Memory is the brain’s process of encoding, storing, and retrieving information. Modern memory science divides memory into multiple systems: Declarative Memory (facts and events expressible in language) vs. Non-declarative Memory (procedural/skill memory, emotional memory, habituation, etc.); Short-term Memory (working memory, capacity ~7±2 chunks) vs. Long-term Memory (theoretically unlimited capacity).
## Spaced Repetition: Scientifically Fighting the Forgetting Curve
German psychologist Hermann Ebbinghaus (1885) discovered the Forgetting Curve: newly learned content is approximately 50% forgotten within 24 hours, ~70% after a week, and ~80% after a month. The key to countering forgetting: review just before the point of forgetting; each review slows the forgetting rate and extends the interval before the next review is needed — this is the core logic of Spaced Repetition.
The SuperMemo algorithm (Piotr Wozniak, 1987) digitized spaced repetition; Anki built an open-source implementation on this foundation. Research shows that for content requiring long-term retention (language vocabulary, medical knowledge, historical facts), spaced repetition outperforms massed practice by 200-400% in long-term retention.
## Active Recall: The Neural Mechanism of the Testing Effect
**Testing Effect**: active retrieval (self-testing) strengthens memory far more effectively than passive reading/re-reading. Psychologists Roediger and Karpicke’s (2006) classic study: studying the same content, groups who took multiple tests before an exam showed 67% retention one week later vs. 40% for groups who re-read multiple times.
Neural mechanism: active retrieval “reactivates” neural assemblies, triggering memory reconsolidation — after reactivation, memories enter an unstable state; the brain then re-consolidates and “updates” them. Each retrieval episode strengthens memory encoding.
**Practical recommendations**: Anki flashcards (combining spaced repetition + active recall) are the most evidence-based long-term memory tool. After reading, “close the book and recall,” do practice problems rather than looking at answers, and explain knowledge in your own words (Feynman Technique) — all are effective active recall forms.
See [Neuroplasticity Guide](https://sunqi.org/neuroplasticity-guide-en/) and [Retrieval Practice resources](https://www.retrievalpractice.org/).
