The notion of memory is so intriguing that we've come up with more metaphors for it than for any other mental phenomenon. Early theories predicted a memory "engram," a literal text written by the body to describe past experiences. Freud popularized descriptions of repressed memories, experiences physically buried in the depths of the subconscious. Modern descriptions are dominated by analogies to computers, in which the human brain is a hard disk that stores experience in electronic files and folders. Typical of biology, the truth is at once more complicated and more beautiful than any of these descriptions.
Fundamentally, memory represents a change in who we are. Our habits, our ideologies, our hopes and fears are all influenced by what we remember of our past. At the most basic level, we remember because the connections between our brains' neurons change; each experience primes the brain for the next experience, so that the physical stuff we're made of reflects our history like mountains reflect geologic eras. Memory also represents a change in who we are because it is predictive of who we will become. We remember things more easily if we have been exposed to similar things before, so what we remember from the past has a lot to do with what we can learn in the future.
Scientists divide memory into categories based on the amount of time the memory lasts: the shortest memories lasting only milliseconds are called immediate memories ,memories lasting about a minute are called working memories , and memories lasting anywhere from an hour to many years are called long-term memories.
Each type of memory is tied to a particular type of brain function. Long-term memory, the class that we are most familiar with, is used to store facts, observations, and the stories of our lives. Working memory is used to hold the same kind of information for a much shorter amount of time, often just long enough for the information to be useful; for instance, working memory might hold the page number of a magazine article just long enough for you to turn to that page. Immediate memory is typically so short-lived that we don't even think of it as memory; the brain uses immediate memory as a collecting bin, so that, for instance, when your eyes jump from point to point across a scene the individual snapshots are collected together into what seems like a smooth panorama.
Another way to categorize memory is to divide memories about what something is from memories about how something is done. Skills like catching a baseball or riding a bicycle are called nondeclarative memories because we perform those activities automatically, with no conscious recollection of how we learned the skills. Declarative memories , on the other hand, are memories of facts and events that we can consciously recall and describe verbally.
Categorizing memory temporally and functionally makes sense from the clinical and biological perspective; patients with various amnesias may have difficulty with one particular type of memory and not with others. Moreover, scientists have discovered that different brain structures are specialized to process each category of memory, suggesting that these categories are not merely convenient for discussion, but are based in the biology of how we remember. Understanding how memories are formed in each category and how some memories move amongst categories can help to focus strategies for improving memory and learning.
Modern computers encode memory as a vast array of independent, digital bits of information that are "randomly accessible." Functionally, this means that your computer can bring up your best friend's phone number without accessing any information about what your best friend looks like or how you met. The human brain stores memory in a very different way; recalling your best friend's phone number may very well bring to mind your friend's face, a pleasant conversation that you had, and the title of the movie that the two of you are going to see. While computer memories are discrete and informationally simple, human memories are tangled together and informationally complex.
Our memories are rich because they are formed through associations. When we experience an event, our brains tie the sights, smells, sounds, and our own impressions together into a relationship. That relationship itself is the memory of the event. Unlike computer memories, a human memory is not a discrete thing that exists at a particular location; instead, it is an abstract relationship amongst thoughts that arises out of neural activity spread over the whole brain.
But how is the memory relationship actually made? The process from both a biological and a behavioral perspective is critically dependent on reinforcement. Reinforcement can come in the form of repetition or practice; we remember that two plus two equals four because we've heard it so many times. Reinforcement can also occur through emotional arousal; most people remember where they were when they heard that John F. Kennedy was shot because of the highly emotional content of that event. Arousal is also a product of attention, so memories can be reinforced independent of context by paying careful attention and consciously attempting to remember.
Sensory information like sight, sound, and touch are independently maintained in immediate memory. By attending to certain aspects of an experience, sensory information can be brought together into an event in working memory, where continued attention can maintain that information "online." Reinforcement and arousal allow events in working memory to be consolidated into long-term memory storage.>insert below pic. If you paid attention during the introduction, the relationship between sight, sound, and awareness is brought together into working memory, somewhere in the prefrontal lobe of the brain. When the event moves from immediate memory to working memory, certain features will be lost. You probably won't remember background conversations from the party, and you may not remember the color of the Mr. Byrd's shoes. The loss of distracting information is an important feature of human memory, and is critical for efficient storage and recollection of experiences.
