Short-Term Memory In Psychology

Short-term memory has three key aspects:

1. Limited Capacity: Only about seven items can be stored at a time.
2. Limited Duration: Storage is fragile, and information can be lost through distraction or the passage of time.
3. Encoding: Information is stored primarily by sound, even when it enters through the eyes.

Capacity: Magic Number 7

The capacity of short-term memory is limited. A classic theory proposed by George Miller (1956) suggests that the average number of objects an individual can hold in their short-term memory is about seven (plus or minus 2 items).

Miller thought that short-term memory could hold 7 (plus or minus 2 items) because it only had a certain number of “slots” to store items.

However, Miller didn’t specify how much information each slot could hold. This is the key to chunking: grouping single items into a larger, meaningful unit lets far more information fit into the same seven slots.

The digits “1”, “0”, “6”, “6” become a single chunk once read as the date 1066. A ten-digit phone number becomes memorable once split into a few short blocks. Chunking is why an expert can hold far more of their subject than a novice can.

Miller’s theory is supported by earlier evidence from Jacobs (1887), using the digit-span procedure. Jacobs presented digit or letter sequences of increasing length and measured the longest string each person could recall correctly. He excluded “w” and “7” because both have two syllables.

People recalled numbers more easily than letters: the average span was 9.3 for digits but only 7.3 for letters.

However, the nature of the items (e.g., simple versus complex) and individual differences can influence this capacity.

Cowan’s Magical Number Four

Some researchers argue that Miller’s seven overestimates true capacity, since his participants were unknowingly chunking items together.

Cowan (2001) reviewed a wide range of memory tasks and found that, once chunking is controlled, capacity is closer to four items.

This lower estimate reframes capacity as a limit of attention rather than a fixed store, a debate still unresolved (see Critical Evaluation).

Duration

Short-term memory typically holds information for about 15 to 30 seconds. However, the duration can be extended through rehearsal (repeating the information).

Atkinson and Shiffrin (1968) built this short duration into their influential multi-store model of memory. Repeating the information verbally, a process called rehearsal, keeps it active in short-term memory for longer.

The classic demonstration of this limit comes from the Brown-Peterson technique, which blocks rehearsal by having participants count backward in threes.

Aim: Peterson and Peterson (1959) tested how long unrehearsed information survives in short-term memory.

Method: Participants saw a trigram of three consonants (e.g., “BRT”). They then counted backward in threes from a random number for 3 to 18 seconds before recalling the letters.

Results: Recall fell sharply as the delay grew: about 70% of trigrams were forgotten after nine seconds, and around 90% after 18 seconds.

Conclusion: Even a tiny amount of unrehearsed information is lost within seconds. This is strong evidence that short-term memory has a brief, limited duration.

If not rehearsed or encoded into long-term memory, the information in short-term memory is susceptible to interference and decay, causing it to be forgotten.

Short-term memory duration varies between individuals, shaped by attention, distraction, and the type of information involved.

Encoding

Encoding in short-term memory primarily involves a transient representation of information, based mainly on the sensory attributes of the input. Here’s a breakdown of how encoding works:

1. Acoustic Encoding: The most common form of encoding in short-term memory. Verbal information is stored by its sound, which is why you might repeat a phone number aloud to remember it.
2. Visual Encoding: The brief storage of visual images, such as glancing at a picture and recalling its details a moment later.
3. Semantic Encoding: Processing information by its meaning. This dominates long-term memory, though it can help some short-term tasks, like recalling a coherent sentence rather than random words.
4. Tactile Encoding: Storage based on touch, which is less common than acoustic or visual encoding in short-term memory tasks.

Various factors, including attention, repetition, and the nature of the information, can influence the effectiveness of encoding in short-term memory.

However, without further processing, the data held in short-term memory can decay or be displaced, emphasizing the transient nature of this memory store.

More durable and elaborate encoding methods, such as deep processing or the formation of associations, are needed to move information from short-term to long-term memory.

Evidence Short-Term Memory Is a Distinct Store

Two lines of evidence show short-term memory works separately from long-term memory. One comes from how people recall lists of words. The other comes from rare neuropsychological patients whose brain damage affected one memory system but spared the other.

The Serial-Position Effect

When people freely recall a list of words, they remember the first and last items best. Murdock (1962) called this the serial-position effect: a primacy effect for early items and a recency effect for the most recent ones.

Researchers think the two effects have different causes. Early words benefit from extra rehearsal, which transfers them into long-term memory. The most recent words are still sitting in short-term memory when recall begins.

Aim: Glanzer and Cunitz (1966) tested whether the recency effect specifically reflects short-term memory.

Method: They had participants recall word lists either immediately or after a 30-second delay filled with a counting task that emptied short-term memory.

Results: Delayed recall abolished the recency effect completely, while the primacy effect stayed intact.

Conclusion: Because only the short-term-based portion of the curve disappeared, this selective loss is strong evidence for a separate, short-lived memory store.

Case Studies: KF and HM

Patients with rare, selective brain damage provide further evidence, showing that short-term and long-term memory can be damaged independently. Patient KF (Shallice & Warrington, 1970) had a severely impaired short-term memory after a brain injury, with a digit span of only about two items.

Yet his long-term learning and comprehension stayed largely intact, the reverse of the usual amnesia pattern. KF’s memory loss was also specific to verbal and acoustic information, since his visual short-term memory was relatively spared.

Patient HM (Scoville & Milner, 1957) showed the opposite pattern after surgery removed tissue on both sides of his brain to treat severe epilepsy. He could still hold information in short-term memory for about 15 seconds, but he could no longer form new long-term memories.

