NRSC 2249 - Memory

Hebbian synapse

• mechanism of learning and memory

• potentiation: if presynaptic cell consistently drives postsynaptic, strengthened

• depression: if presynaptic cell fails to drive postsynaptic, weakened

forgetting function

• illustrates how we forget info over time without reinforcing

• nonlinear: rapidly at first, then levels off

• repetition improves

Bliss & Lomo

• first demonstration of experience-dependent plasticity proposed by Hebb

LTP

• long-term potentiation

• persistent strengthening of synapses

NMDA glutamate receptor

• important for early stages of LTP

• activated by glutamate from presynaptic cells

• voltage-dependent; normally blocked by a magnesium ion

  • when repeatedly depolarized, magnesium ion leaves

  • allows calcium into cell, activating secondary-messenger systems

  • synaptic strengthening

• long-term depression occurs with no response of the post-synaptic cell to repeated input

• coincidence detector

neurogenesis

• thousands of new neurons formed in hippocampus every day

• younger neurons → greater plasticity

• new cells → framework for establishing connection among existing

• in hippocampus → important for storing information

dendritic spines

• potential structural basis for long-term info storage

• grow and change in hours in young

  • less plastic and more stable in adults

• may be important for memory storage

stages of memory

encoding, storage, retrieval

encoding

• first stage of memory

• information transformed into storable format

  • acquisition and consolidation

acquisition

stimuli are captured and processed in short-term memory

consolidation

memory is stabilized into long-term memory

storage

• second stage of memory

• preservation of info in memory

  • categorized into different types

retrieval

• third (final) stage of memory

• process of accessing stored memory

• influenced by:

  • strength of memory

  • context during encoding and retrieval

types of memory

• sensory

• short-term and working

• long-term nondeclarative

• long-term declarative

sensory memory

• brief, immediate storage of sensory inputs

• characteristics:

  • milliseconds to seconds

  • high capacity

  • no conscious awareness

  • lost through decay

short-term/working memory

• temporary storage of information undergoing active manipulation/rehearsal

• characteristics:

  • seconds to minutes

  • limited capacity (7 ± 2 items)

  • conscious awareness

  • lost through interference and decay

characteristics of long-term nondeclarative memory

• minutes to years

• high capacity

• no conscious awareness

• lost through interference

characteristics of long-term declarative memory

• minutes to years

• high capacity

• conscious awareness

• lost through interference

list learning task (serial position task)

• task: listen to a list of words and report back as many as you remember

• primacy and recently effects

• manipulate learning and retrieval variables

  • # of trials, rate of presentation, length of delay, etc.

primacy effect

• people tend to remember items at the beginning of a list

• long-term memory

• serial position task

recency effect

• people tend to remember items at the end of a list

• short-term memory

• serial position task

disrupting recency effect

• length of delay affects short-term memory

• distraction after end of list eliminates recency effect

• disrupts ability to hold final items in short-term memory

disrupting primacy effect

• speeding up rate of word presentation reduces primacy effect

• less time to encode into long-term memory

visual report task

• presented with a mix of letters very briefly

• can usually report 4-5 accurately

• partial report:

  • immediately after removal, tone is played to indicate row

  • usually can report 3-4 correctly from a single line

modal model (Atkinson and Shiffrin multi-store model)

• discrete stages of memory with different characteristics

• serial progression of information through memory stages

• storage

  • attention: sensory → short-term

  • rehearsal/encoding: short-term → long-term

• retrieval

  • control processes: long-term → short-term

    • retrieval/recognition cues

patient KF

• damage to left parisylvian cortex (parietal lobe)

• short-term memory impairments

  • difficulty recalling information/details that he had just heard

  • retained ability to form long-term memories

• challenge to strict serial hierarchy

patient EE

• tumor in left angular gyrus and inferior parietal

• short-term memory impairments

  • poor STM for abstract verbal and transposing numbers

    • normal calculating/other number processing

  • normal visuospatial STM and non-verbal long term

  • strong primacy effect, no recency effect

• STM not a necessary gateway to LTM

working memory

• expands on concept of short-term memory

  • content from recent sensory inputs

  • retrieval from long-term memory

  • conscious recall of recent events

• maintenance/manipulation

  • maintain information through rehearsal

  • manipulation by central executive

• popularized by Baddley & Hitch model

Baddley & Hitch model

• model of working memory

• PFC, ACC, parietal lobe, Broca/Wernicke, occipital lobe

PFC (Baddley & Hitch)

• central executive

• executive control tasks:

  • integrating info for decision-making

  • maintaining/manipulating stored info

  • high cognitive load tasks

  • updating info

ACC (Baddley & Hitch)

• attention controller

• evaluates need for adjustments and adaptations based on task demands

Broca/Wernicke (Baddley & Hitch)

• phonological loop

• left hemisphere: verbal and acoustic information

parietal cortex (Baddley & Hitch)

• episodic buffer

• processing “workspace” for sensory and perceptual

occipital lobe (Baddley & Hitch)

• visuospatial sketchpad

• right hemisphere: visual and spatial information

delayed match-to-sample task

• presented with a stimulus (good/poor), delay, asked to choose the matching stimuli

  • requires holding information in working memory

  • reflects preferred stimulus of neuron

• neurons in IT cortex exhibit elevated delay activity even in a sense of stimulus

