Long-term Memory (LTM) Explained

Long-term Memory (LTM)

• LTM addresses how we acquire, maintain, and utilize information from experience over time to guide behavior and plan actions.
• A key challenge involves determining what to remember and how to retrieve it effectively.
• Learning is intertwined with memory, relying on encoding, storage, and retrieval processes.
• LTM involves semantic, episodic, and procedural types, supported by systems like the hippocampus, cortex, and amygdala.
• LTM tasks include free recall, stem completion, and motor sequence learning.
• This form of memory requires physical changes in the nervous system and is pervasive throughout.

LTM Processes

• Encoding initiates memory traces in the brain from incoming information.
• Consolidation organizes and stabilizes memory traces over time.
• Storage retains memory traces.
• Retrieval accesses and uses stored memory traces.
• Reconsolidation reorganizes and restabilizes memory traces after retrieval.

Dissociating Short-term and Long-term Memory

• Dissociation logic assesses differences between short-term memory (STM) and LTM.
• Dissociation occurs when performance varies across two tasks.
• Single dissociation is when an acquired disability affects one area of functioning without impairing others.
• A single dissociation does not definitively prove separable systems, different brain regions, or distinct cognitive processes.
• Double dissociation is when two related functions are independent and localized in different brain areas.
• This is achieved when impairment in one brain area affects one function but not the other, and vice versa.
• Independent impairment of both systems provides strong evidence they rely on different brain mechanisms.

Memory Task Examples and Explanations

• STM tasks involve hearing a sequence of words (e.g., "time base offer") and immediately recalling them in the same order.
• The length of the list increases as the task progresses.
• LTM tasks involve hearing a list of words (e.g., "hand cook laugh...") and recalling them in any order until all are remembered.
• Patient K.F. had damage to the left temporoparietal cortex, impacting STM while LTM remained intact, exemplifying single dissociation.
• Patient H.M. had damage to bilateral medial temporal lobes.
• They could not form new LTMs, but STM was unaffected.

Alternative Explanations for Memory Deficits

• Partial damage argument suggests a single system for both STM and LTM, with STM tasks being more difficult.
• Partial damage to this system disproportionately affects STM tasks.
• Compensation argument also posits a single system for STM and LTM but suggests that the brain can compensate for LTM tasks when the system is completely damaged.

Double Dissociation Significance

• Double dissociation involves two different areas of damage and opposing single dissociations.
• This pattern cannot be explained by task difficulty.
• It suggests STM and LTM are separable systems.

Characteristics of Short-Term and Long-Term Memory

• Short-term memory:
  • Operates on a timescale of seconds.
  • Has extremely limited capacity.
  • Relies on sustained activation of neurons.
• Long-term memory:
  • Operates on a timescale of minutes, hours, days, or years.
  • Has a massive capacity.
  • Is based on the number and strength of synapses.

Dissociating Implicit and Explicit LTM

• Implicit memory tasks involve perceptual identification.
• In a study phase, participants see 24 words presented for 2 seconds each.
• The test phase involves flashing old (studied) and new words extremely briefly (priming effect), and participants identify the word.
• Explicit memory tasks involve word recognition.
• During the study phase, 24 words are presented for 2 seconds each.
• The test phase involves presenting 24 old (studied) and 24 new words until a response is given.
• Participants respond "yes" or "no" to indicate if the words were from the study phase.

Single Dissociation of Implicit and Explicit LTM

• Patient M.S. had damage to the right occipital lobe.
• This patient showed typical word recognition (explicit memory).
• They did not show a priming effect (impairment in implicit memory).
• Amnesia patients, such as those with Korsakoff's syndrome & epilepsy, exhibit a larger than normal priming effect but impaired word recognition (impairment in explicit memory).

Double Dissociation of Implicit and Explicit LTM

• It involves two different areas of damage and two opposing single dissociations.
• Cognitive and brain systems involved in perceptual identification (implicit memory) differ from those in word recognition (explicit memory).

