PSY260 Lecture 7: Episodic & Semantic Memory

Squire’s Model of Long-Term Memory (LTM)

• Long-term memory → declarative + nondeclarative memory

  • Nondeclarative memory → procedural (motor) memory + implicit memory

  • Declarative memory → semantic memory + episodic memory

Endel Tulving

• 1927-2023

• Cognitive psychologist at UofT

• Distinguished between episodic and semantic memory

• Episodic memory = “mental time travel“

  • Memories for specific events are tied to time and place where the info was learned (context)

  • Hippocampus → binding context + content

    • Essential for episodic memory

Semantic Memory

• General world knowledge

• Fairly stable knowledge, typically shared with the community (common knowledge)

  • Ex. world events

Episodic vs. Semantic Memory

• Distinction between episodic and semantic memory first made by Tulving (1972)

• More recent research has suggested the difference isn’t quite that simple

  • There is overlap

• Both conscious, declarative forms of memory

  • We are aware and can vocalize

Sensorimotor Framework

• A concept is encoded as a specific combination of sensory, motor, and other modality specific features

• Different classes of representations can be modality dependent

  • Semantic memory provides a framework for interpreting and organizing new events or new information

  • Allows us to predict the future with higher accuracy

How Does Conceptual Knowledge Benefit Memory?: Baseball study

• Asked baseball experts vs. non-experts to recall a fictional game

• More conceptual knowledge resulted in:

  • Better memory for information overall

  • More organized recall; events recalled in proper sequence

  • More relevant information remembered

  • Better prediction of the outcome

• The more you know, the more you CAN know

  • Easier to add new information

  • ↑ plasticity in network

Semantic Network Organization

• Semantic memory seems to be stored in laarge, distributed networks of associated concepts and ideas

  • Big networks and co-activation of different brain areas

• Concepts are not understood in isolation

  • When one concept is activated, this activation spreads to related concepts

Semantic Networks

• Semantic interconnectivity

  • More associations → faster retrieval

  • More associations = more potential retrieval pathways

Hub and Spokes Model

• Hub = central concept

• Spokes = related/associated concepts

  • Related concepts end up closer to each other

• ↑ connected concepts → ↑ retrieval speed

Episodic Memory

• Collection of personal experiences that often includes contexts

  • Context: who, what, when, where, why, etc.

• There contextual details can serve as cues

  • If a cue is activated → the entire memory trace can be reactivated

    • Ex. smell something → recall a whole memory

    • Ex. PTSD → cue something that triggers traumatic memory

  • Results from functional neuroimaging studies have shown support for this

    • Can see a cue activating a network

Semantic vs. Episodic Memory: Table

• Episodic

  • Can be communicated flexibly

  • Consciously accessible

  • Tagged with spatial/temporal context

  • Must have experienced event personally

  • Learned in a single exposure

• Semantic

  • Can be communicated flexibly

  • Consciously accessible

  • Not necessarily tagged with spatial/termporal context

  • Can be personal/general information

  • Can be learned in single exposure, strengthened by repetition

Strength with Repeated Exposure

• Semantic → stronger with repetition

  • ↑ details remembered

• Episodic → weaker with repetition

  • ↓ details remembered

Which is First: Episodic/Semantic?

• Episodic may develop only after sufficient semantic memories are formed

  • Ex. can’t remember a grad ceremony if you don’t know what graduation is

• Semantic may develop from repeated episodic

  • Encounter same information in multiple contexts → more whole understanding

  • Specific episodes blend → form a strong semantic memory

• Or, both systems depend one each other

• All have potential for truth

Highly Superior Autobiographical Memory (HSAM)

• Very detailed, episodic memories of everything

  • Hyperthymesia

• Tend to have really vivid visual imagery

  • Hyperphantasia

Severely Deficient Autobiographical Memory (SDAM)

• Tend to rely on semantic descriptions of things that have happened, without feeling like they’re actually reexperiencing anything

  • More like a semantic list framework

• Tend to have very poor or no visual imagery

  • Aphantasia

Semantic Memory in Rodents

• Semantic memories are very difficult to assess non-verbally, but some approaches are possible

• Radial arm maze can indicate semantic memory in rodents:

  • Trained with food always in one arm and rat always started in same arm

  • After training, rat is started from a new arm but navigates directly to the food arm

  • Demonstrates flexible use of memory, a hallmark of semantic memory

• They form a semantic map

• If just relying on episodic memory, they would go the same direction no matter what

Episodic Memory in Rodents

• Using the radial arm maze to assess episodic memory:

  • Half the arms have food, there are no markings to indicate which

  • Rat must sequentially search baited arms, remembering which have been visited and when (declarative memory)

    • Remember which ones they already searched → proof of episodic memory

“Episodic-like“ memory in Scrub Jays

• Facts

  • Store food to eat later

  • Scrub jays prefer worms to peanuts

• Experiment:

  • Were left to store wax worms and peanuts in different locations

  • Searched preferentially for fresh worms after a short period (4hr)

    • Episodic memory of caching

  • Rapidly learned to avoid searching for worms after thee worms were decayed (124hr)

    • Semantic memory that worms would go bad

Encoding

• Initial storage into memory

• Simple repetition isn’t very effective

• Better to relate to prior knowledge

  • Builds web of associations

• Ex. Bransford and Johnson

  • Read a paragraph to one of 3 conditions, revealing topic before, after, or not at all

