Types of Memory
Memory is fundamentally organized into three major systems: working memory (short-term manipulation of information), episodic memory (personal experiences and events), and semantic memory (general knowledge and facts), with additional distinctions based on temporal duration and consciousness of recall. 1, 2
Primary Memory Systems
Working Memory
- Working memory is a system that actively holds and manipulates information in the short term to perform cognitive tasks, distinct from simple storage 2, 3
- This system can be further divided into verbal working memory (language-based information) and visuospatial working memory (spatial and visual information) 2
- Working memory is assessed through tasks requiring active manipulation, such as reverse digit span and N-back tasks, not just passive recall 1, 2
Episodic Memory
- Episodic memory represents the ability to learn and retain new information about personal events and experiences, which is the most commonly impaired memory type in early Alzheimer's disease 1, 2
- This memory type is evaluated through immediate and delayed recall paradigms, allowing assessment of both learning rate and retention over time 1
- Tests include word-list learning (Rey Auditory Verbal Learning Test, California Verbal Learning Test) and paragraph recall (Logical Memory subtests) 1, 4
Long-Term Memory Categories
By consciousness of recall:
- Explicit (declarative) memory: Conscious recollection of facts and events 5, 6
- Implicit (procedural) memory: Unconscious memory for skills and procedures ("knowing how") 5, 6
By content type:
- Semantic memory: General knowledge, facts, and concepts independent of personal experience 1, 5
- Sensory memory: Modality-specific storage including echoic (auditory), iconic (visual), and haptic (tactile) memory 5, 6
Temporal Classification
Short-Term vs. Long-Term Memory
- Short-term memory demonstrates temporal decay and chunk capacity limits, distinguishing it from long-term memory which has essentially unlimited capacity 3, 7
- The distinction between short-term and long-term memory remains neurobiologically and functionally valid, despite claims that short-term memory is merely "activated" long-term memory 7
- Sensory memory serves as the initial gateway, storing information briefly before transfer to short-term systems 6
Clinical Assessment Considerations
Memory Domain Evaluation
- Comprehensive assessment must evaluate multiple memory domains beyond just episodic memory, including attention (which strongly correlates with memory performance), executive functions, language, and visuospatial skills 1
- For nonverbal memory, visual reproduction tasks from the Wechsler Memory Scale assess retention of visual-spatial information 1, 4
Common Pitfalls
- Simple screening tests (like three-word recall in MMSE) are insensitive to subtle memory impairments, particularly in early cognitive decline 4
- Memory encoding is not a passive recording process but involves constructive and reconstructive processes influenced by prior knowledge, beliefs, and schemas 1
- Each recall event triggers reconsolidation, irretrievably altering the original memory through the same neural processes as initial encoding 1
Memory Process Stages
Memory formation involves three critical stages: