Long-term Memory and Priming

LTM Types & Systems

• Long-term memory splits into declarative (explicit) and nondeclarative (implicit) memory
• Declarative memory includes events (episodic memory) and facts (semantic memory)
• Nondeclarative memory covers procedural memory, perceptual representation system, classical conditioning, and nonassociative learning

Priming

• Priming is a change in stimulus processing from prior exposure to the same or related stimulus, without conscious awareness

Perceptual Priming Study (Warrington and Weiskrantz, 1968)

• Warrington and Weiskrantz conducted a study on perceptual priming in patients with Korsakoff's syndrome and amnesia due to severe alcoholism
• The task: showing fragmented pictures (Gollin figures) and identifying the object in as few frames as possible
• Participants improved day to day, despite not remembering the previous day's training; fewer frames are needed to identify the object.

Semantic Priming Study (Rossell, Price, & Nobre, 2003)

• Rossell, Price, & Nobre completed a semantic-priming task
• Participants had to perform a lexical decision (whether a target was a word or non-word)
• Participants are faster at identifying a word if primed with a related word
• Prime words activate other related words

Neural Basis of Priming

• Change in stimulus processing occurs because of prior exposure to the same or related stimulus without conscious awareness
• Priming depends on the region of cortex processing the relevant representations
• Perceptual priming involves sensory cortices, like the occipital lobe for vision
• Conceptual/semantic priming involves unimodal and multimodal association cortices like the anterior temporal, inferior parietal, and prefrontal cortex

Semantic Memory

• Memory for concrete word meanings activates areas of the cortex involved in relevant processing
• Sounds activate the auditory cortex
• Actions activate the motor cortex/somatosensory cortex
• Colors and movements activate the ventral visual stream (occipital/temporal cortex)

Sensory/Functional Theory

• Organization of semantic representations occurs based on relevant sensory and motor features
• Action words activate the region of the primary motor cortex for the specific body part
  • Lick activates face area
  • Pick activates arm area
  • Kick activates leg area

Domain-Specific Theory

• Organization of semantic representations is based on semantic categories
  • Fruits & vegetables : Apple, orange, lettuce
  • Animate living things: Cat, dog, snake
  • Nonliving things: Wrench, hammer, rock
  • Conspecifics: Mom, dad, mailman

Meaning Organization in the Brain

• Tools activate regions tied to actions (ex: premotor cortex)
• Animals activate regions tied to visual/perceptual features (ex: posterior temporal lobe, ventral visual stream)
• Meaning is partly categorical (tools vs. animals)
• Meaning depends on shared sensory and functional features (action-related vs. perceptual)

Forming Semantic Memories

• Semantic memories often start as episodic memories
• Initially, information is tied to a specific learning experience
• Over time, episodic memories can convert to semantic memories through activations and retrievals

Encoding Episodic Memory

• The hippocampus and related structures form indices (pointers) to bind cortical representations
• Cortex represents aspects such as people, things, music, etc
• Hippocampus binds them together

Retrieval of Episodic Memory

• Hippocampus and related structures use indices (pointers) to reinstantiate cortical representations
• Retrieval cues activate indices (pointers) in the HC and activate representations in the cortex

Medial Temporal Lobes

• The parahippocampal cortex encodes spatial layout and visuospatial memory
• The perirhinal cortex handles object recognition/familiarity and binds features of objects
• The entorhinal cortex is an integrative function for input/output to the hippocampus
• The hippocampus forms and consolidates memory and handles spatial navigation
• The fornix is a pathway from the hippocampus to other cortical/subcortical structures

Medial Temporal Lobes & Patient H.M.

• H.M. had a bilateral resection of the hippocampus, entorhinal cortex, and parahippocampal cortex
• H.M. had severe anterograde amnesia, making him unable to form new LTMs after surgery
• Also displayed temporally-graded retrograde amnesia, being unable to recall existing LTMs from just before the surgery

Cognitive Map Theory

• Memory for spatial relationships in the environment
• Place cells fire when in a specific location and form an internal “map” of the environment
• Helps bind where and when events happen

Relational Memory Theory

• Memory for associations in general
• Odor association task
• Lesion to fornix leaves single pairs intact, and disrupts relationships across pairs

Hebbian Learning

• "Neurons that fire together wire together”
• When a presynaptic action potential leads to a postsynaptic action potential, the connection strengthens

Long-Term Potentiation (LTP)

• An increase in synaptic strength
• Exhibits necessary properties for Hebbian learning
• Found in the hippocampus and other brain regions

Early LTP

• There is an increased presynaptic release of neurotransmitter
• The number of postsynaptic receptors increases

Late LTP

• Increased number of dendritic spines and synapses

Long-Term Depression (LTD)

• Neurons If synapses only strengthen, neural firing grows out of control
• Long-term depression (LTD) is also called "Neurons that fire apart wire apart”
• When presynaptic action potential does not lead to postsynaptic action potential, connections weaken
• There is a reduction in neurotransmitter released, the number of receptors, and the number of synapses

Episodic Retrieval

• Retrieval of episodic memories is reconstructive
• Retrieval guesses based on memory trace, genes, past experience, internal state, and environmental context

Lab Activity: False Memory

• Study by Deese (1959), Roediger & McDermott (1995)
• Utilizes the DRM paradigm
• Presented a list of semantically related words
  • Bed, rest, awake, tired, dream, blanket, doze
• Test consisted of recall/recognition memory
  • Words on list (rest, tired, dream) are presented
  • Unrelated distractors (cake, mountain, cloth)
  • Semantically-related lures (sleep)

False Memory and Lures

• Related lures are reported almost as often as words actually on the list
• There is high confidence in their accuracy
• Participants often report specifically remembering the presentation of lures
• Happens even if you know about the effect

Inaccurate Episodic Retrieval

• Episodic retrieval can be inaccurate for many reasons, including semantic relatedness, cultural experience, source misattributions, pragmatic inferences, and misleading post-event information
• Retrieval of episodic memories is reconstruction

Consolidation

• New memories depend on representations in the cortex and links from the hippocampus
• Over time, reactivation due to retrieval and replay during sleep reduces dependence on the hippocampus
• Old memories depend on representations and links in the cortex

Complementary Learning Systems Hypothesis

• Hippocampus learns rapidly (single trial learning) and creates distinct memories for each event/instance, therefore, it's critical for episodic memories
• Cortex learns slowly and extracts generalities across events/instances, therefore, it is important for semantic memories

Consolidation Distinction

• Is the distinction "new" versus "old" or "episodic" versus “semantic"?
• New “semantic” information may start as episodic memory
• Old autobiographical memories may take on semantic qualities

The Process of Consolidation

• Consolidation can either be permanent or is a product of Reconsolidation
• In reconsolidation a memory is retrieved and reformed, and is once again subject to interference

Reconsolidation and Fear Conditioning

• Nader et al. (2000) conducted fear conditioning in rats
• Conclusions:
  • Recall/reactivation leads to reconsolidation
  • If memory re-formation is blocked during reconsolidation, then the memory is forgotten
  • Since this study, reconsolidation has been demonstrated in many LTM systems across many organisms, including humans

Key Aspects of Memory

• Key factors in memory include processes, types, systems, and tasks
• Implicit and explicit long-term memory are important
• Long-term memories have explicit (semantic, episodic) and implicit aspects along with consolidation and reconsolidation