Mechanism of Memory Formation
- Sensory Memory: Occurs as a result of prolonged discharge in neural circuits.
- Primary Memory: A memory trace is formed. It is a newly developed pathway for signal transmission resulting from facilitation of new synapses. It leads to creation of new circuits that keep the experience.
- Secondary Memory: Made by formation of ‘memory engrams’. It is a long lasting memory trace formed by structural changes in pre-synaptic terminals. Memory engrams remain for a very long time. Formation of memory engrams requires protein synthesis.
- Tertiary Memory: Made by formation of permanent memory traces( permanent engrams). These are made by structural changes in the neurons. Changes are similar to that in the secondary memory.
Consolidation of Memory
This is known as the means by which information from the sensory and primary short term memory are transferred into secondary, long term memories. An average of 5mins is needed for minimal consolidation and up to 2hrs for maximum consolidation. Consolidation of memory could be interrupted by deep anaesthesia, brain concussion or electro convulsive therapy (ECT).
If a sensory impression is made then followed within 5 minutes by brain concussion or ECT or deep anaesthesia, the experience will go extinct. This explains why patients who had brain concussions in Road Traffic Accidents cannot remember what happened at the time of the accident. If the concussion occurs two hours after the sensory experience, the memory remains unaffected and if within the two hours, the memory will be affected proportionately. A wide awake person consolidates more than a mentally fatigued person.
Encoding of Memory
This refers to the classification and placement of each memory with other related items in the proper memory store i.e. memory archiving. The part of the brain responsible for this is called the Hippocampus. All information go to the hippocampus first for ‘sorting’ as significant or insignificant. If classified as significant, signals are sent to the appropriate of the cerebral cortex of the brain for onward storage.
Memory Disorders: Amnesia
Simply put, amnesia is the inability to remember or recall past experiences.
- Inability to recall events that occurred shortly before the onset of the brain malfunction without affecting memories of the remote past.
- Occurs in concussion, ECT or deep anaesthesia.
- Inability to form new memories.
- Memories of the past are consolidated.
- Sensory and primary memories are functional but can’t be consolidated
- Occurs when there is bilateral lesion in the hippocampus or other structures involved in encoding.
Psychogenic/ Hysterical Amnesia.
- This is a rare condition characterized by loss of memory for usually all information in the secondary and tertiary memories. This usually follows a severe psychological trauma or it may be an unconscious response to internal conflict or an intolerable life situation.
- It can be differentiated from regular amnesia because all the personal data are forgotten including the patient’s name.
- The amnesia is not affected by key stimuli seeing members of his family, friends or personal effects.
- Inability to remember past events persists though patient can make new memories.
- Complete recovery of memory almost always occurs.
- It is a purely functional disorder without any organic situation.