The different types of memory stores

Memory involves retaining information from past experiences. Psychologists typically identify three main stores: the sensory register, short-term memory (STM), and long-term memory (LTM). These stores vary in terms of duration (how long information is held), capacity (how much information can be stored), and coding (the way information is represented to form a memory trace).

The sensory register

The sensory register acts as a brief storage for incoming sensory information from the environment, including inputs from sight, hearing, touch, taste, and smell.

Features of the sensory register:

• Duration - Extremely short, with information decaying spontaneously unless attention is given to it.
• Capacity - Limited, holding raw sensory data only momentarily.
• Coding - Based on the specific sense involved, such as visual for images, auditory for sounds, or tactile for touch.

Short-term memory

Short-term memory serves as a temporary holding area for information that receives attention from the sensory register.

Features of short-term memory:

• Duration - Brief, typically lasting seconds unless rehearsed.
• Capacity - Restricted, able to hold only a small amount of information at once.
• Coding - Primarily acoustic, focusing on the sound of the information.

Long-term memory

Long-term memory provides a more permanent storage for information transferred from STM through processes like rehearsal.

Features of long-term memory:

• Duration - Potentially lifelong, with memories lasting indefinitely in theory.
• Capacity - Essentially unlimited, capable of storing vast amounts of information.
• Coding - Mainly semantic, based on the meaning of the information, though visual or acoustic forms can occur.

Research studies on the duration of memory

Several studies have explored how long information remains in different memory stores, highlighting variations in duration across the sensory register, STM, and LTM.

Sperling (1960) - Duration of the sensory register

• Method - In a lab setting, participants viewed a grid of three rows with four letters each, displayed for 50 milliseconds. They recalled either the entire grid or one specific row signalled by a tone (high, medium, or low pitch) right after the display.
• Results - Recall of the full grid averaged four or five letters. For a single row, average recall was three letters out of four.
• Conclusions - The sensory register holds a large amount of information briefly, but it fades rapidly during recall, as participants could access most of a row when cued but not the whole grid.
• Evaluation - Strengths include high control over variables, making it replicable. Limitations involve low everyday relevance, as recalling random letters to tones is not a natural task.

Peterson and Peterson (1959) - Duration of short-term memory

• Method - Participants memorised nonsense trigrams (three unrelated consonants) and recalled them after delays of 3, 6, 9, 12, 15, or 18 seconds. An interference task (counting backwards in threes from a number) prevented rehearsal during the delay.
• Results - Recall accuracy was around 80% after 3 seconds but dropped to about 10% after 18 seconds.
• Conclusions - Without rehearsal, information in STM decays quickly, lasting no more than about 18 seconds.
• Evaluation - The controlled design ensures reliability, but the use of artificial trigrams reduces real-world applicability; meaningful items might endure longer in STM.

Bahrick et al (1975) - Duration of long-term memory

• Method - A group of 392 former students listed names of old classmates (free recall). They also matched names to photos (name-recognition test) or identified names from photos (photo-recognition test), with time since graduation ranging up to 48 years.
• Results - Recognition of names and faces was about 90% accurate within 15 years, dropping to 80% for names and 70% for photos after 48 years. Free recall fell to 30% after 30 years and lower after 48 years.
• Conclusions - LTM can retain information for decades in everyday contexts, with recognition outperforming free recall, indicating a vast but sometimes inaccessible store.
• Evaluation - High real-life relevance boosts ecological validity, but uncontrolled variables (like individual rehearsal) reduce reliability. Meaningful content may enhance storage compared to less relevant information.

Research studies on the capacity of memory

Investigations into memory capacity reveal limits in STM, with techniques like chunking potentially expanding effective storage.

Jacobs (1887) - Capacity of short-term memory

• Method - Participants repeated sequences of letters or digits that grew longer until errors occurred, measuring the maximum accurate recall.
• Results - Average capacity was about nine digits and seven letters, with improvements observed as children aged.
• Conclusions - STM holds between five and nine items, varying by individual factors like age or strategies such as grouping items meaningfully.
• Evaluation - The task's artificial nature limits applicability to daily life, where meaningful information might allow for greater effective capacity.

Miller (1956) - Capacity and chunking in short-term memory

• Method - Miller reviewed existing studies on memory span and proposed a theory based on observed patterns.
• Results - People typically recall around seven items (plus or minus two), but chunking—grouping items into larger units—increases this.
• Conclusions - STM's base capacity is "seven plus or minus two," but chunking enables holding more by treating groups as single items, effectively expanding storage.
• Evaluation - The concept explains real-world memory aids, but reliance on reviewed data rather than new experiments may overlook variations in meaningful versus abstract information.

Coding in short-term and long-term memory

Coding refers to how information is transformed for storage. STM often uses acoustic coding (sound-based), while LTM tends towards semantic coding (meaning-based), though other forms like visual can appear in LTM.

Baddeley (1966) - Coding differences between STM and LTM

• Method - Participants memorised lists of words that were either acoustically similar (e.g., sounding alike) or dissimilar, or semantically similar (e.g., similar meanings) or dissimilar. Recall was tested immediately (for STM) or after a 20-minute distracting task (for LTM).
• Results - Immediate recall struggled with acoustically similar words, while delayed recall had issues with semantically similar words.
• Conclusions - STM primarily uses acoustic coding, as sound similarities cause confusion, whereas LTM relies on semantic coding, where meaning overlaps lead to errors.
• Evaluation - The controlled lab setup aids reliability, but low ecological validity arises from artificial word lists; real-life coding may include visual elements not tested here.