Implicit and Explicit Long-Term Memory

• Implicit long-term memory:
  • Is non-declarative.
  • Operates independently of conscious awareness.
  • Encompasses procedural memory, conditioning, non-associative learning, or priming.
  • Functions on a timescale of minutes, hours, days, and years.
  • Has a massive capacity (e.g., thousands of skilled motor sequences).
  • Is based on the number and strength of synapses.
• Explicit long-term memory:
  • Is declarative.
  • Is available to conscious awareness.
  • Includes semantic and episodic memory.
  • Functions on a timescale of minutes, hours, days, and years.
  • Has a massive capacity (e.g., ~20,000 word families in adult vocabulary).
  • Is based on the number and strength of synapses.

Non-Associative Memory

• A change in the strength of response to a single stimulus due to repeated exposure.
• Non-associative learning: a stimulus does not change but response to it does.
• Habituation: reduced response to a repeated stimulus over time like no longer noticing the ticking of a clock.
• Sensitization: increased response to a repeated stimulus, like rubbing your arm hurting more the longer you do it.
• The first evidence of the neural basis of non-associative memory was studied in sea slugs (Aplysia) by Eric Kandel, who won the 2000 Nobel Prize in Physiology or Medicine.

Habituation and Sensitization - Cellular Mechanisms

• Habituation: a decrease in response to a repeated stimulus.
• Touching siphon leads leads to withdrawal of gill, but repeated stimulation, less withdrawal.
• This is a pre-synaptic depression mechanism.
  • Same action potential.
  • Reduced neurotransmitter release.
  • Smaller EPSP.
• Sensitization: increased response to a repeated stimulus.
• Touching the siphon leads to withdrawal of gill, but shocking then touching leads to more withdrawal.
• This is pre-synaptic facilitation.
  • Same action potential.
  • Increased neurotransmitter release.
  • Larger EPSP.

Time scale for habituation and sensitization

• "Short-term" changes:
  • Last for minutes and change the amount of neurotransmitter released.
• "Long-term” changes:
  • Last for hours, days, or weeks and change the number of synapses.
• Note: "Short-term” habituation/sensitization ≠ short-term memory

Classical Conditioning

• Pavlov noticed that a dog learned to salivate in response to a bell that predicted food.
• Unconditioned stimulus (US) = food.
• Unconditional response (UR) = salivation.
• Neutral stimulus (NS) = bell (before pairing).
• Conditioned stimulus (CS) = bell.
• Conditioned response (CR) = salivation.
• Initially, US (food) leads to UR (salivation).
• After pairing, the CS (bell) leads to CR (salivation).

Fear Conditioning

• US = shock.
• UR = freezing.
• CS = tone.
• CR = freezing.

Skill Learning

• Motor skill learning is motor system dependent.
• Motor adaptation uses cerebellar loops.
• Motor sequence learning uses cortico-basal ganglia-thalamocortical loops.

Skill Learning & Motor Adaptation

• Task: Throwing darts with prism glasses, which causes the visual field to shift to the side.
• Control participants adapt to distortion.
• Patients with cerebellar lesions fail to adapt.
• Cerebellum uses a forward model to predict results of motor commands.
• Uses differences between actual results and predicted results for:
  • Online error correction.
  • Motor learning.
  • Feedback control.

Skill learning: Motor sequence learning (Serial Reaction Time Task)

• Each light turns on in a certain order.
• The job is to hit the appropriate buttons in that order.
• R = random order, S = repeating sequence.
• Repetition causes one to get better without conscious awareness.
• Depends on basal ganglia thalamocortical loops & reinforcement learning.
  • Select action expected to lead to maximum reward.
  • Perform action.
  • Compare actual reward to expected reward.
  • Prediction error: actual reward - predicted reward
  • Use prediction error to update expectations
  • Repeat

Dopamine and Prediction Error

• Dopamine signal from substantia nigra pars compacta (SN) represents prediction error.
  • Larger response = better than expected
  • Smaller response = worse than expected

Basal Ganglia and Reinforcement Learning

• Unexpected rewards generate dopamine signals from the substantia nigra pars compacta (SN).
• This excites the direct pathway (via D1 receptors) and inhibits the indirect pathway (via D2 receptors).
• This allows modification of behavior based on reward.

