Short-term memory (STM) refers to the temporary maintenance of information that is no longer perceptually present. It has limited capacity (famously characterized by George Miller as "the magical number seven, plus or minus two") and limited duration (information is lost within about 15-30 seconds unless actively rehearsed). STM is critical for ongoing cognitive processing, bridging the gap between fleeting sensory input and more permanent long-term storage.
Capacity Limits
Miller's (1956) classic paper established that STM capacity is limited to approximately 7 items, though this estimate has been revised downward. Nelson Cowan (2001) argued that the true capacity of STM, when rehearsal and chunking are controlled, is approximately 4 items. The apparent capacity of 7 reflects the benefits of chunking — grouping individual items into meaningful units. A phone number like 8005551234 exceeds raw capacity but can be chunked into 800-555-1234 (three chunks), well within the 4-item limit.
Cowan (2001): ~4 items (pure capacity, without chunking/rehearsal)
Effective capacity = number of chunks × items per chunk
Duration and Forgetting
Peterson and Peterson (1959) demonstrated the rapid forgetting of STM by presenting three consonants (e.g., CHJ) and then requiring participants to count backward by threes (to prevent rehearsal). After just 18 seconds of distraction, recall dropped to near zero. Whether this forgetting reflects temporal decay (the trace fading over time) or interference (the counting task disrupting the memory trace) has been debated for decades, with current evidence suggesting that both mechanisms contribute.
Coding in STM
Conrad (1964) demonstrated that STM relies heavily on phonological (sound-based) coding: recall errors tend to involve letters that sound similar (B/V, P/T) rather than letters that look similar (Q/O). This phonological coding occurs even for visually presented material, suggesting automatic translation into an articulatory or acoustic code. However, STM can also use visual, semantic, and spatial codes, particularly for material that does not lend itself to verbal rehearsal.
The concept of STM has been largely supplanted by working memory in modern cognitive psychology. While STM emphasizes passive storage, working memory emphasizes both storage and active manipulation. Baddeley and Hitch (1974) argued that the single STM store should be replaced by a multi-component working memory system. However, the concept of a limited-capacity, limited-duration store remains valid and continues to influence research on memory encoding, cognitive load, and individual differences.
Neural Basis
STM maintenance is associated with sustained neural activity in prefrontal cortex and sensory association areas. The specific regions involved depend on the type of material: verbal STM engages left prefrontal and parietal regions associated with the phonological loop, while spatial STM engages right-lateralized frontal and parietal regions. Delay-period activity in prefrontal neurons — maintained firing during the retention interval — has been observed in monkey electrophysiology and human fMRI, providing a neural correlate of active information maintenance.