism of rehearsal.
    The long-term memory has virtually unlimited capacity. It allows a person to remember events that have happened years before. Permanent memory takes place through structural changes in nerve cells, caused by patterns of electrical activity in these cells.
    When somebody suffers from amnesia, he forgets events either preceding it or following it. It can last for weeks, months or for years. There are people who have lost memory for life. When memory is restored, one remembers all the forgotten things but forgets every event which took place during the period when one had lost memory. One thing is however certain that in spite of the restoration of memory, some aftereffects do remain, thereby weakening the memory. In cases involving loss of memory, assistance of psychiatrists should be obtained immediately. Normally old age weakens memory, but sometimes peptic ulcer, high blood pressure, asthma etc. also affect memory.