How do we lose our memory?

You must have seen numerous movies in which a person loses his/her memory after an accident or on hearing a tragic news. Such people forget their past and even fail to recognise their close friends and relatives. So much so, they even forget their own names. These incidents happen in real life, too, and in psychology, this condition is called ‘amnesia’. Amnesia can be caused by a number of factors, like a severe head injury, mental shock, extreme tiredness, ill effects of medicines, surgery of brain, psychological processes, old age and may even be due to alcohol. Whatever may be the cause of amnesia, the effect on the brain is mostly the same in each case. Memory is said to be stored in the brain as a ‘memory trace’. What this trace is made of is still unknown. According to one theory, each experience sets up an oscillating pattern or wave of electrical excitation in groups of cells. Each learning experience generates its own pattern of excitation. It is amazing that though a given neuron may participate in thousands of separate memories, yet its removal will not noticeably reduce any of them. Memory is considered a three-part-system – Sensory information store (SIS), Short-term memory (STM) and Long-term memory (LTM). The SIS forms an instant, but very temporary storage of every piece of information that comes 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 for a few seconds only. Two characteristics of short-term memory prevent its use as permanent information store. First, concentration is required to maintain a particular piece of information in it. Second, it is able to store only seven or eight items such as an eight-digit telephone number. For any information to be permanently stored, it has to pass from short-term to long-term memory through the mechanism of rehearsal. Long-term memory has unlimited capacity. It allows a person to remember events that happened years ago. 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 even years. There are people who have lost their memory for the rest of their lives. When memory is restored, one remembers all the forgotten things but forgets every event, which took place during the period of amnesia. One thing is however certain in spite of the restoration of memory, some after effects do remain, thereby weakening the memory.