Basic Neurosciences · Paper A
Memory systems
Because damage to one brain region can erase new facts while leaving old skills and habits intact, memory is not one system but several dissociable ones: declarative memory (facts and events consciously recalled) and non-declarative memory (skills and habits performed without conscious recall). This split matters clinically because a bedside test of one system can look normal while the other is silently impaired. Working memory (a short-term, prefrontal system that holds and manipulates information for seconds) sits apart from both long-term stores.
Declarative memory divides into episodic memory (personally experienced events tied to time and place) and semantic memory (context-free facts), and both depend on the hippocampusA brain structure central to forming new memories.Read more → and medial temporal lobeThe part of the brain by the ears, involved in hearing, language and memory.Read more → to encode new material, the region destroyed in the amnesic patient H.M. Non-declarative memory, covering procedural skills, priming, and classical conditioning, depends instead on the basal gangliaA group of deep brain structures that help control movement and are affected in Parkinson's disease and by antipsychotic drugs.Read more → and cerebellumThe brain structure at the back of the skull that coordinates balance and smooth movement.Read more → and survives even total hippocampal loss. Working memory itself has three parts under Baddeley's model: the central executive (the attention-controlling manager), the phonological loop (a verbal rehearsal buffer), and the visuospatial sketchpad (a buffer for images and spatial layout). E.g., a patient who cannot recall breakfast an hour later (hippocampal loss) can still repeat a phone number back correctly (working memory intact), because the two systems sit in different circuits.
A fresh doctor might treat "memory" as one single skill that either works or fails. Because a single lesion can wipe out someone's ability to recall yesterday's events while leaving their ability to ride a bicycle completely untouched, memory must in fact be organised as several separable SYSTEMS, each depending on a different brain circuit. Hold the phrase DIFFERENT SYSTEM, DIFFERENT CIRCUIT through every section below, because almost every exam question on this topic is really testing whether that mapping is understood.
Long-term memory divides first into DECLARATIVE MEMORY (also called explicit memory: facts and events that can be consciously recalled and described) and NON-DECLARATIVE MEMORY (also called implicit memory: skills, habits, and conditioned responses performed without conscious recall of having learned them). A separate, short-term system called WORKING MEMORY holds and manipulates a small amount of information for only seconds at a time, and is distinct from both long-term stores described above.
A common mistake is assuming a single bedside memory test can rule out a memory problem altogether. Because each system depends on a different circuit, a patient can pass a working-memory test (such as digit span) while having a severe, disabling deficit in forming new episodic memories, or vice versa: clinically, this means memory assessment must probe more than one system deliberately, rather than relying on a single global impression from conversation alone.
The reason memory fractures the way it does is anatomical: each system evolved around a different piece of neural machinery, so damage to one circuit can spare the others completely. This is the single fact that explains every clinical pattern in this topic, from H.M.'s case below to everyday dementia assessment: a "memory problem" is never one thing, and asking WHICH system is affected is the first real diagnostic question.
Declarative memory splits further into EPISODIC MEMORY (personally experienced events tied to a specific time and place, such as remembering yesterday's breakfast) and SEMANTIC MEMORY (context-free general knowledge and facts, such as knowing Paris is the capital of France, without recalling when or where that fact was learned). Both depend on the and surrounding medial structures to encode new material into long-term storage: the region famously destroyed, on both sides, in the patient known as H.M., who could no longer form any new declarative memories after surgery.
Non-declarative memory covers several distinct sub-types: PROCEDURAL MEMORY (motor skills and habits, such as riding a bicycle or typing), PRIMING (a change in response to a stimulus caused by prior, often unconscious, exposure to a related stimulus), and CLASSICAL CONDITIONING (learning an association between a neutral stimulus and an automatic response). These depend mainly on the and rather than the , which is exactly why they survive intact even after severe hippocampal damage.
Working memory is best described by Baddeley's model, which splits it into three linked components: the CENTRAL EXECUTIVE (an attention-controlling manager that directs focus and coordinates the other two components), the PHONOLOGICAL LOOP (a short-term buffer for verbal and sound-based information, used for example when silently repeating a phone number), and the VISUOSPATIAL SKETCHPAD (a short-term buffer for visual and spatial information, such as briefly holding an image of a route in mind). Working memory depends mainly on the , a different circuit again from either long-term memory system above.
For example, a patient with focal hippocampal damage from a hypoxic brain injury cannot recall having had breakfast an hour earlier, yet can immediately repeat a seven-digit number back correctly and, given daily practice, steadily improves at a new mirror-drawing task despite each time insisting they have never attempted it before. This single case demonstrates all three systems at once: working memory (the digit span) is intact, procedural learning (the mirror-drawing improvement) is intact, and only new declarative, episodic encoding is lost.
Anterograde amnesia (the pattern above: new learning impaired, old memories intact) must be distinguished from retrograde amnesia (loss of memories from before the injury, often temporally graded so that older, more consolidated memories survive better than recent ones: known as Ribot's law). Transient global amnesia (a sudden, self-limiting episode of dense anterograde amnesia lasting hours, with an unclear precipitant, that resolves on its own) differs from a fixed amnesic syndrome such as Korsakoff's, where hippocampal-circuit damage is permanent. Dissociative amnesia (psychogenic, often following severe stress or trauma, can be patchy, and can extend to personal identity) differs from organic amnesia by having no structural lesion and, unusually, can spare or affect memory in patterns that don't fit a single anatomical circuit.
Memory systems are probed separately, not with one global test: digit span assesses working memory (the phonological loop specifically), delayed word-list recall and story recall assess episodic memory, and tests of learned skills or conditioning are used to demonstrate intact procedural memory when relevant. This systems framework underlies real diagnostic decisions: amnesic syndrome and the memory domain of dementia (assessed against ICD-11 and DSM-5 criteria) are specifically defined around loss of new declarative learning, and cognitive assessment tools recommended in NICE dementia guidance are designed to probe episodic memory, working memory, and other domains separately rather than assuming one score represents "memory" as a whole; structural imaging such as MRI is used to look for hippocampal atrophy when a degenerative cause is suspected.
Management of memory impairment in dementia, such as cholinesterase inhibitors (donepezil, rivastigmine, galantamine) or memantine in more severe or intolerant cases, in Alzheimer's disease per NICE and Maudsley Prescribing Guidelines, targets the declarative-memory circuit specifically and does not restore procedural learning , which is typically preserved regardless. In Wernicke-, urgent thiamine replacement (commonly as Pabrinex) is the critical, well-established management step, since prompt treatment of is what prevents progression to the fixed amnesic pattern of Korsakoff syndrome.
Questions frequently describe a patient who cannot recall recent conversations but still performs a learned motor task well, and expect the candidate to recognise spared procedural memory alongside impaired declarative memory rather than concluding memory is globally intact or globally impaired. Another trap conflates working memory with long-term memory generally, when a patient with entirely normal digit span can still have a severe amnesic syndrome for new declarative learning, because the two depend on different circuits.
Memory is organised into separable systems, each tied to a distinct circuit (declarative (), non-declarative ( and ), and working memory ()) which is why focal damage can spare some memory abilities completely while devastating others, and why NICE-aligned assessment and dementia management are structured around testing, and treating, each system separately.