What is short-term memory?
If you have looked up a telephone number in a directory, you already know a great deal about short-term memory. You repeat the number to yourself until you dial, and then you forget it. If the number is busy, you may have to look it up again.
Short-term memory is the component of the memory system that holds information the individual is consciously thinking about at the moment. Its capacity appears to be limited to about seven items, and unless the information is rehearsed, it will be lost from short-term memory in about fifteen seconds.
Evidence proving the existence of short-term memory comes from studies of the serial position effect, a phenomenon in which a person’s ability to recall items from a list depends on the item’s position in the list. The primacy effect occurs when items at the beginning of the list are recalled well; the recency effect occurs when the items toward the end of the list are recalled well. Those words that come in the middle of the list are usually not recalled very well, unless they are highlighted in some way, perhaps by using boldface or a yellow marker. The reason for these serial position effects is that words at the beginning of the list have been committed to long-term memory, and words near the end are still in short-term memory. If the subject is distracted, those items at the end of the list that had been in short-term memory will be forgotten. Under these circumstances, there will be no recency effect at all. Studies like these make it clear that human beings do have short-term memory.
Capacity of short-term memory
The amount of information that can be held in short-term memory is an intriguing question. Your own experience should tell you that short-term memory’s capacity is limited, perhaps to the amount of information in a seven-digit telephone number. Experiments that test a subject’s ability to recall a series of items usually report similar results.
These findings created quite a puzzle for psychologists because it was difficult to define the abstract term “item”. People could remember seven numbers, seven proper names, seven letters, seven faces, seven Shakespearean verses, or seven proverbs. Clearly, the amount of information in each of these “items” is very different. Cognitive psychologist George A. Miller addressed this problem in 1956 in a paper called “The Magical Number Seven, Plus or Minus Two”. He suggested that short-term memory could hold about seven chunks of information ± two. A chunk refers to anything that is represented in long-term memory as a single unit. This is frequently referred to as Miller’s Law.
The capacity of short-term memory can be dramatically increased by reorganizing information into larger chunks. One student with an average memory tried to improve his ability to repeat a sequence of digits read to him. At first, he could only correctly repeat seven or eight digits, but after two years of practice, he was able to correctly repeat almost eighty digits.
The researchers found that the student’s improved skill was entirely due to the reorganization of information into increasingly larger chunks. For example, if the sequence 3492 appeared, it was recorded from four chunks into one as “3 minutes and 49.2 seconds, near world record time” (for the mile). Most of his larger chunks were running times for races, but he also recorded strings of numbers into dates or ages. His remarkable ability could only be demonstrated on number strings, however. When they read him a list of letters, his memory span dropped to six. This shows that the capacity of his short-term memory had not changed — only the size of his number chunks. The experiment is described in The Genius in All of Us by David Shenck:
Ericsson and Chase recruited an undistinguished college student for an epic experiment. The student, known by his initials, S.F., tested normal for intelligence and normal for short-term memory performance. Memory-wise, he was just like you or me. Then they began the training. It was grueling work. In one-hour sessions, three to five sessions per week, researchers read sequences of random numbers to S.F. at the rate of one digit per second: 2… 5… 3… 5… 4… 9… At intervals, they stopped and asked him to echo their list back. “If the sequence was reported correctly,” the researchers noted, “the next sequence was increased by one digit; otherwise it was decreased by one digit.” 2… 5… 3… 5… 4… 9… 7… At the end of every session, S.F. was asked to recall as many of that day’s numbers as possible. 2… 5… 3… 5… 4… 9… 7… 6…
Instead of jumping off a bridge or transferring to another college, S.F. kept returning to the memory lab. In fact, he continued to participate most days of the week for more than two years — more than 250 hours of lab time. Why? Perhaps because he was seeing results. Almost immediately, his short-term memory performance started to improve: from seven digits to ten after a handful of sessions, then to an amazing twenty digits after several more dozen training hours. Already he had clearly escaped the normal bounds of short-term memory. From there, the improvements continued unabated: to thirty digits, forty, fifty, sixty, seventy, and finally to a staggering eighty-plus digits before the team concluded the experiment.
