Lecture 4 - Memory

Architecture of Memory and the Multi-Store Model

• Information Processing Framework:

  • Cognitive memory systems operate as a multi-stage pipeline transitioning from fleeting environmental sensations to permanent neural records.

  • Sensory Input: Environmental stimuli are captured across visual, auditory, somatosensory, and other sensory modalities.

  • Sensory Memory: High-capacity but ultra-brief storage buffer. Information in sensory memory decays rapidly within milliseconds to seconds; any unattended information is permanently lost.

  • Attention: The critical filter mechanism that selects a subset of sensory input for transfer into conscious cognitive workspaces.

  • Short-Term Memory (STM): A capacity-limited buffer that retains information across seconds. Retains sensory, motor, and cognitive representations through active neural firing. Unrehearsed information is rapidly forgotten.

  • Maintenance Rehearsal: The process of cyclically repeating or actively manipulating information within short-term memory to prevent decay and prolong retention.

  • Encoding: The active neurobiological consolidation operations that transfer information from short-term holding stores into permanent storage.

  • Long-Term Memory (LTM): The virtually unlimited, enduring memory store supported by structural and biochemical synaptic modifications. Information may still be lost over time through trace decay, interference, or retrieval failure.

  • Retrieval: The cognitive and neural process of accessing and reactivating stored long-term representations back into short-term memory for conscious awareness and behavioral execution.

Fundamental Forms of Learning and Neural Systems

• Taxonomy of Basic Learning Forms:

  • Perceptual Learning:

    • Definition: The capacity to recognize stimuli that have been perceived previously (such as sights, sounds, smells, or tactile patterns).

    • Neural Substrate: Involves structural and synaptic modifications within neural circuits of the sensory association cortex dedicated to detecting particular stimuli.

  • Motor Learning:

    • Definition: The establishment of alterations within neural circuits that control specific motor sequences and bodily behaviors.

    • Neural Substrate: Driven by changes in the primary motor cortex, supplementary motor areas, premotor cortex, cerebellum, and basal ganglia.

  • Stimulus-Response ($S\text{-}R$) Learning:

    • Definition: The learned ability to execute a specific behavioral response when a particular stimulus is presented.

    • Circuit Interaction: Establishes functional connectivity linking the perceptual circuits that detect the stimulus directly to the motor circuits that control the behavior.

    • Primary Subdivisions: Encompasses both Classical Conditioning and Operant (Instrumental) Conditioning.

Stimulus-Response Learning: Classical Conditioning and Conditioned Emotional Responses

• Mechanisms of Classical Conditioning:

  • An association formed between two stimuli where an organism learns to predict environmental events.

  • Unconditioned Stimulus ($US$): A stimulus that naturally, biologically, and automatically evokes a physiological reflex without prior learning (e.g., corneal air puff, painful foot shock).

  • Unconditioned Response ($UR$): The reflexive, unlearned physiological reaction triggered by the $US$ (e.g., corneal blink reflex, autonomic freezing).

  • Conditioned Stimulus ($CS$): An initially neutral environmental stimulus (e.g., tone, light) that elicits no specific target response prior to conditioning.

  • Conditioned Response ($CR$): The learned behavioral or physiological response evoked by the $CS$ alone following repeated contingent pairings of the $CS$ and $US$.

• Eye-Blink Classical Conditioning Paradigm:

  • Investigated extensively in human and rabbit paradigms utilizing electromyography (EMG) recording systems.

  • Apparatus: Subject is fitted with headgear holding an air jet delivery tube adjacent to the cornea and EMG electrodes placed on the periorbital musculature.

  • Procedure: An acoustic tone ($CS$) is presented via an audio speaker, followed immediately by a puff of air ($US$) directed to the cornea, which reflexively elicits an eye blink ($UR$).

  • Conditioning Outcome: After repeated associative pairings of the tone and the air puff, presentation of the tone alone ($CS$) becomes sufficient to elicit an anticipatory eye blink ($CR$).

• Conditioned Emotional Responses (CER) in the Amygdala:

  • Occurs when an auditory tone ($CS$) is contingently paired with an aversive foot shock ($US$).

  • Lateral Nucleus of the Amygdala: Acts as the primary convergence zone receiving weak auditory afferents ($CS$) and strong somatosensory nociceptive afferents ($US$).

  • Synaptic Strengthening: Coincident activation of weak auditory synapses and strong shock synapses depolarizes the postsynaptic membrane of pyramidal neurons in the lateral nucleus, driving long-term potentiation ($LTP$).

