A short-term memory's conversion to long-term memory requires the passage of time, which allows it to become resistant to interference from competing stimuli or disrupting factors such as injury or disease. This time-dependent process of stabilization, whereby our experiences achieve a permanent record in our memory, is referred to as "consolidation."
Memory consolidation can occur at many organizational levels in the brain. Cellular and molecular changes typically take place within the first minutes or hours of learning and result in structural and functional changes to neurons (nerve cells) or sets of neurons. Systems-level consolidation, involving the reorganization of brain networks that handle the processing of individual memories, may then happen, but on a much slower time frame that can take several days or years.
Memory does not refer to a single aspect of our experience but rather encompasses a myriad of learned information, such as knowing the identity of the 16th president of the United States, what we had for dinner last Tuesday or how to drive a car. The processes and brain regions involved in consolidation may vary depending on the particular characteristics of the memory to be formed.
Let's consider the consolidation process that affects the category of declarative memory—that of general facts and specific events. This type of memory relies on the function of a brain region called the hippocampus and other surrounding medial temporal lobe structures. At the cellular level, memory is expressed as changes to the structure and function of neurons. For example, new synapses—the connections between cells through which they exchange information—can form to allow for communication between new networks of cells. Alternately, existing synapses can be strengthened to allow for increased sensitivity in the communication between two neurons.
Consolidating such synaptic changes requires the synthesis of new RNA and proteins in the hippocampus, which transform temporary alterations in synaptic transmission into persistent modifications of synaptic architecture. For example, blocking protein synthesis in the brains of mice does not affect the short-term memory or recall of newly learned spatial environments in hippocampal neurons. Inhibiting protein synthesis, however, does abolish the formation of new long-term representations of space in hippocampal neurons, thus impairing the consolidation of spatial memories.
Over time, the brain systems that support individual, declarative memories also change as a result of systems-level consolidation processes. Initially, the hippocampus works in concert with sensory processing regions distributed in the neocortex (the outermost layer of the brain) to form the new memories. Within the neocortex, representations of the elements that constitute an event in our life are distributed across multiple brain regions according to their content. For example, visual information is processed by primary visual cortex in the occipital lobe at the rear of the brain, while auditory information is processed by primary auditory cortex located in the temporal lobes, which lie on the side of the brain.
When a memory is initially formed, the hippocampus rapidly associates this distributed information into a single memory, thus acting as an index to representations in the sensory processing regions. As time passes, cellular and molecular changes allow for the strengthening of direct connections between neocortical regions, enabling the memory of an event to be accessed independently of the hippocampus. Damage to the hippocampus by injury or neurodegenerative disorder (Alzheimer's disease, for instance) produces anterograde amnesia—the inability to form new declarative memories—because the hippocampus is no longer able to connect mnemonic information distributed in the neocortex before the data has been consolidated. Interestingly, such a disruption does not impair memory for facts and events that have already been consolidated. Thus, an amnesiac with hippocampal damage would not be able to learn the names of current presidential candidates but would be able to recall the identity of our 16th president (Abraham Lincoln, of course!).
The role of sleep in memory consolidation is an ancient question dating back to the Roman rhetorician Quintilian in the first century A.D. Much research in the past decade has been dedicated to better understanding the interaction between sleep and memory. Yet little is understood.
At the molecular level, gene expression responsible for protein synthesis is increased during sleep in rats exposed to enriched environments, suggesting memory consolidation processes are enhanced, or may essentially rely, on sleep. Further, patterns of activity observed in rats during spatial learning are replayed in hippocampal neurons during subsequent sleep, further suggesting that learning may continue in sleep.
In humans, recent studies have demonstrated the benefits of sleep on declarative memory performance, thus giving a neurological basis to the old adage, "sleep on it." A night of sleep reportedly enhances memory for associations between word pairs. Similar overnight improvements on virtual navigation tasks have been observed, which correlate with hippocampal activation during sleep. Sleep deprivation, on the other hand, is known to produce deficits in hippocampal activation during declarative memory formation, resulting in poor subsequent retention. Thus, the absence of prior sleep compromises our capacity for committing new experiences to memory. These initial findings suggest an important, if not essential, role for sleep in the consolidation of newly formed memories.