What's magic about magic numbers? Chunking and data compression in short-term memory.

Area of Science:

• Cognitive Psychology
• Information Theory
• Neuroscience

Background:

• Short-term memory capacity is traditionally defined by Miller's (1956) 7±2 items or modern 4±1 chunks.
• The definition of a 'chunk' as a collectively treated unit lacks quantitative precision.

Purpose of the Study:

• To propose a new, quantitatively precise definition of a 'chunk' based on Kolmogorov complexity and compressibility.
• To investigate the relationship between stimulus sequence compressibility and short-term memory capacity.

Main Methods:

• Stimulus sequences with variable-length patterns were used to manipulate compressibility.
• Subjects' digit span (short-term memory capacity) was measured in relation to pattern compression.

Main Results:

• Measured digit span correlated with the number of distinct sequences after compression.
• The effective limit was found to be approximately 3-4 distinct chunks.
• This is equivalent to about 7 uncompressed items, aligning with Miller's findings.

Conclusions:

• A chunk is defined as a unit within a maximally compressed code.
• Short-term memory capacity is fundamentally limited by the number of distinct, compressed chunks.
• This provides a more precise understanding of memory limitations, reconciling classic and modern findings.