WHY WE SHOULD
KNOW HOW MEMORY WORKS ?
Understanding the physiology of memory is the ultimate challenge to neuroscience research. It is, indeed, through the hard work conducted in this field that we have managed to make important progress in comprehending as to how human memory operates. The brain is complex, and so is learning and remembering. Despite the fact that the individual changes that occur within the cells of the brain may be relatively simple, the brain consists of many billions of neurons. Consequently, it is exceedingly difficult to isolate and identify the particular changes which are responsible for a particular memory. Learning is the process by which experiences change our nervous system and, hence, our behaviour i.e. how we perceive, perform, think and plan. Undoubtedly, the primary function of the ability to learn is to develop behaviours that are adapted to an ever-changing environment.
WHAT ARE THE STAGES OF MEMORY ?
Memory involves three stages:
It should be borne in mind that the three stages of memory do not operate in the same way in all memory situations and that memory can fail at any of these three stages.
Furthermore, the three stages may operate differently in situations that require us to store material for a matter of seconds – Short Term Memory (STM) which is considered active – than in situations that require us to store material for longer intervals – Long Term Memory (LTM) which is considered passive.
STAGE OF MEMORY:
Encoding entails that one attends to specific information which is deposited in one’s memory in a certain form or code. This code could be visual – a mental picture – or acoustic sound.
Research shows that Short Term Memory (STM) tends to favour an acoustic code for verbal materials like digits, letters and words but it sometimes uses a visual code. However, with verbal materials, the visual code fades quickly and is soon dominated by the acoustic one. The dominance of the acoustic code may apply mainly to verbal materials as other research suggests that when it comes to storing non-verbal items, like pictures that are difficult to describe, the visual code may become more important. It is also possible to store information in codes linked to other senses, such as touch and smell.
STAGES OF MEMORY:
STORAGE & RETRIEVAL
The most striking fact about STM is that its storage capacity is limited to 7 (±2) items or chunks. When this limit is reached, a new item can enter short-term memory only by displacing an old one. Therefore, what is at the forefront of our memory must soon give way to subsequent information. However, a process called rehearsal constitutes the only exception to the displacement process. Rehearsal entails that the same items are recycled for example, phone numbers. What happens during rehearsing is that we cannot encode new items at the same time we are rehearsing old ones. Therefore, the more we rehearse, the less we encode which means that the more difficult is for displacement to occur. Another reason we forget is that, as time goes by, regardless of whether or not new information follows, information decays with time. However, rehearsing an item that has partly faded may bring it to full strength again.
According to contemporary theories of human memory, once input information has been processed or interpreted in a reasonably deep fashion the underlying abstraction is stored away in this so-called long-term memory.
Thus, LTM is presumed to be without limit either incapacity to store information or in duration of that which is stored. Most of the research on LTM is highly specialized. The search is for the organizational principles and the form(s) of memorial representation that will explicate memory generally. However, the work itself focuses on specific types of information storage. Special cases of LTM are the semantic memory, the episodic memory, lexical memory (also known as reference memory), associative memory and image.
Definitions of the foresaid which are considered to be special cases of the long-term processing system (also known as the long-term store (LTS), secondary memory and permanent memory) are:
- The method of loci – It was used by Greek and Roman orators. It consists of mentally placing each one of a list of “to-be-remembered items” in a well-known locus. Then, a “mental walk” serves to retrieve items in the correct order. The trick consists of learning a newly ordered series of items by relating it to an already known ordered series of locations.
- Peg Words – This method focuses on learning a word list which entails mastering a peg list of rhyming pairs. Then, “the list to be remembered” is hung on the pegs by means of interactive imagery.
- Initial letters mnemonics – A sentence with the same initial letters e.g. “Richard of York gains battles in vain” to remember “red”, “orange”, etc.
- The PORS
- Preview – examine the chapter looking at the headings
- Question – formulate questions so you know what information you are aiming to extract
- Read – read the material answering questions
- Summarize – summarize what you have read
Most cases of forgetting in LTM are due to retrieval failures which means that, even though the information is stored, it cannot be found. Retrieval failures are more likely to occur when there is interference from similar items and when emotional factors somehow disrupt the usual retrieval processes. It is to be borne in mind that interference, which operates in two directions, decreases retrieval.
- Retroactive interference occurs when the learning of new materials interferes with our memory of old ones.
- Proactive interference occurs when previously learned material interferes with our attempt to remember new information (after having the same parking place for a year, your place is changed. Initially, it may be difficult to retrieve from memory your new parking location). The best ways to improve retrieval are to organize the material (hierarchical organization), attempt to restore the learning context at the time of retrieval and practice retrieving information while learning it.
Memory for complex materials, like sentences and stories, involves a constructive process. For example, we often take sentences as incomplete descriptions of events and use our general knowledge of the world to construct a more complete description. Construction may involve adding inferences to the material presented or fitting the material into stereotypes and schemata.
This theory assumes that information is transferred from short-term to long-term memory, often by the process of rehearsal. The theory offers an explanation of several memory disorders. For example, Retrograde Amnesia is considered to be a loss of memory for events immediately preceding a head injury, is thought to be due to a disruption of STM;
Anterograde Amnesia, an inability to learn new information after removal of the hippocampus (an area deep in the brain’s temporal lobes), is assumed to be caused by a breakdown of the transfer process. Furthermore, this theory accounts for the results of experiments on free recall:
- items at the end of a list are remembered well because they are still in STM,
- while items at the beginning of a list are remembered well because they are rehearsed more often.
