1.4 Memory

Stages of memory

Memory runs through three processes: encoding, storage and retrieval.

• Encoding is the first stage, told apart from attention, which selects input, and from consolidation, storage and retrieval, which follow it.

• Synaptic consolidation takes minutes to hours; systems consolidation takes months to years as the neocortex takes over support of the memory.

• Head injury, electroconvulsive therapy, benzodiazepines and alcohol can disrupt memory formation.

Strengthening encoding

• Levels of processing (Craik and Lockhart): processing meaning improves recall more than processing sound or appearance, even when the person is not trying to learn.

• Elaborative rehearsal is deep, meaning-based, associative processing that transfers material into long-term memory.

• Maintenance (rote) rehearsal only holds items in short-term memory.

• Encoding strategies shape how material is laid down: organising, chunking, elaborating and imagery mnemonics.

• Cue-based strategies act at retrieval instead, through the encoding specificity principle.

• Focused attention, spaced rather than massed study, and retrieval practice rather than repeated study all strengthen memory.

The multi-store model

Atkinson and Shiffrin's multi-store (modal) model (1968) has three structurally separate stores in series: the sensory register, a brief limited-capacity short-term store and a permanent long-term store.

Rival accounts are working memory, levels of processing, and Tulving's and Squire's models.

Sensory memory

• Sensory memory is the first store: it holds a large amount of raw, unprocessed (precategorical), modality-specific input for well under a second, which decays unless attended to.

• Echoic persistence is longer than iconic, and new input in the same modality masks it.

A hold lasting seconds and kept alive by rehearsal, such as a short digit string, is short-term or working memory, not the iconic register.

Short-term memory

• Capacity is classically seven plus or minus two items (Miller); unrehearsed items last about 18 seconds.

• Chunking regroups many small items into fewer meaningful units, so more material fits within the span.

• The Brown-Peterson task blocks rehearsal with a filler task, such as counting backwards, during the retention interval; recall falls as the delay grows.

• Recall of verbal material after a filled delay of minutes tests long-term memory, because unrehearsed short-term traces fade within seconds.

Serial position curve

Amnesia impairs the primacy effect.

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Working memory

Working memory holds information briefly and manipulates it for the task in hand, as when reversing digits or using newly heard information at once. Forward span needs only passive holding.

Baddeley and Hitch's model has four components.

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• The central executive is distinct from the storage subsystems, so normal forward span with failed manipulation or dual tasking localises the deficit to it.

• Working memory depends chiefly on the prefrontal cortex, especially its dorsolateral part.

• Capacity is not fixed: it grows through childhood, differs between people and responds to practice.

• Frontal pathology reduces capacity, and anxiety uses it up through worry.

• Dysfunction impairs tasks that hold and manipulate information online, such as calculation, planning and reasoning, but spares retrieval of well-established knowledge from long-term memory.

Long-term memory systems

• Declarative (explicit) memory is consciously recalled and has two subtypes, episodic and semantic.

• Non-declarative memory is the same as implicit memory: it shows through performance without awareness ("knowing how") and covers procedural memory, priming and conditioning.

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• Recalling the occasion on which a skill was learned is episodic; the skill itself is procedural.

• Procedural memory improves with practice without recollection of the sessions and depends on the basal ganglia and cerebellum.

• Procedural memory survives medial temporal damage: patient H.M. improved at mirror drawing without recalling the practice.

• Priming survives the diencephalic damage of Korsakoff syndrome.

• Semantic amnesia removes general knowledge, word meaning and concepts while episodic memory is relatively spared, as in semantic dementia with anterior temporal lobe atrophy.

Prospective and flashbulb memory

• Prospective memory is memory for intentions: carrying out a planned action at a future time (time-based) or on a future cue (event-based).

• Prospective memory relies on self-initiated retrieval, so it is sensitive to frontal executive dysfunction; it can fail in attention deficit hyperactivity disorder, depression and dementia.

• Flashbulb memory is a vivid, lasting, confidently held recollection of the personal circumstances of learning shocking public news (Brown and Kulik, 1977), laid down under emotional arousal.

• A flashbulb memory's vividness and confidence do not guarantee accuracy, and it can change without the person noticing.

• Flashbulb memory is not eidetic (photographic) memory.

Retrieval

• Encoding specificity principle (Tulving): cues work best when they match the conditions at encoding.

• Context-dependent memory: recall improves when the external setting present at learning is reinstated, as when divers recalled material learned underwater best underwater.

• State-dependent (mood-dependent) memory: retrieval is best when the internal mood or drug state at recall matches that at encoding.

