Short-term memory capacity (7 ± 2) predicted by theta to gamma cycle length ratio

The number of items that can be held in human short-term memory (STM) is limited to 7 (± 2) elements. Lisman and Idiart's theoretical model of STM proposes that this value depends on the number of gamma cycles that can fit in one theta cycle. Previous studies on animals and humans provided support for this hypothesis but direct evidence from human EEG scalp recordings has not previously been reported. We recorded spontaneous EEG activity from 17 participants and measured their verbal STM capacity with a modified digit span task from the Wechsler battery. The strong and positive correlation we found between verbal STM capacity and theta/gamma cycle length ratio thus provides a direct argument in favor of this STM theoretical model. In this study we also demonstrated a new method for assessing individual theta and gamma frequencies by detecting functional coupling between these oscillations.