This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
Short-term (Working) Memory
Neurologists divide memory into long-term and short-term memory, or "working" memory. Some are beginning to argue for the existence of a middle-term memory as well (7). Short-term memory, or STM, "is the brain's system for remembering information "in use." Most people can only hold five to nine items in their short-term memory at one time. If they try to remember more than that, they will often end up forgetting the "middle" items (10).
STM works like this. First, "the brain's cerebral cortex receives nerve messages from eyes, ears, and touch sensors. This sensory stimulus is held for a fraction of a second in the sensory memory. Unless an individual pays attention to the image for about eight uninterrupted seconds to encode the stimulus into short-term memory, it will be lost" (10). The memory then is stored on something akin to an electronic tape loop (although some scientists debate the existence of that loop). "Once a complete loop is made, three things can happen: (1) the information can be "rehearsed" (repeated) silently or aloud, which will provide auditory cues; 2) the information goes into long-term memory; or 3) the information will be lost" (10).
John Mackin conceives of working memory as composed of three functions: "the central executive (an attentional controlling system) and two slave systems: the visuospatial sketch pad, which manipulates visual images, and the phonological loop, which stores and rehearses speech-based information" (7). (More information on the process of short-term remembering is included in "The Prefrontal Cortex--Site of Working Memory," below).
Long-term memory, or LTM, is that part of our "memory storage system that has unlimited capacity to retain information over an extended time" (10). At least three different types of memory are included in LTM. "Procedural memory represents motor or skill learning which is memory without verbal mediation and thus without record. It includes learning how to drive a car or tie your shoelace. Such memories are slow to acquire but more resistant to change or loss. "Declarative memory is memory for facts, such as names and dates. It is fast changing, quick to acquire but quick to be lost. Much of the loss is by design." This is because "[c]onsiderable information activates the receptors but is not retained. We attend to meaningful or relevant stimuli and ignore unchanging or uninformative information" (6).
"Remote memory simply refers to memories that were acquired early...They represent the foundation memories upon which more recent memories are built...Since early acquired information is the foundation for new memories and may be linked to many more new memories, such memory is less subject to change and/or loss" (6).
The Anatomy of Long-term Memory
Both long- and short- term memory are composed of three processes: encoding, storage, and retrieval (8). These processes take place in various locations in the brain, often simultaneously. Not much is known about the physiology of long-term memory, although scientists speculate that the hippocampus is involved in the creation of long-term memory. It is unclear where long-term memories are stored, although there is some evidence that a single memory may be broken down into various elements and stored in many places at once. As Irving Kupferman explains, "long-term memories are stored in multiple regions throughout the nervous system. (In other words, they are not localized but stored through circuitry)" (7). Furthermore, "reflexive and declarative memory formation may involve different circuits in the brain. Reflexive memory relies on the cerebellum and amygdala; formative, on the hippocampus and temporal lobes" (7).
Another area of dispute involves the generation of new proteins during long-term memory formation. "Though long-term memory has ...been shown to require protein synthesis, it is not known for sure whether these newly synthesized proteins are used only in strengthening existing synapses or in growing new ones. Circumstantial evidence may point to new synapse formation in learning: it has been demonstrated that an enriched environment leads to denser dendrite growth in rats. Still however, the conclusive evidence that specific long-term memory formation relies on dendritic growth and structural synaptic changes has hitherto proven elusive" (7). .
The Prefrontal Cortex--Site of Working Memory
Since the 1970's, scientists have speculated that the prefrontal cortex, located in the forehead area of the brain, plays a central role in working memory. (1). Experiments using PET scans and functional MRI on primates, coupled with observations of human brain injuries, point to the fact that "the prefrontal cortex always seems to be "busy" when target information is kept "in mind" (1). Writer Tim Beardsley explains, "with neural connections to almost all the areas of the brain that process sensory information, [the prefrontal cortex] is well situated to maintain a flexible store of information relevant to any task at hand" (1).
Neurologist Patricia Goldman-Rakic of Yale University has begun to map the various areas of the prefrontal cortex into various regions associated with the different senses. Her laboratory has found evidence that information about spatial location is confined to the sub-region of the prefrontal cortex, while processes related to visual appearance are in a separate area below that. Her findings, however, are still controversial (1).
Short-term memory is the subject of various other arguments as well. "Short-term memory...may be either plastic or dynamic in nature, and this is still a matter of debate. In the plastic scenario, short-term memories are formed by brief changes in synaptic transmissions. In the dynamic theory, it may arise out of a reverberating feedback circuit, where a memory is held electrically within a loop. Thus, no physical changes are made, and synaptic connections are not modified. " Long term memory, [on the other hand,] may be encoded by plastic changes in existing synapses" (7).
Perhaps the most fascinating questions about memory have to do with the connection between human consciousness and memory. Psychologist Michael Dawson notes that what makes memories feel so real is that "real-time experience is just as indirect" as remembered experience (5). In other words, the neurobiological process of recollecting an experience is in some ways identical to the process of experiencing it in the first place! Dawson goes on to point out that all consciousness can be said to be recent memory, due to the time lag between experience and the perception of experience (5). One can imagine this lag as akin to the fifty second delay of a "live" football broadcast.
New discoveries about the nature of memory and the workings of the mind are cropping up daily. In order to keep up with the dizzying barrage of new information, I've included a list of websites used as source material for this paper. If you find yourself overwhelmed, just tell yourself that none of this is really happening. It's just a memory, that's all.
World Wide Web SourcesSerendip