Memory Systems

Isabelle Rapin

Seminar in Developmental Disabilities

February 20, 2013No conflict of interest

Multiple Memory Systems

Short term memory systems• Sensory buffers (for each modality +

endogenous inputs from memory)

• Working memory

Long term memory systems• Declarative (or explicit)

• Non-declarative

Retrieval systems

1. Schema of STM systems

Short Term Memory

Sensory buffers Working memory

Sensory cortices Prefrontal cortex

Short Term Memory

Components: buffers + working memory• Sensory buffers: in continuous on-line reciprocal

contact with working memory circuitry• Working memory: processes only information that

reaches awareness, does so in the light of other inputs and priorities*

Duration range: < 1 - few seconds Requirement: repeated inputs or rehearsal Fate: erasure or potential storage

*Information that does not reach awareness may have non-declarative priming effects

Working Memory Circuitry

Prefrontal cortex: in continuous on-line reciprocal connections with• Sensory cortices for each modality (specific buffers)• Limbic circuits• Arousal circuits• Motor output circuits

* * * Note: Hippocampus circuitry: (data from amnestic

patients) • Not critical on-line, • Critical for long term storage

LTM declarative systems

Declarative (or explicit) memories

Episodic memory(time-bound facts of the

individual’s past)

Semantic memory(knowledge of

the world)

Medial temporal cortices + Medial temporal cortices +

Declarative (explicit) Memory1. Episodic (Tulving)

Uniquely human capacity to look back to the past and predict the future

Powerful tool Retrieve specific facts/events in time/place

context (unique, autobiographical) Late to develop evolutionarily (man only?) and

ontogenitically (infant amnesia) Fragile to degeneration Critically dependent on (not limited to!)

hippocampal/medial temporal cortices

Declarative (explicit) Memory 2. Semantic Memory

Knowledge (as opposed to remembrance of specific facts)

“Picked-up” knowledge from exposure to mostly forgotten past events/experiences

Starts at birth, long before episodic memory

Broad and powerful Much more resilient than episodic memory

2 - LTM non-declarative systems

Non-declarative (implicit) memories/learning nodes

Priming Conditioning

Sensory corticesEmotional: amygdalaMotor: cerebellum +

Procedural(skills, habits)

Basal ganglia(putamen), etc.

Non-associative

Reflex pathways

Hippocampus system

Binds inputs from all sensory modalities with limbic and prefrontal executive inputs

Reciprocally connected with relevant cortical and subcortical circuitry

Required for declarative memory• For fresh and midterm declarative memories

• Not for very long term “ “

Not required for non-declarative memories

Brain molecular/cellular underpinnings of memory

(Kandel, etc.)

Molecular – short term manipulation of incoming information to guide on-going behavior• Alteration in strength/effectiveness of already existing

synapses Neurotransmitter release/uptake

Cellular – long term storage • Requires protein synthesis and growth of new

synaptic receptors dendrites

Brain circuitry – long-term (yrs) consolidation

Retrieval

Long-term explicit memory storage: • mainly in modality-specific relevant

neocortical, ± limbic areasRecognition -- cue, strong, bottom-upRetrieval of unique item -- top/down, effortful

Long-term implicit memory storage:• mainly in cortical/subcortical circuitry relevant

to task/skill performance (e.g., motor cortex, basal ganglia, cerebellum, brainstem, spinal cord)

Amnestic syndrome

Result of bilateral hippocampal damage Not necessarily irreversible except for peri-

lesional time-window Impairs new explicit learning Impairs explicit but not implicit memories Does not impair procedural memory/skills Does not preclude learning new skills! Occasional cases in kids, even infants

Evidence for neural circuitry of memory

Lesion studies• Behavioral evidence• Imaging• Autopsy

Electrophysiology Functional imaging (PET, fMRI…) Subtraction: task vs. no task (“rest”) Subtraction: impaired group vs. “typical”

group

Problems with functional studies

Smallness of samples Homogeneity of samples Failure to replicate Multiplicity of nodes in widely distributed

pathways Requirement for cooperation Great difficulty/impossibility of testing young

children Cost Time required for data analysis Fancy statistical analyses required