Types of Memory

Memory is the process of encoding, storing and retrieving information received by the senses. Encoding describes how information in the form of stimuli reaches the senses, storage is concerned with maintaining information over time and retrieval is the process of locating stored information and bringing this data into consciousness. Memory also refers

to the act of remembering and the act remembered. Memory is not exclusive to human beings; memory is an organism’s ability to store, retain and recall experiences and information. The study of memory started in the field of philosophy and was later included in the fields of cognitive psychology and cognitive neuroscience. Memory can be classified according to information type and whether the information to be recalled is from the past (retrospective) or to be remembered in the future (prospective).

 Sensory Memory

Sensory memory is memory of the 200-500 milliseconds after an item is perceived. Remembering what an item looked like after just a second of observation is an example of sensory memory. This type of memory degrades within a few hundred milliseconds and cannot be extended by practice. George Sperling conducted the first experiments in sensory memory by showing subjects a grid of letters arranged in rows. Different tones were afterwards presented to the subjects as cues. Sperling concluded that sensory memory has a capacity of about 12 items, but the memorydecayed in a matter of seconds.Types of sensory memory include iconic memory and echoic memory. Iconic memory briefly stores visual information that has been perceived in a very short period of time, while echoic memory briefly stores auditory information.

Short Term Memory

Short term memory is a temporary period of recall from several seconds to one minute. The name of a tune just played on the radio and a random phone number are short term memories. Working memory is a type of short term memory that organisms manipulate while it is stored, such as remembering pricesand working out percentages at the supermarket. Like sensory memory, short term memory capacity is limited. Studies show that an average person can only hold about seven bits—give or take two bits—of information in mind at one time. Recent estimates peg the bits lower, generally at four to five. Another characteristic of short term memory is susceptibility to interruptions. For example, if you’re trying to remember the name of a plumbing company and your friend walks into the room and engages you in conversation, the name of the plumbing company flies out the window.

Long-Term Memory

A person may remember the time his neighbor’s dog chased him down the street, but not the name of the telemarketer who called him this morning. The brain stores important memories while getting rid of trivial ones—the process is akin to automatically clearing out spam mail from an e-mail inbox. How to get home, drive a car and operate a computer are long-term memories. The music playing on the radio the night a family member died and the smell of frying bacon in the kitchen when you were a child are other examples. Long-term memory differs from short-term memory not only by how long memories last, but also by how much information a memory system can handle. The brain can process only a small number of short-term memories simultaneously, but it can store a relatively large number of long-term memories. This is why people can always learn and store new information, provided the brain is not diseased or injured. Long term memories are also more durable, but they are also changeable. When something interrupts your train of thought for example, you still know how to operate a mobile phone.Previously-learned memories can also remain intact even in the early stages of dementia.

Maintaining long-term memory requires practice. That is, memory has to be revisited to retain it. Unused or irrelevant long-term memories eventually fade or become distorted. The inability to recall the plot or characters of a book that you had once read and loved is one example. Some long-term memories are persistent, however, regardless of how many times you use them. Memories of a first romance or a childhood punishment are remarkably durable, but research shows that the way people perceive these memories can change based on information acquired during the intervening years. Long term memories can be classified into declarative and procedural memory.

Declarative Memory

Also called explicit memory, some conscious process must call back the information in declarative memory. Declarative memory can be further subdivided into semantic and episodic. Semantic memories are abstract memories independent of context. World capitals and place names are examples. Episodic memories are memories associated with a timestamp or a specific period, for example your wedding day or the day you graduated from college. Autobiographical and flashbulb memories can be classified under episodic. Autobiographical memory is memory for specific events in an individual’s own life. The term “flashbulb memory” is used to describe a strong memory of an unexpected and emotionally-charged event. The Japan earthquake and the events of 11 September 2001 are examples. Some emotionally-charged memories are stored and recalled much easier than other memories. New information is also more likely to be retained when it is related to information already stored. Stronger associations between old and new memories mean clearer and easily retrieved memories.

Procedural Memory

Also called implicit memory, procedural memory is associated with learning routines and motor skills. This type of memory is not based on conscious recall of information; people use procedural memories to perform actions automatically like walking, swimming or driving a car. These activities required conscious effort and practice at one time, but once mastered, people perform them without thinking about each individual step. Procedural memory is durable, even with illness or aging. Those suffering from Alzheimer’s disease can perform many routine tasks, and some patients with amnesia can also learn new skills like playing computer games. Researches attribute this durability partly to the widespread distribution of procedural memories in the brain.

Other types of memory include topographic, retrospective and prospective. Topographic memory is the ability to follow an itinerary and orient oneself in space. Retrospective memory (for past information) includes semantic, episodic and autobiographical memory, while prospective memory (for future information) is the process of remembering to remember. Prospective memory can be event- or time-based.

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