Memory—A Primer

May 9, 2013

The brain’s ability to change with experience gives us memory, a faculty nearly as essential to mental life as breathing is to physical survival. Memory is the bedrock of our sense of ourselves and the world, and our ability to manage the moment-to-moment demands of existence.

Thanks to a century of research, we know a good deal about its operation: what happens in the brain when we store facts, experiences, and skills in memory; what happens when we recall them. We can map in some detail the structures, circuits, and molecular processes underlying memory.

Memory systems, ii, iii

Thoughts, perceptions, emotions, and actions are generated by groups of neurons firing together; memories are the patterns this activity leaves behind. There are two systems to preserve such neural networks:  

Explicit or declarative memory is what we can recall consciously and describe verbally. It encompasses two types of memory. Episodic memory refers to the data of specific experiences: the sights, smells, sounds, and feelings of a day at beach, for example. Semantic memory comprises what we have learned about the world, such as the meanings of words and objects, public facts like the name of the U.S. president during World War I, and information of personal relevance like the appearance of a friend’s face.

Implicit or non-declarative memory is for things we do without thinking and typically do not put into words. The most familiar manifestation is procedural memory, used to perform automatic actions like riding a bicycle or tying shoelaces. Habits and conditioned reflexes also rely on implicit memory.

The stages of memoryiv, v, vi, vii

Much of the time, we need to store information only briefly—while dialing a phone number or reading a paragraph from beginning to end. The brain accomplishes this with short-term memory, which holds data for seconds to several minutes.

Scientists use the term working memory to describe the faculty for thinking about things while holding them in short-term memory, or switching between tasks that use short-term memory (i.e. “multitasking”). Taking notes—listening to a speaker while writing down what he or she said 10 seconds earlier—uses working memory, as does talking on the phone while reading e-mails.

If we need to retain data for longer, it is stored in long-term memory. This is actually a multistage process that unfolds over time. Initially, data from an experience or bits of factual information are simply encoded—the brain selects and connects key elements for storage. As days, months, or years pass, the memory is consolidated, i.e. integrated with other experiences and facts into the framework of things you know. The process of consolidation establishes memories more firmly, making them less subject to misremembering or forgetting.

While distinctions between short-term and long-term memory have traditionally been applied to explicit memory, similar processes are at work in implicit memory as well.

The formation of explicit long-term memories engages the hippocampus and surrounding structures in the medial temporal lobe, which connect input from different parts of the brain that respond to an experience or register a fact. As consolidation occurs, the memory linkages become independent of the hippocampus: they are stored in the circuits of the brain that originally processed the information.

Implicit long-term memories are registered via direct modification of areas of the brain (cerebellum, motor cortex, striatum) that regulate movement, without the involvement of the hippocampus or any analogous mediating region.

Molecules of memoryviii, ix, x

Both short- and long-term memory modify the synapses through which neurons connect with one another. This process typically strengthens existing synapses, but may also establish new ones.

Short-term memory temporarily alters neurotransmission. The release of neurotransmitters into the synapse starts a biochemical cascade of enzymes and energy-transferring compounds that increases the flow of charged particles into participating neurons, making them more excitable. It lasts only for the brief period that the released molecules remain active.

Long-term memory actually changes the structure of the synapse. A key process in this remodeling is long-term potentiation (LTP), which increases the number of and activity in certain receptors of neurons joined in the synapse, making them respond more strongly to neurotransmitters. LTP initially rearranges and reshapes bonds between proteins in the synapse. Further consolidation of memory modifies gene expression in the partipating neurons to synthesize new proteins that strengthen memory-encoding synapses in a more enduring way.

Sleep appears to play a key role in memory consolidation, by promoting the underlying chemical processes.

Remembering and forgettingiv, v, xi, xii, xiii, xiv, xv
To use stored memories, they must be retrieved. Among the factors that determine how readily we can recall a memory are the conditions surrounding its initial storage: we are more likely to remember a situation, face, or fact later if we paid close attention to it at the time—particularly if we had the intention of committing it memory—and if we had a strong motivation to remember it.

Emotion has a powerful effect on memory. Because strongly affecting experiences activate the amygdala, they register in memory quickly and deeply—some put “emotional memory” in a class by itself—and are readily recalled. All too readily, sometimes: fearful or traumatic memories can be intrusive, as in phobias and PTSD.

Preservation of a memory is affected by how well it is integrated with other facts and experiences already stored in the brain. The more links with your general body of knowledge, the more reliably it will be recalled. Long-established memories may be retrieved more easily than new ones for this reason. On the other hand, memories tend to decay over time, particularly if they are rarely recalled.

Forgetting can also reflect active processes; new memories may interfere with old ones, and we may be motivated by the desire to suppress painful emotions or events. Selective erasure of memory may be as important as recall for effective mental functioning, and researchers are mapping out the molecular processes that make it possible.

Studies in the last decade have shown that when a memory is retrieved it becomes unstable for a time, until it is “reconsolidated.” Research suggests that during this window of opportunity, a fear memory can be erased either by rewriting it in a non-fearful version, a process known as extinction, or with drugs that block reconsolidation. This finding may have important implications for treating PTSD and other anxiety disorders.

Age, disease, and memory v, iv, xvi, xvii, xviii, xix, xx, xxi
Like other brain functions, memory changes over the life cycle. Long-term declarative memory is rudimentary in the first few years of life, and the process that turns short-term to long-term memories continues to develop, using progressively more of the brain, into a person’s 20s.

Memory decline begins in the 30s. It is typically selective: words used less often, like proper nouns, are particularly vulnerable, and it may be progressively difficult to recall when things were learned or events are scheduled. This appears in large part due to the decay of memories over time, new memories interfering with old, diminished sleep quality, and neuron loss in critical brain areas. Age-related declines in working memory are small in magnitude but particularly bothersome: they may reflect the brain’s diminished ability to switch rapidly between networks.

Injury can cause much more profound memory deficits. Depending on the area affected, it may impair the ability to store new memories (anterograde amnesia), or recall established ones (retrograde amnesia).

Neurological disorders often devastate memory. The prime example is Alzheimer’s disease, where the sequence of memory loss (recent episodic memories go first; older and semantic memories are affected later) follows a pattern of neuron loss that begins in the hippocampal region and then spreads widely.

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