Implicit Memory & Priming

• Priming:
  • Change in stimulus processing due to prior exposure to same or related stimulus without conscious awareness
• Perceptual priming: based on the form of the stimulus and is enhanced by the match between the early and later stimuli.
• Conceptual priming: technique and process that engages people in a task or exposes them to stimuli.

Perceptual Priming Studies

• Warrington and Weiskrantz (1968)
  • Studied Korsakoff's syndrome patients and amnesia due to severe alcoholism.
  • Task: Show fragmented pictures (Gollin figures)
  • Identify 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 objects.

Semantic priming studies

• Rossell, Price, & Nobre (2003): Semantic-priming task
• Lexical decision: word or non-word?
• Will participants be faster at identifying a word if primed with a related word vs an unrelated word?... YES!
• Prime words activate other related words in mind

Neural Basis of Priming

• Change in stimulus processing due to prior exposure to same or related stimulus without conscious awareness.
• Depends on region of cortex processing relevant representations.
• Perceptual priming happens in sensory cortices (e.g., occipital lobe for vision).
• Conceptual/semantic priming happens in unimodal & multimodal association cortices (e.g., anterior temporal, inferior parietal, prefrontal cortex).

Semantic Memory: Activation of Cortex

• Concrete words activate areas of cortex involved in relevant processing.
• Actions: motor cortex/somatosensory cortex.
• Sounds: auditory cortex.
• Colors & movements: ventral visual stream (occipital/temporal cortex).

Organization of Semantic Representations

• Sensory/functional theory is based on relevant sensory and motor features.
  • E.g., Action words activate region of primary motor cortex for specific body part.
  • Lick: Face
  • Pick: arm
  • Kick: leg
• Domain-specific theory is based on semantic categories.
  • E.g.: categorize fruits & vegetables, animate living things, nonliving things, conspecifics, etc.

Organization of Meaning in Brain

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

Semantic Memory Formation

• New semantic memories often start as episodic memories.
• At first one is recalling something recently learned, like the capital of British Columbia.
• Over time and after many activations and retrievals, they convert into semantic memories so when one recalls the capital of BC, they have no idea how they learned that.

Explicit Memory: Episodic Memory

• Hippocampus and related structures (HC) form indices (pointers) to bind cortical representations.
• E.g. an episodic memory for costume party can have many aspects.
  • Saw Sam dressed as Elmo
  • Heard dance music
  • Ate lots of sweet M&Ms
• Cortex represents Sam, Elmo, dance music, M&Ms, etc..., and the hippocampus binds them together.

Retrieval of Episodic Memory

• HC and related structures use indices (pointers) to re-instantiate cortical representations.
• E.G. when asked, “Remember when Sam dressed as Elmo?” - Sam & Elmo act as retrieval cues in cortex.
• This activates indices (pointers) in HC and representations in cortex with visual, auditory, and taste information.

Medial temporal Lobes

• Parahippocampal cortex is for encoding spatial layout and visuospatial memory.
• Perirhinal cortex is for object recognition/familiarity, and binds features of objects.
• Entorhinal cortex has integrative function for input/output to hippocampus
• Hippocampus is for formation and consolidation of memory + spatial navigation
• Fornix = pathway from hippocampus to other cortical/subcortical structures.
• H.M.: Bilateral resection of the hippocampus, entorhinal cortex, and parahippocampal cortex led to amnesia.
• Removal of bilateral medial temporal lobes led to severe anterograde amnesia, and unable to form new LTMs after surgery.

Cognitive Map Theory

• Cognitive map theory: memory for spatial relationships in environment.
• Place cells fire when in a specific location.
• help form an internal "map" of your environment and help bind where and when events happen.

Relational Memory Theory

• Memory for associations in general, as shown with rats trained to prefer certain smells, so each smells in a chain have a relative spatial or relational meaning.
• Rats prefer A over B, but then B over C, etc. but if the fornix is lesioned it disrupts the single pair relationships.

Hebbian Learning

• Hebbian Learning states: "Neurons that fire together wire together.”
• When presynaptic action potential leads to postsynaptic action potential, the connection is strengthened.
• Long-term potentiation results in increased number of dendritic spines and synapses.