  • Understanding topic before helped with memory of paragraph

Encoding: Levels of Processing Framework

• The more deeply you process information → the better it is encoded

• Ex. memorize list of words with different depths of processing

  • Low-level: structural processing

    • Ex. capital letters, colour of words

  • Mid-level: phonemic processing

    • Ex. identifying rhymes

  • High-level: semantic or multisensory information

    • Ex. word meaning, imagery, making connections to own life

Consolidation

• Process of making a memory more stable for storage

  • Makes it resistant to disruption

  • Solidified into storage form

Consolidation: Recency

• Recent memories are more vulnerable to disruption

• Trained rats with two different sounds in two different contexts with foot shock to elicit freezing, then repeated both sounds in a new context

• Results:

  • Rats were much less afraid of more recent tone, short-term memory disrupted

  • Hippocampal damage → drop in freezing % of recent sound

Consolidation: Sleep

• Sleep plays a key role in memory consolidation

  • Replaying new memories (REM/nREM)

  • Integrating new memories with existing semantic knowledge (nREM)

    • Connections and adding to frameworks

  • Procedural memory (REM)

  • Forgetting/pruning irrelevant information

    • Keep what’s meaningful

Reconsolidation

• Each time we recall a memory, it becomes temporarily labile (susceptible to modification)

• Memories can be updated with new information (true/false), associated with other memories, or weakened

• Potential outcomes:

  • May get distorted

  • Could be externally influenced

  • But can have therapeutic benefits

Retrieval

• Re-activating the memory for further processing

• Accessing memories that have been encoded and consolidated into long-term storage

• Making information accessible for conscious processing again

• Different ways to test retrieval:

  • Recognition → yes/no

  • Recall → details

    • Cued recall

      • With a cue → activates trace

    • Free recall

      • As much information freely recalled without cues

    • Serial recall

      • Recall a list in order that it was encoded

Retrieval: Role of Cues

• The more cues provided, the easier it is to recall a memory

• Ex. what form of memory is for facts and general knowledge?

  • Free recall: ________

  • Cued recall: s______

  • Recognition: a) semantic, b) implicit, c) episodic

Retrieval: Testing to enhance memory

• Testing Effect, Retrieval Practice, Test-Enhanced Learning

• Roediger & Karpicke (2006)

  • Encoding

    • Students studied two short paragraphs

    • Later, re-read one paragraph (Reading condition), and wrote all they could remember about the other one (Testing condition)

  • Retrieval

    • Reading: 40% recalled

    • Testing: 55% recalled

Cerebral Cortex & Semantic Memory: Storage

• Semantic memories seem to be stored in a distributed fashion throughout the cerebral cortex

• Sensory cortices:

  • First cortical processing center for a sense

  • Somatosensory cortex, auditory cortex, visual cortex

• Association cortices:

  • Links across senses

  • Get activated between sensory cortices and show links

“Jennifer Anison“ neurons/”grandmother cells”

• Recordings from the human cortex show that some neurons are tuned to specific semantic categories

• One example:

  • A neuron in the right hippocampus of an awake human fires to pictures of Steve Carell but not photos of other celebrities

  • Some only fire to Jen Aniston

  • Some fire and are specific to anything about your grandmother

Frontal Cortex: Storage & Retrieval

• May play an organizing role in declarative memories

  • Selecting information to be encoded into LTM

  • Suppressing hippocampal activity to inhibit memories

  • Frontal lobe damage causes problems of source memory (ex. not remembering if something was a dream or real), suggesting problems of retrieving complex memories

    • Accuracy depends on frontal lobe

Medial Temporal Lobes and Memory

• Consolidation seems to depend on MTL

  • Hippocampus and surrounding cortices

Hippocampus

• Lots of evidence to support role of hippocampus in memory consolidation (STM → LTM)

• Activity during encoding of words is greater for words that are later remembered compared with those forgotten

  • ↑ hippocampal activity → ↑ memory of words

• Lesion in hippocampus:

  • Rodents cannot do episodic radial arm maze → search same arms over and over again

  • Scrub jays don’t remember where they cached food

  • Human patients (HM, Clive Wearing) cannot consolidate new experiences into LTM

    • HM: removed both hippocampi to treat epilepsy → lost consolidation ability, couldn’t form new memories, had some retrograde amnesia

    • Clive Wearing: memory reset every 30 secs

Hippocampus & MTL complex

• So there is strong evidence that the MTL (especially hippocampus) consolidates declarative memories

• 2 current theories:

1. Standard Consolidation Theory

2. Multiple Trace Theory/Multiple Trace Memory Model

Standard Consolidation Theory

• During learning, the MTL relays information to the cortex

  • MTL → cortex

  • During consolidation

• Over time, however, the cortex gets the message and the memories become independent of the MTL

  • Cortex ←→ Cortex (builds webs)

  • After consolidation

• Explanations

  • Explains why brain disruption usually damages recent memories (still undergoing consolidation) but not older memories (fully consolidated)

Multiple Trace Theory

• The MTL helps organize together the distributed semantic facts into specific episodic memories

• True episodic memories still rely on the hippocampus and MTL cortex for retrieval

• Explanations

  • Explains cases of severe retrograde amnesia

  • Also suggests that spared memories after MTL damage are actually semantic rather than true episodic

LTP in the Hippocampus

• What is happening at the level of the synapse when we form new episodic memories?

• Remember LTP

  • Lots of Hebbian plasticity in hippocampus

  • Firing in neuron A causes joint activity in A and B - causes LTP in the connection between these neurons

    • Connection strengthens