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.

Physiology of Memory 

Certain areas of the brain are believed to be involved in memory processing, like the amygdala (emotional memory), hippocampus(declarative learning) and mammillary bodies.To determine which areas of the brain are involved in memory processing, researchers study animal models and patients with damage to those brain areas. Memory, however, is not dependent on a particular region; adjacent areas and pathways and synaptic changes are also implicated in learning and memory. Memory building starts with encoding, which is the perception of data through smell, sound, taste, touch and sight. Information is then encoded and stored in the form of electrical charges and chemicals.

Communication between neurons (brain cells) occurs via the synapse, the spaces between neurons and center of brain activity. The firing of pulses across the synapse triggers the release of chemical messengers called neurotransmitters, which attaches to neighboring cells. These power linkages form a network between cells and groups of cells, but are temporary. Stronger connections between cells are possible through repetition. In fact, practice is the basis of how the brain organizers itself. For example, drawing or playing a piece of the music everyday enforces brain connections that ensures you get better at the task with time. Skipping a day or a week of practice, however, slows down the process to perfection. The brain “forgets.”

Parts of the Brain Involved in Memory Processing

Hippocampus. The hippocampus (Greek term for ‘seahorse’, which the structure resembles) is part of the brain’s limbic system. It contains mental maps and plays a part in the formation of explicit memory, memory consolidation and the conversion of short term memory into long term memory. Pre-processed information from different parts of the cortex feeds into the hippocampus, which also sends out information to different areas of the brain. Without the hippocampus, it is possible to form semantic but not episodic memories.Damage to the hippocampus may cause loss of memory and impaired memory storage.

Cerebellum. The cerebellum is a structure at the base of the brain that looks like a smaller replica of the cerebral cortex. Learning to drive a car and play the guitar are motor skills needing procedural memory. The cerebellum also coordinates balance and movement.

Amygdala. The amygdala is located below the hippocampus and is associated with emotions and memory. Neurons in the amygdala help in encoding emotional memories. Studies show that intense emotional activity associated with events increase the chances

of the events being remembered. For instance, people with damage to the amygdala do not show this memory enhancement effect.Research showsthat injection of hormones cortisol or epinephrine (causing stimulation of the amygdala) enhances memory storage. However, excessive cortisol may impair memory storage.

Basal ganglia. The basal ganglia are located in the medial temporal lobe and are associated with learning, memory and unconscious memory processes. Damage to the basal ganglia results in some type of motor function impairment.

Frontal Lobe. The frontal lobe of the cerebral cortex is important in working memory; it helps select memories relevant to an event. If you need to go to part of town you’ve never seen before, you combine information that you already have (traffic routes, information from a map) to get to your destination. The frontal lobe helps us remember what needs to be done in the future, like attending a concert or showing up for a doctor’s appointment.

Parietal Lobe. Thisregion performs many functions, including sensation, perception and construction of a coordinate system or a map to represent the world. Along with frontal lobe regions like the dorsolateral prefrontal cortex, the parietal lobe is also involved in short term memory processing (neuronal communication). Damage to the left parietal lobe results in writing and mathematical difficulties, while damage to the right parietal lobe causes sufferers to treat body parts and some objects in the visual field as though they do not exist.

Temporal Lobe. This area is associated with autobiographical and recognition memory, the ability to identify an item that was recently encountered, such as a person. Temporal lobe damage can affect long-term memory.

Occipital Lobe. The occipital lobe plays a key role in vision. It is found in the back of the brain underlying the occipital bone. Damage can result in hallucinations, impaired movement and color discrimination. Left side damage can cause language problems while right side damage can cause non-verbal problems.

The Engram

An engram is a hypothetical memory trace in the form of biochemical or biophysical changes in the brain. They can also be neural networks or memory fragments, reinforcing the notion that memories are not localized in a particular area of the brain. Much of engram research focuses on its exact mechanism and location. Many neuroscientists believe thatwhile certain types of information are localized in specific brain regions, memories involved in complex tasks (such as rats running in mazes) are likely spread out among neural systems. Richard Thompson studied the engram in the cerebellum using classical conditioning in rabbits. He introduced air on the rabbit cornea along with a tone that produced a blinking response. Eventually, the rabbits blinked when only the tone was introduced, allowing Thompson to study possible engram locations in the rabbits’ brain.

Thompson’s team found that deactivation and reactivation of the lateral interpositus nucleus (LIP) affected the conditioning response, showing that the LIP is involved in the engram for the eyelid response. Since the cerebellum is a more primitive region, the study only examined automatic responses. Recent studies performed on mice have also that is possible to localize certain memories into specific neurons, explaining how and why the brain organizes memories and allocates a specific memory to a group of neurons.According to an MIT study, a specific memory can be expressed in a mammal by physically activating a specific group of brain cells by light. This shows that some memories are localized in certain regions of the brain. Other researchers have shown that the limbic system and cortical regions are involved in declarative memory processes, but there is still much to be done in this area. Studies on humans are limited or do not exist, making it unclear what neuronal changesoccur with higher cognition processes.

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