long-term memory and amnesias

Long term memory

Declarative memory (explicit)

“Knowing what, why, where and when”

Facts, events, locations, autobiographical knowledge

Reminiscence of personally experienced events

Hippocampal-dependent

Subdivision of declarative memory:

Episodic and semantic

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

Forms of long-term memory that are expressed as a change in behaviour without conscious recollection.

Tests of non-declarative memory are referred to as implicit memory tests. Tests that do not require description of the contents of memory, but rather reveal memory processes indirectly (implicitly) through observed changes in performance.

“Knowing how”

Motor skills (eg riding a bike)

Habits (procedural used memories - driving the route to work without thinking)

Cognitive skills (eg reading)

Non-hippocampal dependent

Non-declarative memory is revealed when previous experience facilitates (improves) performances 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

Subdivision of non-declarative memory

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

• Eg. Associative/semantic priming

• The prior presentation to 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 formerly neutral stimulus because it has become associated with a meaningful stimulus

Operant conditioning (associative learning)

• learning to produce/avoid beaviour because it has become associated with rewarding/punishing consequences

Non-associative learning:

• habituation: learning to ignore a stimulus because it is trivial (eg, screening out background noise)

• sensitization: learning to attend to a potentially threatening stimulus

Amnesia

Amnesia is 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

Two types of amnesia

Retrograde and Anterograde amnesia

Retrograde:

• 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)

Role of hippocampus

Hippocampus crucial for the consolidation of new declarative information.

Severe anterograde amnesia that results form removal of hippocampus bilaterally.

Consolidation of declarative memories

Case study of patient H.M.

Henry Gustav Molaison (1926-2008)

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

HM’s general condition has been described as dense anterograde amnesia, as well as temporally graded retrograde amnesia.

Unable to form new long-term memories of events since the surgery or retina new semantic knowledge - he lived in the past.

Since HM did not show any memory impairment before the surgery, the removal of the medial temporal loves can be held responsible for his memory disorder

• 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 (shows that STM or working memory does not depend on medial temporal lobes structures)

Evidence from cases of amnesia that support the distinction (dissociation) between declarative and non-declarative forms of memory

When presented with a short list of numbers and asked to remember them for 30 seconds. HM performs as well as those with intact medial temporal lobes.

This observation indicates that STM/working memory, information that is maintained over a period of seconds or minutes, does not depend on medial temporal lobes structures.

HM also had preserved long-term memory for general information acquired before his operation. He remembered his name and former occupation, and he retained a command of language, including vocabulary, indicating a preservation of previously acquired semantic memory. Remote episodic memories were also spared: he could recall childhood events in detail, inducing riding in a car with his parents when he had his first major seizure on his 16th birthday.

HM suffered from severe anterograde amnesia, the inability consciously to remember information encountered after brain damage. Thus although HM could briefly retain a short list of numbers (because his STM/working memory was intact), he would immediately and completely forget them as soon as the information was no longer attended and so was lost from STM/working memory.

This catastrophic forgetting reveals an inability to form, retain and retrieve new declarative memories.

Brenda Milner’s interrogation

“My amnesiac 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 do that fit with your theory?”

Learning in amnesia - dissociation of declarative and non-declarative memory

Anterograde amnesiacs are capable of new procedural learning, for example 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

Declarative and non-declarative (specifically procedural) long-term memory can be distinguished (dissociated) in anterograde amnesia

Preserved non-declarative memory in anterograde amnesia

Furhter 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 sensitization

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