Long term memory and amnesias

mbb1

Divisions of LTM

• declarative memory (explicit)

• non-declarative memory (implicit)

Declarative memory (explicit):

• “Knowing what, why, where, and when”

• Facts, events, locations, autobiographical knowledge

• Reminiscence of personally experienced events

• Hippocampal-dependent

Non-declarative memory (implicit)

• “knowing how”

• Motor skills (e.g., riding a bike)

• Habits (proceduralised memories - driving the route to work without thinking)

• Cognitive skills (e.g., reading)

• Non-hippocampal dependent

sub-divisions of declarative memory: episodic and semantic memories

• Endel Tulving proposed that declarative memory can be subdivided into the episodic and semantic memory systems

• episodic memory

• semantic memory

Episodic memory

• Vivid first-person recall of personally experienced events

• When/where memories

• Contextualised memory

• ‘Mental time travel’

Semantic memory

• General knowledge of facts about the world and yourself

• What/Why memories.

• Abstract knowledge (includes abstract self-knowledge)

Non-declarative memory

• Non-declarative memory is revealed when previous experience facilitates (improves) performance on a task

• The improvement in performance does not require conscious recollection of the prior learning experiences.

  • We get better at things with experience and practice.

  • We learn associations between recurring stimuli in the environment

Subdivisions of non-declarative memory

• procedural memory

• priming

• classical conditioning

• operant conditioning

• non-associative learning

  • habituation

  • sensitization

Procedural memory

• learning and performance of motor and cognitive skills

Priming

• demonstrated by a change in the ability to identify a stimulus as the result of prior exposure to that stimulus, or a related stimulus.

• E.g., Associative/semantic priming

• the prior presentation of the word “nurse” facilitates subsequent identification of the word “doctor”

• More frequently encountered words are easier to perceive and comprehend - the resting level of activation in memory is higher (activated more quickly).

classical conditioning (associative learning)

Learning to attend to a formerly neutral stimulus because it has become associated with a meaningful stimulus.

Operant conditioning (associative learning)

Learning to produce/avoid a behaviour because it has become associated with rewarding/punishing consequences

Habituation

learning to ignore a stimulus because it is trivial (e.g., screening out background noise).

sensitization

Learning to attend to a potentially threatening stimulus

The architecture of LTM

• declarative

  • facts

  • events

  • Both facts and events combined in medial temporal lobe

• non declarative

  • Priming

    • cortex

  • Procedural (skills and habits)

    • striatum

  • Associative learning: classical and operant conditioning

    • emotional responses

      • amygdala

    • skeletal musculature

      • cerebellum

  • Non associative learning: habituation and sensitization

    • reflex pathways

The amnesias

• Deficits in memory caused by brain damage, disease, drug abuse, or psychological trauma.

• The selective deficits in memory processes seen in cases of amnesia provide support for the proposed division between the declarative and non-declarative memory systems.

Retrograde amnesia

• An inability to remember episodes acquired before the brain injury

• Usually temporally graded

Anterograde amnesia

• An inability to recall any declarative knowledge experienced after the time of the brain injury

• Inability to learn and retain (consolidate) new declarative knowledge (episodic and semantic)

The case of H.M

• Henry Gustav Molaison (1926-2008)

• Removal of the medial portion of both temporal lobes, including the hippocampi, to treat epilepsy

• Epileptic seizures controlled But….

• Temporally-graded retrograde amnesia

  • Memory worst for personally experienced events from years just before the operation

• Severe anterograde amnesia

  • Could not consolidate or retrieve new episodic memories

  • Severely impaired ability to learn new semantic facts.

  • For example, H.M. did not acquire new vocabulary introduced since 1953 despite frequent exposure to radio and TV.

• Normal sensory and working memory (STM)

  • For example, normal digit span ability

Fergus Craik’s contention

• memory is the outcome of the normal processes of perceiving and comprehension

Brenda Milner’s interrogation

• “My amnesic patients have no trouble perceiving and comprehending events, they are clearly capable of processing to deep semantic levels—yet they don’t remember things.

• How does that fit with your theory?”

Role of hippocampus in consolidation of declarative memories

• The severe anterograde amnesia that results from removal of hippocampus bilaterally indicates that these structures must be crucial for the consolidation of new declarative information.

• Craik (2020) conceded that cases like H.M. forced him to adjust his account of elaborative encoding to include not just the process of interacting meaningfully with information in working memory, but also an additional process of consolidation, mediated by the hippocampus

Learning in amnesia: Dissociation of declarative and non-declarative memory

• Anterograde amnesiacs are capable of new procedural learning (learning a new motor skill).

• For example, the mirror-tracing task

• Despite stating that they have never performed the task before, they show an improvement over time.

• This demonstrates that procedural learning can proceed independently of the brain systems required for declarative memories

Preserved non- declarative memory in anterograde amnesia

• Further studies show that patients with anterograde amnesia show other types of preserved non-declarative memories

  • Intact classical and operant conditioning

  • Intact priming effects.

  • Normal habituation and sensitisation

• Also, patients with Korsakoff’s syndrome, depressed patients undergoing bilateral ECT, patients with anoxic encephalopathy, who all suffer anterograde amnesia