LTP (Long-Term Potentiation)

• Increase in synaptic strength.
• Exhibits necessary properties for Hebbian learning.
• Found in hippocampus (and other brain regions)

Early LTP: increased presynaptic release of neurotransmitter and an increased number of postsynaptic receptors.

Late LTP: Increased number of dendritic spines and synapses.

LTD (Long-Term Depression)

• If synapses only strengthen, neural firing will grow out of control.
• When presynaptic action potential does not lead to postsynaptic action potential, connection is weakened.
• Reduction in neurotransmitter released, number of receptors, and number of synapses.

Episodic memory retrieval & distortion

• Episodic retrieval is reconstructive (based on memory trace, genes, past experience, internal state, and environmental context).
• Episodic retrieval can be inaccurate for many reasons, including semantic relatedness, cultural experience, source misattributions, pragmatic inferences, misleading post-event information.
• A key method scientists use to study distortions is false memories (Deese, Roediger, & McDermott (1995)).
  • Present list of semantically related words: Bed, rest, awake, tired, dream, blanket, doze...
  • Recall/recognition memory test: look out for words on list (rest, tired, dream), unrelated distractors, or semantically-related lures - Participants often report specifically remembering presentation of lures, even if know about the effect.
  • Related lures are reported almost as often as words actually on the list, with high confidence in the accuracy.

Consolidation

• There representations in cortex for both new and old memories. The new memories depend on both representations in cortex and links from hippocampus, with reactivation due to retrieval and replay during sleep reduces dependence on hippocampus.
• Over time reduced reliance on the hippocampas shifts to higher reliance on the corte, and researchers ask whether “Is the distinction “new” vs “old” or “episodic” vs “semantic”?”
• New “semantic” information may start as episodic memory initially, and old autobiographical memories may take on semantic qualities over time.

Complementary Systems Hypothesis

• The hippocampus rapidly learns and creates distinct memories for each event and instance, and is more important for episodic memories.
• The cortex learns slowly, extracts generalities across events/instances, and is more important for semantic memories.
• Reconsolidation: when a memory is retrieved, it is reformed and is once again subject to interference.
  • Nader et al. (2000) demonstrated this effect of reconsolidation on fear conditioning in rats.

Language Definition and Societal Importance

• Language is a systemic means of communicating information using conventionalized sounds, gestures, marks, or signals having understood meaning.
• The function of human language is to influence people's behavior by changing what they know, think, believe, or desire.
• Language is the basis for society - it allows us to interact with each other in a way that goes beyond our immediate surroundings.
• It has the power to communicate information quickly, facilitate an interactive social network, store knowledge outside individuals, allows wisdom to accrue over generations, refer to any time or place, real or imaginary, enable creative expression due to generativity and compositionality.

Language Hierarchy

• Hierarchical organization of language: phoneme, morpheme, word, phrase, sentence
• Phonemes
  • Smallest unit of perceived speech.
  • Different phonemes in different languages.
  • Combine and form sounds, often using language specific rules.
• Morphemes
  • Smallest unit that signals meaning (combination of phonemes) made in a language-specific combination (prefixes, suffixes, roots, or entire words).

Words Phrases and Sentences

• Words are the smallest stand-alone units of meaning and a combinations of one or more morphemes use language specific rules.
• Phrases are an organized grouping of one or more words that play a particular role use almost limitless language-specific rules.
• Sentences are a set of words/phrases that (in principle) tells a complete thought and use almost limitless language-specific rules to form larger linguistic units sentences.

Generative Grammar, Syntax, and Semantics

• Children combine words in novel ways to express novel ideas so researchers understand languages cannot be based solely on imitation, association, and reinforcement.
• Thus, we must learn a set of rules (grammar) that can be applied in a generative way that is studied by Noam Chomsky, an academic.
• Rules for language structure are defined by morphology, syntax, and semantics that defines how meaning is derived from morphemes, words, phrase, and sentences, also known as phrase structure.