At this point you might rehearse the event by saying the name to yourself, or by making up a mnemonic (John Byrd, who has a big hook nose like a bird). The mnemonic and the rehearsal cause the memory to move from working memory into long-term memory, a change that starts in the brain's hippocampus . The process of converting working memory into long-term memory is called consolidation , and again, it is characterized by the loss of distracting information. Several days after meeting Mr. Byrd you may not be able to remember what color his tie was or whether he wore a wristwatch, but you will still remember his face, his name, and the person who introduced you to him. The consolidation phase of memory formation is sensitive to interruption; if you are distracted just after meeting Mr. Byrd, you may have trouble remembering his name later.
So to recap, the event of meeting John Byrd started out in immediate memory, spread out in various modality-specific regions of the brain. Reinforcement through attention caused the relationship between sight, sound, and context to consolidate into working memory in the prefrontal lobe. Further reinforcement through practice caused more consolidation, and the most critical relationships in the event (the name, the face, and the context) were tied together in the hippocampus. From there, the memory relationship is probably stored diffusely across the cerebral cortex , but research on the actual location of memory relationships is still inconclusive.
Memory loss is the inability to recall people, objects, places, or events that took place in the recent or distant past. It is most often a temporary condition and covers only a part of a person's experience, such as immediate memory. The causes of amnesia range from psychological trauma to brain damage caused by a blow to the head or conditions such as a brain tumour, a stroke or swelling of the brain.
In severe cases of amnesia, memory loss can result from a two-sided or bilateral (both hemispheres) damage to parts of the brain vital for memory storage, processing, or recall. These parts involve the limbic system, which includes of the hippocampus in the medial temporal lobe of the brain).
The hippocampus encodes memory for events and episodes. Memories, either short term or long term, are not stored in the hippocampus. But instead, information gets processed into memory in the hippocampus. Therefore, people suffering hippocampal damage might still have the storage capacity available for use, but memories cannot be processed for storage. Hippocampal injury disrupts the ability to convert short-term memories into long-term memories .
Some sources of brain alteration include aging, chronic emotional stress, car accidents, head injury, intense fear, physical traumas, drugs, rape, assault, sexual molestation during childhood, cold baths, surgery, stroke, hypoxia (lack of oxygen), closed head injury, electroconvulsive therapy (ECT)-usually temporary, Korsakoff's syndrome (alcoholism), or Alzheimer's disease. Severe stress, emotional shock, as well as a blow to the head or a stroke can induce sudden and drastic alterations in neurotransmitter and arousal levels in the same respect.
Amnesia can be either functional, i.e. based on psychodynamic factors (such as defense mechanisms), or organic, i.e. based on brain disorders or lesions.
Functional amnesia deals with disorders of memory that seem to result from psychological trauma, not an injury to the brain. Though traumatic events affect the brain in some way, people with functional amnesia appear to have nothing physically wrong with their brains.
Organic amnesia involves memory loss or traumatic forgetting caused by specific brain damage. Tumors, strokes, head trauma, or degenerative diseases (e.g. Alzheimer's disease) may involve organic amnesias. They may be temporary or permanent. The most well known amnesic patient is H.M. who had brain surgery in 1953 for epilepsy leaving him with dramatic anterograde amnesia. H.M. was unable to remember new information and events that occurred after his operation. His hippocampus was destroyed surgically in the course of operation to relieve the effects of epilepsy. The brain damage did not result in loss of learned motor skills, but the problem was with declarative memory. H.M. more specifically had organic amnesia with bilateral damage to his parahippocampal cortex, entorhinal cortex, and perirhinal cortex.
Compared to neuroscientists who are mainly interested in the anatomic structures and activities of the brain and the neurological alterations occurring in cases of amnesia, when philosophers approach this issue, one of their main concerns is the question of identity; if a person�s thinking is altered, is the person the same person or a different one? Is a person with amnesia, whose thinking is altered chemically, considered to be a different person?