  • Receptor Dynamics in Lateral Amygdala: LTP induction requires the activation of $N$-methyl-$D$-aspartate ($NMDA$) glutamate receptors, leading to subsequent insertion of $\\alpha$-amino-$3$-hydroxy-$5$-methyl-$4$-isoxazolepropionic acid ($AMPA$) glutamate receptors into the postsynaptic membrane.

  • Effector Circuitry: The lateral nucleus projects directly and via the basal nucleus to the central nucleus of the amygdala.

  • Expression of CER: The central nucleus projects to downstream brainstem and diencephalic centers:

    • Hypothalamus: Regulates autonomic sympathetic activation and endocrine hormonal cascades.

    • Midbrain (Periaqueductal Gray): Mediates behavioral arrest and freezing responses.

    • Pons and Medulla: Modulate autonomic cardiorespiratory reflexes and startle potentiation.

Stimulus-Response Learning: Operant Conditioning and Reinforcement Systems

• Principles of Operant (Instrumental) Conditioning:

  • Involves the acquisition of novel goal-directed behaviors based on environmental outcomes.

  • Establishes functional associations between a discriminative stimulus, an emitted behavioral action, and subsequent consequences.

  • Permits an organism to adaptively modify behavior based on consequences:

    • Reinforcer: A stimulus or outcome that increases the future frequency or probability of the preceding behavior.

    • Punisher: A stimulus or outcome that decreases the future frequency or probability of the preceding behavior.

• Operant Neural Circuit Architecture:

  • Perceptual circuits detect the discriminative stimulus (e.g., the visual sight of an operant lever).

  • Motor circuits execute the voluntary behavior (e.g., forelimb lever pressing).

  • Execution of the behavior produces a reinforcing stimulus (e.g., consumption of food).

  • The reinforcing stimulus activates specialized reinforcement systems within the brain.

  • The reinforcement system delivers neuromodulatory feedback that strengthens the specific synaptic connections between the perceptual circuit and the motor circuit.

• The Mesolimbic Dopaminergic Reinforcement System:

  • Ventral Tegmental Area (VTA): A cluster of dopaminergic cell bodies located in the ventral midbrain whose projecting fibers give rise to the mesolimbic and mesocortical dopamine systems essential for reinforcement and motivated behavior.

  • Nucleus Accumbens (NAC): A basal forebrain structure situated near the septum that receives dense dopaminergic projections from the VTA. It processes reward prediction, incentive salience, and behavioral reinforcement.

  • Microdialysis Quantification of Dopamine Dynamics:

    • Extracellular dopamine concentrations measured in the nucleus accumbens of rats pressing a lever delivering reinforcing electrical stimulation to the VTA.

    • Baseline: Standardized at $100\%$.

    • Initial Stimulation ($10\text{--}25\,\text{minutes}$): Dopamine levels elevate rapidly, peaking at $400\%$ of baseline at $15\,\text{minutes}$.

    • Post-Stimulation Decay: Following cessation of stimulation, dopamine concentrations drop to $250\%$ at $30\,\text{minutes}$, $220\%$ at $45\,\text{minutes}$, and stabilize between $150\%$ and $160\%$ across $60\text{--}105\,\text{minutes}$.

    • Re-stimulation ($115\text{--}125\,\text{minutes}$): Delivery of secondary electrical stimulation triggers an immediate rebound in accumbens dopamine levels to $325\%$ of baseline at $120\,\text{minutes}$.

Basal Ganglia Circuitry and Automaticity

• Dual Pathways Mediating Operant Behaviors:

  • Perceptual processing originating in the sensory association cortex connects to motor output systems in the motor association cortex via two anatomical routes:

    1. Direct Transcortical Connections: Direct corticocortical pathways that operate in concert with hippocampal declarative memory systems during early motor learning when behaviors require deliberate, conscious execution.

    2. Basal Ganglia-Thalamocortical Pathways: Complex subcortical loops that progressively assume control as behaviors are mastered through repetition.

  • Behavioral Transfer to Automaticity:

    • Initial execution of complex motor tasks demands attentive cognitive control.

    • With extensive practice, behavioral control is "transferred" from cortical-hippocampal systems to the basal ganglia, turning actions into automatic motor habits.

• Circuit Architecture of the Basal Ganglia:

  • Neostriatum: Composed of the caudate nucleus and putamen.

  • Cortical Afferents: The caudate nucleus and putamen receive massive excitatory (glutamatergic) projections from the primary motor cortex, supplementary motor area (SMA), premotor cortex, and primary somatosensory cortex.

