To a large degree, we are what we remember. We create our sense of self out of recollections of our experience, and we decide what to do in everyday situations on the basis of “what happened last time.” The information storage systems in our brains, and how we evaluate and make use of our memories, make us what we are.
There are three steps to memory storage: encoding, storage, and retrieval. These three functions work together so that we can remember what we learned in class, how to drive, or our cousin’s children’s names. A problem with any one of these systems can lead to a problem with memory.
Encoding is the term for how the brain stores away memories. The first step may happen in the hippocampus, which, together with the frontal cortex, analyzes information from the senses and decides whether a memory is worth recording. The hippocampus is thought to be involved in declarative memory and spatial memory. Declarative memories are the memories that can be put in words, such as the events of a summer vacation, or knowledge about calculus, if any. Spatial memory is thought to be that form of memory that describes one’s location in space. On the other hand, the amygdala plays a role in encoding memories of emotionally stressful events.
The hippocampi are a pair of small organs, shaped somewhat like seahorses, located in the left and right medial temporal lobes. People whose hippocampus is severely damaged generally cannot encode new declarative memories. However, it is still possible for a person without a functioning hippocampus to learn a new skill, such as jumping rope, because procedural memories are encoded through a different route(the basal ganglia and the cerebellum, probably). Spatial memory is also mediated by the hippocampus, and experiments on taxi-drivers have shown that the part of the hippocampus involved in navigation actually grows larger in experienced drivers.
The amygdalae are almond-shaped groups of neurons, and are also located in the medial temporal lobes. They hold a primary role in the encoding of emotional memories. Under perceived threat, the amygdalae send messages to the hypothalamus to activate the sympathetic nervous system, to the thalamic reticular nucleus to improve reflexes, and to other parts of the nervous system to generally prepare the organism to counter a threat. The amygdala also processes information from the senses, especially the sense of smell. It encodes those emotional memories that often modify the organism’s behavior. Research has shown that memories associated with strong emotional content tend to be retained longer than mere facts.
In any case, if information is encoded, it is encoded as an electrical charge in the synapses. Synapses are specialized junctions at which a neuron (a nerve cell) passes a message to another cell. The neuron releases a chemical that diffuses across a gap to the next cell, where it activates a receptor in the cell. The place where the chemical is released is the axon, the gap it diffuses across is the synapse, and the receptor is in the dendrite. Messages in the brain pass from the axon of one cell to the dendrite, the many-branched tree-like receptor, of the next cell this way, so that brain cells are connected in networks. The way the networks are connected is the stored information.
There are three types of stored information. They are sensory memory, short term memory and long term memory. Sensory memory is extremely short term, and outside of conscious control: it happens automatically. It is a sensory impression which is very accurate, but very brief, under two seconds at the most. Short term memory, though, is held somewhat longer, perhaps 30 seconds. It is sometimes called telephone memory, because it is the length of time one may be able to hold a phone number in mind before dialing. This is information in an active state, ready to be used.
Long term memory is what we think of as real memories: the first day of kindergarten, the birth of a child. Many scientists believe that long term memory involves an actual physical change in neurons which were once temporarily reconfigured by a short term memory. Processes involved in this permanent alteration might include rehearsal and association. Rehearsal is the way people learn lines of poetry, but it is also the way we learn the path to school. It’s
mental practice. Association is the connection of separate ideas into a complete concept, for example the spectrum of associations a person might have when he or she thinks of mother.
The way to have good information storage is to have a peaceful, predictable, but not dull life. Stress damages information storage, at least temporarily, and long term debilitating stress is believed to damage the hippocampus. On the other hand, exploring and exercise may improve its function. Many scientists believe that sleep is essential to good information storage. Physical trauma to the brain, such as repeated concussion, may also damage memory. Our memories are precious; fortunately, we know ways to preserve them.