There was no indication as the sessions ended that he had reached any sort of boundary. “With practice,” Ericsson and Chase concluded, “there is seemingly no limit to memory performance.”
How did he do it? Through interviews with S.F., Ericsson and Chase realized that their subject had neither trapped into a hidden memory gift nor somehow transformed the brain circuitry of his short-term memory. Rather, he had simply employed clever strategies that enabled him to get around his — and all of our — natural limits.
S.F. happened to be a competitive runner. Early on, after trying in vain simply to remember as many random numbers as possible, he realized that when he pictured an unconnected string of three or four digits as one single race time — for example, converting the numbers 5 – 2 – 3 – 4 into five minutes and twenty-three point four seconds — the numbers would come back to him quite easily.
This was not a new technique; attaching disconnected pieces of information to older memories goes back all the way to the Greek “memory palaces” of the fourth century B.C. The trick is to assign new information to some system or image that’s already in your head. For example, a classroom teacher could mentally “place” the face and name of each new student in a different room in her home: Lucas in the dining room; Oscar in the pantry; Malcolm standing in the kitchen sink. The advantage of this technique, explained Ericsson and Chase in their report, “is that it relieves the burden on short-term memory because recall can be achieved through a single association with an already-existing code in long-term memory.” S.F., like every impressive mnemonist before him, had not transformed his natural memory limit; instead he had changed the way he formed new memories to take advantage of a different, less restrictive memory system.
But how did the researchers know for sure that S.F. had not actually altered his short-term memory capacity? Simple: between number sessions, they also tested him with random alphabet letters: U… Q… B… Y… D… X… Whenever they did this, his memory performance immediately reverted to normal. Without specific mnemonic tricks and lots of contextual practice, his short-term memory was again as ordinary as yours and mine.
Besides a limited capacity, the short-term memory also has a limited ability to hold items for any length of time. In one classic experiment, subjects tried to remember three letters of the alphabet. After eighteen seconds, they could not remember the letters. These people were not slow-witted; they were simply not allowed to rehearse because they were counting backward by threes during those eighteen seconds. Unless the chunks in short-term memory are rehearsed, they are forgotten fairly rapidly.
These limitations on short-term memory are very puzzling considering how much work it must do. For example, with a short-term memory that can only hold seven items at a time, how can we read so quickly and comprehend what we are reading? The answer appears to lie in the idea that short-term memory is not just a passive storehouse for a limited amount of information. It is a dynamic information processor, one that performs computations, screens information, and interacts constantly with long-term memory. Many psychologists prefer to call short-term memory “working memory” to emphasize its information processing role. To show how this might work, and how short-term memory must interact with long-term memory, read the sentence below once. Then look up and try to recall the sentence.
“In the failing days of the Roman Empire, the emperors were increasingly protected by German Troops recruited or enslaved through military conquests.”
When you tried to recall the sentence you were probably unable to repeat it word for word — too many chunks. But your short-term memory was working with your long-term memory’s storehouse of information on the Roman Empire so you could reproduce much of the meaning. Perhaps some of the information in your long-term memory even leaked into your recollection even though it wasn’t in the original sentence. In contrast, people who have little or no knowledge of the Roman Empire would have a much harder time trying to recall this sentence since there would be little in their long-term memory related to it. This is one of the reasons why it is so difficult to design tests of reading comprehension that are valid measures for people who grow up in difficult cultures. A person reared in China might know the Chinese emperors very well but know nothing about the Roman ones. The less information you have in your long-term memory about a subject, the smaller will be your chunks and the more trouble you will have to recall the details of a paragraph.
Difference between short-term and working memory
The distinction between short-term memory and working memory is an ongoing debate, as the terms are often used interchangeably.
The term ‘working memory’ was coined in 1960 by Miller, Galanter and Pribram in their classic book Plans and the Structure of Behaviour, used in 1968 by Atkinson and Shiffrin in an influential paper “Human memory: A proposed system and its control processes”, and adopted as the title for a multicomponent model by Baddeley and Hitch in 1974.