  • Internal Connections:

    • The neostriatum projects inhibitory (GABAergic) efferents to both segments of the globus pallidus: the globus pallidus external ($GPe$) and globus pallidus internal ($GPi$).

    • $GPe$ sends inhibitory GABAergic fibers to the subthalamic nucleus ($STN$).

    • The subthalamic nucleus projects excitatory glutamatergic afferents back into $GPi$.

    • $GPi$ serves as the major basal ganglia output nucleus, projecting inhibitory GABAergic efferents to the ventral anterior and ventrolateral ($VA/VL$) motor nuclei of the thalamus.

    • The $VA/VL$ thalamus provides excitatory glutamatergic drive back to the supplementary motor area, premotor cortex, and primary motor cortex to facilitate or inhibit motor execution.

    • The substantia nigra provides dopaminergic inputs to the striatum to modulate these motor loops.

Taxonomy and Systems Neuroscience of Memory

• Conceptual Definitions of Memory:

  • Engram / Memory Trace: The physical, structural, or biochemical modification left within neural systems following an experiential episode.

  • Memory Representation: The pattern of neural activation elicited during memory retrieval that reconstructs the neural state present during the original experience.

  • Distributed Network Theory: Memories are not stored within discrete individual units; rather, knowledge resides in altered connection weights across neural networks.

• Declarative (Explicit) Memory:

  • Refers to memory systems supporting conscious, intentional recollection.

  • Subdivisions:

    • Facts (Semantic Memory): Generalized, context-free knowledge, concepts, names, and vocabulary.

    • Events (Episodic Memory): Context-rich autobiographical memories of personal experiences anchored in specific spatial and temporal frameworks.

  • Anatomical Substrates: Dependent on the Medial Temporal Lobe (hippocampus, entorhinal cortex, perirhinal cortex, parahippocampal cortex) and Diencephalon (mammillary bodies, anterior thalamus).

• Nondeclarative (Implicit) Memory:

  • Refers to behavioral manifestations of past learning that operate without conscious awareness.

  • Subdivisions and Associated Neural Substrates:

    • Skills and Habits: Striatum (caudate nucleus and putamen).

    • Priming: Neocortex.

    • Simple Classical Conditioning:

      • Emotional Responses: Amygdala.

      • Skeletal Musculature / Motor Reflexes: Cerebellum.

    • Nonassociative Learning (Habituation and Sensitization): Reflex pathways.

    • Emotional Memory: Features both conscious and unconscious attributes involving attraction, avoidance, and fear.

• Dichotomous Terminology Comparing Memory Systems:

  • Conscious (Explicit) vs Unconscious (Implicit)

  • Declarative vs Nondeclarative

  • Fact vs Skill

  • Memory vs Habit

  • Knowing that vs Knowing how

  • Locale vs Taxon

  • Conscious recollection vs Skills

  • Elaboration vs Integration

  • Memory with record vs Memory without record

  • Autobiographical vs Perceptual

  • Representational vs Dispositional

  • Episodic vs Procedural

  • Semantic vs Nonassociative

  • Working vs Reference

Complementary Learning Systems: Hippocampus and Neocortex

• Dual-System Model (Kwok & McClelland, 2001):

  • Fast-Learning Hippocampal System: Rapidly binds disparate elements of an experience into a single episodic trace in one trial. Binds contextual information to target content.

  • Slow-Learning Neocortical System: Gradually integrates information across repeated exposures to prevent catastrophic interference and extract statistical regularities.

  • Semantic Memory Formation: Built up progressively as identical content is experienced across varied contexts, allowing representations to become context-independent within the neocortex.

  • Lesion Consequences: Bilateral damage to the hippocampus disrupts the fast-binding mechanism, preventing the formation of new episodic memories, while well-consolidated cortical semantic memories remain intact.

• Neuromodulatory Control and Plasticity:

  • Hippocampal processing is modulated by ascending projections of dopamine, noradrenaline (norepinephrine), serotonin, and acetylcholine.

  • Drives both initial synaptic consolidation (converting short-term traces to permanent long-term stores) and reconsolidation (restabilizing previously consolidated traces upon reactivation).

• Adult Hippocampal Neurogenesis:

  • Occurs in the subgranular zone of the dentate gyrus within the adult mammalian brain.

  • Neural stem cells undergo successive mitotic divisions into progenitor cells.

  • Progenitor cells differentiate into developing neurons that extend neurites and migrate into the granule cell layer.

  • These mature into functional granule neurons that integrate into existing dentate gyrus neural circuits.

• Spatial Learning and the Morris Water Maze:

  • Used to assess relational spatial memory versus stimulus-response learning in rodents.