• Mood-congruent recall favours material that matches current mood, so depressed people retrieve negative events more easily.

Forgetting

• Retrieval failure, an intact trace that the available cues cannot reach, is the commonest cause of long-term forgetting, ahead of encoding failure and failed consolidation.

• Ebbinghaus's forgetting curve, from nonsense syllables, shows steep early loss, with more than half lost within nine hours, after which the curve levels off.

• Jost's law: of two memories of the same current strength, the older decays more slowly. This is normal decay, not Ribot's gradient after brain damage.

• Theories of forgetting rest on decay, interference, repression and state dependency; Maslow's need hierarchy is a theory of motivation, not of forgetting.

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• Similar material interferes most.

• Whether extensive autobiographical memories can be wholly repressed and later accurately recovered remains contested.

Everyday lapses

• Absent-mindedness, one of Schacter's seven sins of memory, is a failure at the interface of attention and memory: attention is elsewhere at the critical moment, so intact memories go unused.

• Transience is fading of memory over time.

• Blocking is a retrieval failure in which a stored memory is temporarily inaccessible despite cues; the tip-of-the-tongue state is the standard example, with an intruding related word (the "ugly sister" effect).

• Source amnesia is loss of memory of when, where or from whom a fact was learned while the fact itself is kept; it follows frontal lobe damage and is milder in normal ageing.

Emotion and memory

• The amygdala interacts with the hippocampus to strengthen the encoding and consolidation of emotionally arousing memories.

• Weapon focus shows that arousal strengthens memory for central emotional content but weakens peripheral detail: a gun is recalled better than the assailant's face.

• Extreme arousal and glucocorticoid release can impair contextual and sequential encoding, leaving traumatic memories poorly ordered.

Reconstruction and distortion

• Bartlett found that repeated recalls of The War of the Ghosts shortened and normalised the story, with schemas supplying plausible additions.

• Loftus found that asking how fast cars "smashed" rather than "hit" raised speed estimates and false reports of broken glass, so post-event information can be remembered as witnessed.

• Repetition or pressure raises confidence in a memory without making it more accurate.

Recognition differs from déjà vu because it rests on a genuine earlier encounter.

Brain basis and the lifespan

• The hippocampus consolidates new declarative memories into long-term storage, so damage (for example hypoxic-ischaemic injury or temporal lobe surgery) causes anterograde episodic amnesia.

• Hippocampal damage spares working memory, remote memory and procedural learning.

• Long-term potentiation at glutamate synapses is triggered by the calcium-permeable N-methyl-D-aspartate (NMDA) receptor and is the proposed cellular basis of Hebbian memory formation.

• Infantile (childhood) amnesia is the normal, universal, non-pathological absence of retrievable autobiographical memories from about the first three to four years of life, reflecting immature memory systems and narrative language.

• Age-associated memory impairment falls chiefly on delayed, uncued free recall of new material, while immediate span, recognition, orientation and semantic knowledge are relatively preserved.

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Amnesia

• Ribot's law: in organic retrograde amnesia, recent memories are lost before remote ones, giving a temporal gradient that runs through episodic memory.

• Implicit memory survives retrograde amnesia because it does not depend on the hippocampus and medial temporal lobe.

• Amnesic syndrome selectively damages episodic memory, with immediate (short-term), working, semantic and procedural memory and priming intact or little affected.

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Organic amnesias

• Korsakoff syndrome is the chronic amnesic state after thiamine deficiency, which damages the mammillary bodies and dorsomedial thalamus.

• Korsakoff syndrome gives dense anterograde and variable retrograde amnesia, apathy and confabulation, with working and procedural memory spared.

• In suspected Wernicke encephalopathy, give parenteral high-potency thiamine before any carbohydrate or glucose load, to prevent worsening and progression to Korsakoff amnesia.

• Alcohol-induced blackouts are anterograde amnesia from failed consolidation of new memories during intoxication, with earlier memories spared.

• In schizophrenia, procedural memory is relatively spared while declarative and executive memory systems are impaired.

• Post-traumatic amnesia ends when continuous memory returns.

Psychogenic amnesias

Organic medial temporal amnesia mainly impairs new learning and keeps identity; dissociative amnesia mainly loses the past, sometimes including identity, and spares new learning.

Testing memory

• Poor span suggests impaired attention or arousal rather than isolated amnesia.

• Retest after days if accelerated forgetting is suspected.

• Below-chance forced-choice recognition argues against a simple organic deficit.

• Mood, education, language and sensory function affect performance, and an informant shows whether the patient under- or overestimates the difficulty.