What is memory consolidation?
The conversion of labile memory into a more permanent one is understood as memory consolidation. The permanent memory however is liable to frequent reactivation and recall which constitute reconsolidation. Recent trends have changed the previous view of a single speedy process of consolidation to one which involves the stages of stabilization, enhancement, and integration lasting over the years. The processes are believed to occur during the wake-sleep states. Each recall is believed to shift the memory back to unstable form and a reconsolidation occurs to remodel the memories. The initially encoded memories keep improving with each recall. The processes of consolidation and reconsolidation involve molecular, cellular, and system-level processes. Memory is declarative and non-declarative.
The memory stages start with a fast initial coding taking milliseconds or seconds when it is prone to lose or change. Consolidation involving protein synthesis is necessary to maintain it as long-term memory. Electroconvulsive shock (ECS) and protein synthesis inhibitors have proved that consolidation occurred during sleep at night. Consolidation has the power to improve memory and thereby accuracy of performance even without practice. For an initial period of stabilization, the consolidation process causing enhancement of a motor learning task was 10 times the stabilization time. For a visual discrimination task, it lasted 2 days. The stabilization process allows the memory to last for years. Recall would make the memory labile again. It then undergoes reconsolidation to maintain it the same or in a better condition. If the destabilized memory is not stabilized, it fails or maybe inaccessible to recall.
Short-term memory which is mediated through Ca+ influxes will not be adequate for long-term purposes. Similarly, processes that support long-term memory like protein synthesis cannot be rapidly encoded for short-term memory. Episodic memories which are made possible by special network structures will not be able to support other types of memory. Consolidation processes are believed to facilitate behavior through offline memory organization.
Role of Sleep
The brain does not remain in a single physiological state all through the day; neural and metabolic activity occurs in diverse proportions during the states of wake and sleep. Sleep is further divided into REM (rapid eye movement) and non-REM (nonrapid eye movement) stages which alternate all through sleep. The distinct stages have differing neurochemical and physiological changes.
Some stages of memory development are believed to occur during sleep and others can occur even when awake. Motor memory can occur when awake. However, the enhancement phases of motor and sensory memory consolidation occur during overnight sleep. Memory consolidation of motor skill tasks has been associated with specific physiological characteristics and the stage 2 and slow-wave sleep components of the NREM sleep. The memory consolidation of the visual skill tasks is related to the REM sleep and the short wave component of the NREM. Therefore consolidation enhancement for different memories is dependent on different sleep-stage brain states.
Reconsolidation can become more difficult as time elapses probably due to degradation of the memory trace. The process of recall goes through a destabilization followed by a reactivation which may vary in duration. Positive results have not been elicited from rat experiments regarding the molecular, behavioral, or cellular level changes. Degradation has been only behaviorally assessed. Reconsolidation has been identified as the process which prevents memory degradation.
Memory consolidation and reconsolidation occur at functional synapses of the hippocampus which exhibit molecular plasticity necessary for the encoding of new information which modifies older memories. This plasticity involves the processes of
‘memory organization, reorganization, and refinement’. The initial stabilization is uncomplicated but the following process all depends on the sleep-wake system. Degradation of memories also could be occurring during sleep.
Analysis
Sleep deprivation at specific stages could affect memory consolidation. Experiments showed that early night sleep which had plenty of short wave sleep was good for memory consolidation but not the REM-filled periods later in the night. Hippocampal activation was found to increase during the short wave sleep period substantiating the argument of active processing during SWS. Neuroimaging studies had demonstrated the same activity of the brain during the training period in the subsequent sleep in rat hippocampal neurons. Reorganization of memory had also been detected by this technique. The molecular processes involved show an increase of IEG zif 268 in the hippocampal dentate gyrus, the piriform, and frontal cortices following a sensorimotor exposure. This zif 268 conversely reduced when there was no such exposure. The different studies at the brain, neuronal, molecular, and behavioral levels all show that memory consolidation occurred at different wake-sleep states. All stages except stage 1 of NREM sleep had been associated with memory consolidation.
Why at night?
It is noticed that all sleep experiments were conducted in the night, presuming probably that all people slept only during the night and the memory consolidation occurred overnight. However, it is time to notice that half the intelligent world in this era of globalization is working at night and sleeping some time during the day. They are people who have to continuously learn and be sharp always. It is difficult to think that their powers of memory consolidation have been affected by the clock change. Studies may be conducted in rats who may be taught to sleep during the day or in humans who work at night.
Reason for lability of reactivated memory
Subsequent reactivation being labile may be explained by the Pavlovian fear conditioning in the long-term memory of animals. These conditioned fear associations if indelible may have caused an animal to lead a life of fear always remembering its previous experiences. However, this is not so. These conditioned fear associations are not always expressed. In the absence of the causes which could be a hunter animal running after it, the responses slowly get extinguished and the animal is bolder day by day. The responses spontaneously recover with time. The conditioned association is never erased from memory but the conditioned response will be erased.
Schedule for future experiment
Human experiments have shown the relationship of sleep to memory reconsolidation. The memory consolidation after a day’s training of a short motor sequence showed a reconsolidation overnight leading to speed and accuracy. A second competing sequence after 10 minutes caused an interference for the first sequence the next day. However, a second sequence after 6 hours did not interfere with consolidation. Reinforcing the first sequence before starting the second showed that consolidation had taken place but resulting in less accuracy. Reconsolidation could be blocked by another behavioral sequence and not necessarily chemical or electrical stimuli. Late-night interference training worsened the accuracy the next day.
A study may be conducted on people who sleep during the day and work at night
Memory consolidation can be tested to see if the responses to a motor skill sequence trained at night have any effect on the accuracy the next night after asleep. The variations of an interference sequence 10 minutes after the first and 6 hours after the first and the effect of reinforcing the first sequence before the second may be tested the next day. This experiment could follow the lines of the mentioned experiment but involving the night workers. If this experiment has the same results which say that the consolidation took place during sleep, the concept of sleep at night would lose its significance and become just ‘sleep’.