  • Apparatus: A circular tank filled with opaque water containing a submerged, hidden escape platform.

  • Relational Task (Variable Start Positions):

    • Rats are released from varying locations across trials, requiring spatial navigation via distal extramaze cues to find the hidden platform.

    • Control rats rapidly acquire a spatial cognitive map: escape latency declines from $\\sim 55\text{--}60\,\text{s}$ on trial 1 down to $\\sim 10\text{--}15\,\text{s}$ by trials $13\text{--}18$.

    • Rats with bilateral hippocampal lesions fail this task: mean latency remains high ($\\sim 50\text{--}60\,\text{s}$), displaying disorganized search trajectories without a directed path to the platform.

  • Stimulus-Response Task (Constant Start Position):

    • Rats are consistently placed into the pool from the exact same starting position, requiring only an egocentric motor habit (e.g., swim straight ahead) rather than a relational map.

    • Both control and hippocampal-lesioned rats master this task at comparable rates, decreasing escape latencies from $\\sim 90\text{--}100\,\text{s}$ on trial 1 to $\\sim 5\text{--}10\,\text{s}$ by trial 12.

    • This dissociation confirms that hippocampal lesions spare non-relational stimulus-response habit learning.

Long-Term Potentiation (LTP) and Hebbian Synaptic Plasticity

• Core Concepts and Synaptic Rules:

  • Long-Term Potentiation (LTP): An enduring increase in the excitability and synaptic efficacy of a neuron resulting from repetitive, high-frequency stimulation of its afferent inputs.

  • Expressed electrophysiologically by enhanced amplitude of excitatory postsynaptic potentials (EPSPs).

  • Temporal Summation Requirement: Low-frequency stimulation produces isolated EPSPs that fail to depolarize the postsynaptic membrane to the critical threshold. High-frequency stimulation (tetanus) produces rapid summation of successive EPSPs, surpassing the threshold required to trigger LTP.

• Hebb's Postulate (Donald Hebb, 1949):

  • Rule: "Cells that fire together, wire together."

  • Theoretical Premise: When an axon of cell A is close enough to excite cell B and repeatedly or persistently takes part in firing it, metabolic or structural growth processes occur in one or both cells such that A's efficiency in firing B is enhanced.

  • Underlies associative synaptic plasticity: when presynaptic neurotransmitter release occurs concurrently with postsynaptic membrane depolarization, the intervening synapse strengthens.

• Hippocampal Trisynaptic Circuitry and LTP Induction:

  • Anatomical Circuit Sequence:

    1. Perforant Path: Axons originating from pyramidal neurons in the entorhinal cortex project through the subicular complex to form synapses onto granule cells of the dentate gyrus.

    2. Mossy Fibers: Axons of dentate granule cells project to pyramidal neurons in hippocampal field CA3.

    3. Schaffer Collaterals: Axons of CA3 pyramidal cells branch into Schaffer collaterals to innervate pyramidal neurons in hippocampal field CA1 (as well as sending Schaffer commissural axons contralaterally).

  • Electrophysiological Measurement:

    • Stimulating electrodes are placed into the perforant path; recording electrodes are positioned within the dentate gyrus.

    • Population EPSP: An extracellular field potential measuring the summed excitatory postsynaptic potentials generated across dentate granule cells by perforant path activation.

    • Following high-frequency electrical stimulation of the perforant path, the population EPSP amplitude increases immediately and remains elevated over baseline.

    • Long-Term Duration: Once established, potentiation of the population EPSP persists across days, recorded stably at $1\,\text{hour}$, $24\,\text{hours}$, $48\,\text{hours}$, $72\,\text{hours}$, and $96\,\text{hours}$.

• Associative LTP and Dendritic Spikes:

  • Associative LTP occurs when weak and strong synapses converging onto the same postsynaptic dendrite are activated concurrently.

  • Activation of a strong synapse produces robust local EPSPs that trigger an action potential in the soma and axon of the pyramidal cell.

  • Dendritic Spike: An active, backpropagating action potential that travels backward through the dendritic arbor.

  • This backpropagating spike depolarizes the postsynaptic membrane of distant dendritic spines, unblocking their voltage-dependent NMDA receptors.

  • If a weak synapse on a primed dendritic spine releases glutamate at that same moment, calcium enters and the weak synapse undergoes LTP, strengthening its synaptic efficacy.

Neuropharmacology and Molecular Dynamics: NMDA and AMPA Receptors

• The NMDA Receptor as a Coincidence Detector:

  • Abundant on postsynaptic dendritic spines throughout hippocampal field CA1 and the lateral amygdala.