Together, KF and HM form a double dissociation, strong evidence that short-term and long-term memory are separate systems. This split shows up in the brain too. The prefrontal cortex sustains short-term maintenance, while the hippocampus, damaged in HM, supports the transfer to long-term memory (D’Esposito & Postle, 2015).

Working memory

Short-term memory and working memory are closely related but not identical. Short-term memory is the passive, temporary storage of information, while working memory also involves manipulating and processing it, the brain’s active “workspace” for reasoning and problem-solving.

Baddeley and Hitch (1974) reworked the short-term store into this more active system. They argued that a single passive buffer could not carry out the reasoning and comprehension that short-term memory had been credited with.

In place of one store, they proposed a central executive directing a rehearsal buffer for sound-based information, later called the phonological loop. A separate visual store, the visuospatial sketchpad, handles visual information.

Baddeley (2000) later added an episodic buffer to link these systems with long-term memory.

This view explains findings a single store cannot, like holding a verbal load while reasoning through a different task at the same time.

Critical Evaluation

Short-term memory is one of psychology’s most tested ideas, backed by robust classic findings. It also faces real, unresolved questions about exactly what it is and how it works.

Strengths

• Robust, replicated findings: The 7±2 capacity limit (Miller, 1956) and the 15-30 second duration (Peterson & Peterson, 1959) are among the most frequently replicated findings in memory research, giving the model clear boundaries.
• Explains expert performance: Chunking shows how a fixed-capacity store can still handle rich material: grouping digits into a memorable date lets far more pass through the same seven slots, which is why an expert can hold more than a novice.
• Converging evidence: Several independent methods agree: acoustic-based coding, the serial-position effect (only the recency portion collapses under a filled delay), and the KF and HM case studies, where the two systems were damaged separately, showing they can be dissociated both behaviorally and neurologically.
• Practical value: Because the model makes specific, numerical predictions rather than vague claims, it translates directly into practice: chunking-based study techniques, cognitive-load-aware lesson design, and clinical digit-span assessment that helps localize memory impairment all rest on the same capacity-and-duration framework.

Yet several of these classic, well-established findings are now the subject of active ongoing debate:

Limitations

• Not a single, passive store: The working memory model shows short-term memory is multi-component and active, and KF’s selectively verbal deficit shows it is not one uniform system.
• Capacity is disputed: Estimates range from about four chunks (Cowan, 2001) to seven items (Miller, 1956), partly because chunking makes “one item” hard to define.
• Duration findings are confounded: The Brown-Peterson paradigm cannot fully separate memory decay from interference caused by its own counting task, so the true cause of forgetting stays uncertain.
• Separate store debated: Some theorists argue short-term memory is simply activated long-term memory (Cowan, 2008), while others defend it as a genuinely distinct system (Norris, 2017).
• Relies on artificial evidence: Much of the classic evidence comes from meaningless lab tasks and rare, atypical patients, so applying it to everyday memory needs caution.

Contemporary Research

• Attention-based models: Newer accounts, reviewed by Cowan (2008) and D’Esposito and Postle (2015), see short-term storage as temporary activation of long-term memories, controlled by attention rather than a separate box.
• The distinct-store debate continues: Norris (2017) reviewed decades of behavioral evidence and concluded that short-term and long-term memory remain functionally different systems, a position that embedded-processes accounts (Cowan, 2001; Nairne, 2002) still dispute.
• Brain-imaging evidence: D’Esposito and Postle (2015) reviewed neuroimaging and patient studies showing that some short-term content is held in temporary “activity-silent” changes to brain connections, and can be decoded from the same sensory brain regions that first perceived it.

Baddeley (2012) reviews this capacity-versus-attention debate as one of the field’s liveliest open controversies.

References

Atkinson, R. C., & Shiffrin, R. M. (1968). Human memory: A proposed system and its control processes. In K. W. Spence & J. T. Spence (Eds.), The psychology of learning and motivation (Vol. 2, pp. 89–195). Academic Press.

Baddeley, A. D. (2000). The episodic buffer: A new component of working memory? Trends in Cognitive Sciences, 4(11), 417–423.

Baddeley, A. D. (2012). Working memory: Theories, models, and controversies. Annual Review of Psychology, 63, 1–29.

Baddeley, A.D., & Hitch, G. (1974). Working memory. In G.H. Bower (Ed.), The psychology of learning and motivation: Advances in research and theory (Vol. 8, pp. 47–89). New York: Academic Press.

Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24(1), 87–114.

Cowan, N. (2008). What are the differences between long-term, short-term, and working memory? Progress in Brain Research, 169, 323–338.

D’Esposito, M., & Postle, B. R. (2015). The cognitive neuroscience of working memory. Annual Review of Psychology, 66, 115–142.

Glanzer, M., & Cunitz, A. R. (1966). Two storage mechanisms in free recall. Journal of Verbal Learning and Verbal Behavior, 5(4), 351–360.

Jacobs, J. (1887). Experiments on “prehension”. Mind, os-12(45), 75–79.

Miller, G. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. The psychological review, 63, 81-97.

Murdock, B. B. (1962). The serial position effect of free recall. Journal of Experimental Psychology, 64(5), 482–488.

Nairne, J. S. (2002). Remembering over the short term: The case against the standard model. Annual Review of Psychology, 53, 53–81.

Norris, D. (2017). Short-term memory and long-term memory are still different. Psychological Bulletin, 143(9), 992–1009.

Peterson, L. R., & Peterson, M. J. (1959). Short-term retention of individual verbal items. Journal of experimental psychology, 58(3), 193-198.

Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery, and Psychiatry, 20(1), 11–21.

Shallice, T., & Warrington, E. K. (1970). Independent functioning of verbal memory stores: A neuropsychological study. Quarterly Journal of Experimental Psychology, 22(2), 261–273.