Problems with Surface and Deep Structure

• Phrase structure has limits to relying on phrase structure alone, as exemplified by ambiguous sentence structures.
• Ambiguity can provide insight into cognitive processing language by creating sentences with similar surface or deep structure.
  • Lexical ambiguity: word has two different meanings (Lexical but not syntactic ambiguity).
  • Syntactic ambiguity: same words can be grouped together to lead to ambiguous sentences where phrases have multiple meanings. -Syntactic and lexical ambiguity: phrases and words have multiple meanings.
  • Referential ambiguity: some words or phrases can refer to different things ("it, she, ...").

Speech Production

• Speech production
• Speech is fundamentally a motor act dependent on hierarchical planning that depends on prefrontal areas.
• Broca's area is in the left hemisphere only (in most individuals).
• Broca's Aphasia is due to damage to Broca's area and causes labored, slow & nonfluent speech with awkward articulation.
  • Patients make with phonemic errors and written output shows same errors as speech.
  • Better fluency for memorized phrases, but problems with language planning and production (not a motor problem).
  • Can't speak with greatest difficulty with verbs, articles, pronouns, inflections, but the sentences generated, for the most part, make sense.
  • Have difficulty reading and producing function words and understanding or using syntax.

Speech Comprehension and Wernicke's Aphasia

• Speech comprehension is fundamentally, a perceptual process that depends on the ventral "what" stream in Wernicke's Area in left hemisphere only of brain.
• Wernicke's Aphasia is fundamentally, a perceptual process that depends on the ventral "what" stream in Wernicke's Area in left hemisphere.
• Wernicke's aphasia is due to damage to Wernicke's area in the brain, which causes the brain to generate phonetically & grammatically normal but meaningless speech.
  • Has fluent, unlabored, and well articulated intonation, however uses uses inappropriate, nonsense words, and expresses what they mean using roundabout ways.
  • Causes severe comprehension and causes problems translating auditory input into phonological forms that can then access semantics or language.

Hemispheric Specialization

• Left hemisphere specializes in language production & syntax in Broca's Area and language comprehension & semantics in Wernicke's area.
• Right hemisphere specializes mental rotation, emotion perception, music, and melodies for detecting prosody in language.
• Split-brain studies corpus callosum severed to each hemisphere acts somewhat independently or differently.
• Typically split split-brain studies: left hemisphere can name objects, right hemisphere cannot or split-brain patients can perform two different tasks simultaneously.

Lateralization

• Handedness is right-handed 80-90%, left-handed ~10% or cross-dominant/Mixed-handed: ~10%.
• Language is dominantly lateralized with most people, even left-handed, have left-hemisphere dominance for language.
• Prosody (intonation, tone, stress, and rhythm) that is used for emotional state is primarily processed in the right hemisphere

Interactive Language Network

• Localization & distribution of processing:
  • Bottom-up & top-down influences
  • Recurrent & interactive processing
• Interactive Activation Theory: McClelland and Rumelhart show model of bottom-up and top-down processes interacting.
  • Examples include the McGurk effect. when misinterpretation due to conflicting stimuli.

fMRI - functional Magnetic Resonance Imaging

• FMRI: Functional Magnetic Resonance Imaging
• Stimulus causes Neuronal activity that leads to Neurovascular coupling that leads to a Hemodynamic response that is detected by the MRI scanner as the fMRI BOLD response. FMRI has very good spatial resolution (millimeters) with an temporal resolution (seconds) and is non-invasisve, and generally low risk Meaning in the Brain

Decision Making

• Decision-Making is vast, variable, and important.
• Examples include navigation, substance relationships, time management, money...
• Descriptive approach for how should people decide?
• Expected Value: Is clear prescription for “correct” choices and leads people, on average, maximizes monetary gains, but does not explain all choices that people make.

Behavioral Economics & Subjective Utility

• Behavioral economics describes how do people decide using probability, expectations, subjective, utility.
• People transfer objective value into subjective utility because utility = usefulness or desirability of an outcome, using diminishing marginal utility ( subjective utility increases more slowly)
• People don't like loses because they feel worse than gains, and have small probability aversion when facing uncertainty.
• People make decisions based on individual
  • Reference dependence, gain & loss framing