Some scholars claim that some kind of manipulation of remembered information is needed to qualify the task as one of working memory. Repeating digits in the same order they were presented would thus be a short-term memory task, while repeating them backward would be a working memory task.
Another viewpoint is that of Nelson Cowan, who says short-term memory refers to the passive storage of information when rehearsal is prevented with storage capacity around four items. When rehearsal is allowed and controlled attention is involved, it is a working memory task and the capacity is closer to seven items.
Losing information from short-term memory
A long-running controversy exists over how information is lost from short-term memory. The decay theory of forgetting maintains that information simply decays or fades over time, usually within fifteen seconds, unless it is constantly rehearsed. The interference theory of forgetting proposes that information remains in short-term memory until new information interferes with or replaces it.
Attempts to confirm one of these theories and discard the other have been frequent but unsuccessful. For example, the study in which the subjects forgot three letters after just eighteen seconds because they could not rehearse would suggest the decay theory is correct. They were not trying to add new information, but they forgot the letters anyway. Another study, however, found that subjects could recall three words quite easily after a fifteen-second delay if the task used to prevent rehearsal involved detection of an auditory stimulus rather than counting. Perhaps the counting task provided some interference and the auditory task provided none.
If short-term memory is really doing a great deal of its own information processing, then information is lost purposefully, not simply because it decays or is replaced. This means that the loss of information would depend partly on its interest and value and would not be uniform.
Transfer to long-term memory
The manner in which information in short-term memory is rehearsed is important to whether it will eventually be transferred to long-term memory. If you simply want to remember a phone number for a short period, perhaps until you dial it, you use maintenance rehearsal; using this technique, a person maintains information in short-term memory by continually repeating it. When the person stops repeating the information, it is lost. Elaborative rehearsal facilitates the transfer of information from short-term memory to long-term memory. This process involves organizing the information and integrating it with the knowledge that already exists in long-term memory.
A fascinating study demonstrates how important the integration process is in the transfer of information from short-term memory to long-term memory. A group of people read the story under the title “Watching a Peace March from the 40th Floor.” Another group read the same story under the title “A Space Trip to an Inhabited Planet.” Each group performed a distracting task for a short time afterward, and then each group was asked to recall the story.
The researchers were very interested in how well each group was able to recall the underlined sentence about the gentle landing. This sentence did not fit in well with the “Peace March” story, but it was an important element if the story was about a space trip. Only eighteen percent of the group that read the “Peace March” story recalled anything about this sentence. But fifty-three percent of the people who read the story with a space trip title recalled something from this key sentence. This simple experiment demonstrates how people use an organizational framework when they transfer information from short-term memory to long-term memory.
The distinction between maintenance and elaborative rehearsal shows the importance of the level of processing you use to encode the information in the first place. A shallow level of processing might only involve noting some visual features of textual material, such as the font or capitalization. A deeper level would involve processing the meaning of the information, and relating it to ideas or concepts that already exist in long-term memory. The deeper the level, the more it involves long-term memory, the more likely the new information will be transferred and stored. For example, if you view a sequence of word pairs, such as “lobster-shorts,” and are asked to count the vowels, your level of processing will be shallow and you’ll do poorly on a test that asks you to recall the second word when only “lobster” is presented. If instead you’re asked to create a sentence using two words, your level of processing will be deeper because the task requires you to analyze the meaning and relate the meanings of the two words to one another. You’ll do better on a later recall test even though you were exposed to each word pair for the same amount of time. Clearly, the way you study is at least as important as the amount of time you devote to it.
Poor short-term memory a barrier to learning success
Poor short-term memory may lead to difficulties in processing, understanding, and organization. Students who have deficits in registering information in short-term memory often have difficulty remembering instructions or directions they have just been given, what was just said during conversations and class lectures and discussions, and what they just read, says Glenda Thorne, Ph.D. Research has confirmed that verbal short-term memory deficits are a common characteristic of children with reading problems and may markedly increase the difficulty of learning to read.
Unfortunately, even if a student has excellent long-term memory, his ability to use it is limited by how much information has ‘leaked’ while it was in his short-term memory. In this way, short-term memory can cripple someone who has many areas of strength but is unable to use them to their fullest potential.