  • reflects WM for object attributes

disrupting working memory

• adding distractions during delay period

• accuracy decreases with # of intervening distractors

working memory - parietal cortex

• delay activity for stimulus location

working memory - PFC

• reflects memory for object attributes and location

  • ventrolateral: patterns/object selectivity

  • dorsolateral: spatial selectivity

• not attenuated by intervening stimuli: keeps track of behavioral goals

nondeclarative memory: procedural

• learning/remembering skills and habits

  • automatic over time

  • often motor skills

• not necessarily consciously available

• cerebellum, basal ganglia, motor cortex

nondeclarative memory: perceptual

• learning/remembering perceptual information

  • no conscious awareness or intention

  • often repeated exposure to stimuli (shapes/patterns/sounds)

• influences actions and performance in perceptual skills

  • priming, perceptual learning, sensory adaptation

• sensory cortex, occipital lobe

nondeclarative memory: classical conditioning

• type of implicit long-term memory

• learning and remembering associations between stimuli and responses

  • no conscious awareness or intention

• conditioned and unconditioned responses, reflexive actions

• amygdala, cerebellum

nondeclarative memory: operant conditioning

• learning driven by outcomes of actions (reward/punishment)

• linking specific behaviors with outcomes (BF Skinner)

• basal ganglia, associated structures

nondeclarative memory: habituation

• responses to a stimulus decrease with repeated exposure

  • ignore repetitive, non-harmful stimuli

• CNS changes, not peripheral adaptation (muscle fatigue)

declarative memory: semantic

• type of explicit memory that stores facts and concepts

• consciously accessible, deliberate recall

• hierarchically organized

  • general knowledge → specific information

  • lacks details about where fact was learned

• medial temporal lobe (hippocampus)

declarative memory: episodic

• type of explicit memory that stores autobiographical events or experiences

• consciously accessible, subjective experience

• chronologically organized

• medial temporal lobe (hippocampus)

amnesia

• substantial memory deficits

• injury/trauma to the brain or neurodegenerative diseases

• may regain over time or be permanent

anterograde amnesia

• inability to form new memories

• most amnesia is this type

retrograde amnesia

• inability to remember events before a specific point in time

Ribot’s law

• principle in neuropsychology

• in cases of retrograde amnesia, memories are lost in reverse chronological order

  • most recent likely to be lost, older likely to be preserved

patient HM

• bilateral resection of MTL and hippocampus to treat epilepsy

• anterograde amnesia

  • inability to form new LTM

  • access to old memories, partial loss of recent memories

• nondeclarative memory:

  • could learn and improve, but no memory of having done the task before

patient KC

• damage to multiple brain regions, including MTL

• episodic amnesia (retrograde/anterograde):

  • semantic (facts) memory intact, slow acquisition

  • only very early episodic memory

patient Clive Wearing

• extensive damage to hippocampus and surrounding

• anterograde/retrograde amnesia

  • only sporadic recollection of past life

  • could play piano and conduct, but not remember having done so

• near-continuous state of reawakening to the present moment

Korsakoff’s Syndrome

• thiamin deficiency due to poor nutrition, chronic alcoholism

• confusion, disorientation, confabulations, apathy

• anterograde and retrograde amnesia

  • more severe than MTL or dorsomedial thalamus lesions

• mammillary bodies, thalamus

patient PZ

• Korsakoff’s amnesia

• wrote autobiography prior to amnesia - “ground truth”

• early recollection more accurate than late (pre-amnesia)

  • clear temporal gradient (Ribot’s law)

Alzheimer’s disease

• progressive, irreversible neurological disorder, causes dementia

• plaques and tangles in hippocampus leading to deter nation

• stages of disease:

  • early: difficulty with recent events, conversations, names

  • advancing: recent and known individuals and places

• disrupts connectivity between hippocampus and PFC

chronic stress

• damage to brain cells, reduced hippocampus size

  • dentaste gyrus and CA1 particularly affected due to high density of cortisol receptors

• impaired memory formation, forgetfulness, learning difficulties

• inhibited neurogenesis, dendritic hypotrophy

• interference with sleep quality

Patient S

• could not forget: could repeat hundreds of words/numbers

• inflexible memory

  • trouble discerning importance, extracting gist, following plot, understanding poetry/art/metaphors

  • difficulty with faces (changing)

• unable to generalize

Patient MS

• right occipital lobe removed due to epilepsy

• preserved explicit memory: intact recall and recognition

• impaired implicit memory: impaired visual priming

declarative theory (hippocampus)

• theory of hippocampal function

• crucial for all forms of consciously recalled memory

  • episodic, semantic

  • recollection, familiarity

multiple-trace theory (hippocampus)

• theory of hippocampal function

• each time a memory is recalled, hippocampus creates a new trace or representation of the memory

dual-process theory (hippocampus)

• theory of hippocampal function

• retrieval contingent on two independent processes:

  • familiarity (oldness)

  • recollection (events and context)

relational theory (hippocampus)

• theory of hippocampal function

• representations of associations linking elements of experience or knowledge to construct cohesive memory

cognitive map theory (hippocampus)

• theory of hippocampal function

• constructs/maintains cognitive maps

  • spatial representation of environment

  • navigation and spatial memory

consolidation

• process of stabilizing memories

  • days to weeks to years

• MTL essential for early consolidation and storage of episodic/semantic

• over time, memories re-encoded in cortex (immune to hippocampal damage)

neural basis of consolidation

• representations of an event throughout cortex → MTL, bound by hippocampus

  • retrieval of info slowly transferred to neocortex

• consolidation after repeated reactivación of the memory to create direct connections within cortex

  • no longer require hippocampus

• long-term memories require many iterations of activation, consolidation, reactivation, reconsolidation