However, recent findings concerning rehearsal and depth of encoding suggest the theory is incomplete. A new approach, called depth-of-processing, is developing. It assumes items are analyzed to various levels and that deeper levels of analysis lead to better memories.
As already mentioned, amnesias occur after some disturbance to the brain, such as head injury or certain degenerative brain diseases. The first explanation for anterograde amnesia was that the ability of the brain to consolidate short-term memories into long-term memories was damaged. However, studies show that ordinary perceptual, stimulus-response and motor learning do not appear to be impaired; people can learn to recognize new stimuli, they are capable of instrumental and classical conditioning and they can acquire motor memories. Despite the foresaid, people with anterograde amnesia are not capable of declarative learning – of describing events that happen to them. Due to this difficulty, anterograde amnesia has also been called a deficit in explicit memory. Additionally, people with anterograde amnesia exhibit relational learning difficulties i.e. they are unable to master the meanings of words they did not know before the brain damage took place.
Further studies in this field show that anterograde amnesia can be caused by the theanine deficiency that sometimes accompanies chronic alcoholism (Korsakoff’s syndrome) or malnutrition. Alcoholics that suffer from this syndrome exhibit confabulation, which is the reporting of memories of events that did not take place, without the intention to deceive. Confabulation may be caused by disruption of the frontal lobes. Studies show that Korsakoff’s syndrome is caused by damage to the mammillary bodies, which receive input from the hippocampal formation via the fornix and relay it to the anterior thalamus. Generally, traumatic or surgical damage to the fornix, mammillary bodies, or the connection between the mammillary bodies and the anterior thalamus also produce anterograde amnesia.
Furthermore, anterograde amnesia appears to be a loss of the ability to learn about the relations among stimuli, including the time and place in which they occurred and the order of their occurrence. Apart from its accompanying chronic alcoholism, it can also be caused by damage to the temporal lobes such as bilateral medial temporal lobectomy. For this reason, neurosurgeons stopped performing them and are now careful to operate on only one temporal lobe because such an operation causes minor memory problems. Before a patient receives a unilateral temporal lobectomy, a short-acting anaesthetic is injected into the carotid artery that serves the hemisphere that is to be operated on. If the patient is able to learn and remember items that are presented while that hemisphere is anaesthetized, we can conclude that the other hemisphere contains a functioning hippocampal formation and the one in the anaesthetized hemisphere can safely be removed.
Olton (1983) suggested that lesions of the hippocampus or its connections impair working memory but leave reference memory relatively intact. Working memory consists of information about things that have just happened, information that is useful in the immediate future but may change from day today. Thus, it is “erasable” memory that is replaced on a regular basis. On the other hand, reference memory is permanent, long-term memory, produced by consistent conditions. Hippocampal lesions disrupt the ability to keep track of and remember spatial locations.
Biochemical bases of LTM Storage
Research shows that changes in the brain underlie the encoding and storage of memories. These changes occur at the synaptic connections between brain cells as well as the biochemical nature of the brain cells themselves.
First of all, the nucleus of a cell contains deoxyribonucleic acid (DNA) which is the primary vehicle of heritable characteristics i.e. our genes are composed chiefly of DNA molecules and of Bmolecules and the genetic code is written along the DNA molecule. It is to be borne in mind that the DNA never leaves the cell’s nucleus but rather directs the cell’s activities by manufacturing its own “assistants” to which it then “delegates” responsibilities. The assistants are various forms of ribonucleic acid (RNA) which after having been manufactured in the nucleus, move out to the cytoplasm, where they control the essential functions of the cell. Studies support that the RNA composes the biochemical basis of memory.
Flexner (1967): An animal which had learned a particular response was injected with an RNA-inhibitor and then tested for its memory of the learned response. Specifically, mice that had learned to avoid shock in a maze were injected with a known RNA-inhibitor and lost their memory of the maze.
Hyden (1969): showed that learning produces a change in the RNA of specific cells. Young rats learned to balance on a thin, slanting wire in order to obtain food. The vestibular nerve cells are those that play a critical role in the memory of this balancing skill. Cells of this kind in mice that had learned the balancing skill were found to contain more RNA and, also, RNA of different composition than the cells of control animals that had not learned the skill. Therefore, the acquisition of a specific memory was accompanied by an increase in specific cells.
Science has still to research further as to why damage to the hippocampal formation does not disrupt recall of memories that occurred early in a person’s life but does disrupt recall of more recent memories. The period of retrograde amnesia of a person with severe anterograde amnesia can be as long as fifteen years.
A battery of Memory Tests
The following tests are administered in order to evaluate memory status:
- Wechsler Memory Scale-III and the Russell modification
- The Wechsler subtests of Digit Span, Digit Symbol, and Information
- Benton Visual Motor Retention Test-Revised
- Graham-Kendall Memory for Designs Test
- Wide Range Assessment of Memory and Learning
- Rey Auditory Verbal Learning Test
- Rey-Osterreth Complex Figure Test with Meyers’s norms Clock Test
- Rivermead Behavioural Memory Test; and
- Denman Neuropsychology Memory Scale
- Carlson, R.N. (1977) Physiology of Behaviour (6th ed), Boston: Allyn & Bacon
- Hilgard, E.R., Atkinson, R.L., Atkinson (1979) Introduction to Psychology (7th ed), New York: Harcourt Brace Jovanovich, Inc.
- Reber, A.S., The Penguin Dictionary of Psychology (1985), Harmondsworth: Penguin Books Ltd
- Zuckerman, E.L., (2000) Clinician’s Thesaurus (5th ed), New York: The Guilford Press