Test your short-term memory
Test your short-term memory using the test below.
Digit span test
Read each sequence as if it were a telephone number, then close your eyes and try to repeat it back. Start with the four-digit numbers, and continue until you fail on both sequences at a given length. Your span is one digit less than this.
5 8 1 6
3 7 1 9
9 2 7 4 5
9 3 6 4 8
5 1 3 4 8 2
8 4 2 7 1 6
3 8 4 2 5 9 1
7 9 2 8 5 6 3
7 5 1 2 9 4 8 3
5 1 7 9 8 4 3 6
6 8 7 9 4 5 2 1 3
3 1 5 9 7 8 4 6 2
Digit span is a classic short-term memory task, in that it involves holding a small amount of material for a short period.
The most obvious fact about digit span is that it is limited to about six or seven digits for most people, although some people can manage up to ten or more.
How Edublox can help
Edublox Online Tutor (EOT) houses several multisensory cognitive training programs that enable learners to overcome learning obstacles and reach their full potential.
EOT is founded on pedagogical research and 30+ years of experience demonstrating that weak underlying cognitive skills account for the majority of learning difficulties. Underlying cognitive skills include short-term memory. Specific cognitive exercises can strengthen these weaknesses leading to increased performance in reading, spelling, writing, math, and learning.
In one research study, Edublox improved visual memory by 1.3 years in 5 days. Chiropractor Dr. Jaidan Mays compared the effects of Edublox training versus Edublox training combined with cervical spinal manipulative therapy on visual short-term memory and visual sequential memory.
Thirty-four Grade 5, 6 and 7 students from an inner-city school participated in Mays’s study. Two subtests of the Test of Visual Perceptual Skills were used to assess the visual short-term memory and visual sequential memory of the students. They were then divided into two equal groups.
The members of the first group (the Edublox Group) did Edublox for 22.5 hours over five days. The members of the second group (the Edublox and Adjustment Group) received the same Edublox training as the first group. But this second group also received cervical adjustment therapy every morning for the five-day period. The assessment was repeated after the five days.
The results: The mean Visual Memory Skills Test POST score across both groups was significantly higher than the mean Visual Memory Skills Test PRE score. The mean score across both groups improved from 6.2 years to 7.5 years. As the graph below illustrates, the Edublox Group improved slightly more than the Edublox and Adjustment Group (an improvement from 6.3 to 7.8 years versus an improvement from 6.2 to 7.1 years):
In a recent, soon-to-be-published research study, 64 2nd grade students at an inner-city school were divided into three groups: group 1 consisted of 22 students who did Edublox Online Tutor (Development Tutor) for 28 hours over a period of three weeks, while group 2 consisted of 21 who played computer games, and the rest continued with school. The Test of Auditory Processing Skills (TAPS-3) was used to assess short-term auditory memory of numbers, words and sentences, before and after the three weeks. Preliminary results show that the short-term auditory memory of the Edublox group (blue line) improved significantly:
EOT has been optimized for children aged between 7 and 13, is suitable for the gifted and less gifted, and can be used at home and in school. The program is effective for a variety of learning difficulties including dyslexia, dysgraphia, and dyscalculia.
Baddeley A. “Working memory.” Current Biology. February 23, 2010, 20(4).
Baddeley A, Eysenck, MW, Anderson, MC. Memory. Psychology Press. 2009.
Cowan N. “The magical number 4 in short-term memory: A reconsideration of mental storage capacity.” Behavioral and Brain Sciences. 2001, 24.
Horne T, Wootton S. Teach Yourself: Training Your Brain. Mc Graw Hill. 2008.
Mays JL, Effects of Edublox Training versus Edublox Training Combined with Cervical Spinal Manipulative Therapy on Visual Memory and Visual Sequential Memory. M.Tech. thesis, University of Johannesburg. 2013.
Shenck D. The Genius in All of Us. Doubleday. 2010.
Thorne G. “10 Strategies to enhance students’ memory.” ReadingRockets.org. 2006.