  • Controls a calcium ($Ca^{2+}$) ion channel that produces postsynaptic depolarization when activated.

  • Magnesium ($Mg^{2+}$) Blockade: At resting membrane potentials ($\sim -70\,\text{mV}$), an extracellular $Mg^{2+}$ ion is electrostatically held inside the channel pore, blocking the influx of $Ca^{2+}$.

  • Glutamate binding alone is insufficient to open the channel while the $Mg^{2+}$ ion is present.

  • Voltage-Dependent Unblocking: Strong depolarization of the postsynaptic membrane (driven by adjacent AMPA receptor activation) repels and ejects the positively charged $Mg^{2+}$ ion from the pore.

  • Dual Activation Requirement:

    1. Glutamate must bind to the extracellular receptor site.

    2. The postsynaptic membrane must be concurrently depolarized to expel $Mg^{2+}$.

  • Once both conditions are fulfilled, the channel opens, allowing an influx of $Ca^{2+}$ ions into the dendritic spine cytoplasm.

• AMPA Receptor Mobilization and Trafficking:

  • AMPA receptors are ligand-gated ion channels permeable to sodium ($Na^{+}$) that mediate baseline fast excitatory neurotransmission.

  • $Ca^{2+}$ influx through NMDA channels initiates intracellular enzymatic cascades, including the activation of calcium/calmodulin-dependent protein kinase II (CaMKII).

  • Enzymatic activation drives a two-step insertion of AMPA receptors into the postsynaptic density (PSD):

    1. AMPA receptors present in adjacent nonsynaptic dendritic membrane move laterally into the dendritic spine membrane.

    2. Intracellular reserves of AMPA receptors housed within endocytic vesicles inside the cell interior are mobilized to replenish the adjacent membrane.

  • The increased number of functional AMPA receptors in the postsynaptic membrane produces larger $Na^{+}$ currents upon subsequent glutamate release, resulting in larger EPSPs.

Presynaptic, Retrograde, and Morphological Remodeling in LTP

• Retrograde Signaling via Nitric Oxide ($NO$):

  • Postsynaptic $Ca^{2+}$ entry activates nitric oxide synthase (NOS).

  • NOS produces Nitric Oxide ($NO$), a soluble gas that functions as a retrograde messenger.

  • $NO$ diffuses backward across the synaptic cleft into the presynaptic terminal button.

  • Inside the presynaptic terminal, $NO$ stimulates guanylyl cyclase, increasing cyclic guanosine monophosphate (cGMP) and promoting sustained glutamate release during subsequent action potentials.

  • Free-floating $NO$ gas also diffuses to neighboring synapses, lowering the threshold for local cooperative LTP induction.

• Structural Remodeling of Dendritic Spines:

  • Dendritic spines exhibit distinct morphological classes along a structural continuum:

    • Thin Spines: Characterized by a long, slender neck and a small head; plastic and flexible ("learning spines").

    • Filopodia: Elongated, slender dendritic protrusions lacking a clear bulbous head; highly motile precursors.

    • Stubby Spines: Short protrusions with wide bases and no distinct neck.

    • Mushroom Spines: Large bulbous head ($H_D$) atop a constricted neck ($N_D$); house large postsynaptic densities with high AMPA receptor densities ("memory spines").

  • Following LTP induction, existing thin spines and filopodia enlarge and transform into wide mushroom-shaped spines.

  • Physical growth, spine enlargement, and the formation of new synaptic contacts continue for up to $19\,\text{hours}$ post-induction, supported by local protein synthesis.

Amnesia, Clinical Neuropsychology, and Patient H.M.

• Clinical Classifications of Amnesia:

  • Anterograde Amnesia: Inability to learn new information or form new memories after the onset of brain damage. Represents a selective impairment in complex relational memory and episodic memory consolidation.

  • Retrograde Amnesia: Inability to retrieve or recall memories for events that occurred prior to the onset of brain damage.

  • Korsakoff's Syndrome: Severe anterograde amnesia accompanied by confabulation; caused by thiamine (vitamin $B_1$) deficiency linked to chronic alcoholism, damaging the mammillary bodies and mediodorsal thalamic nuclei.

• The Neuropsychological Profile of Patient H.M. (Henry Molaison):

  • In 1953, patient H.M. underwent bilateral medial temporal lobe resection to treat severe, pharmacologically intractable epilepsy.

  • Surgical Lesion Extent: Bilateral removal of the anterior two-thirds of the hippocampus, parahippocampal cortex, entorhinal cortex, and amygdala, as observed in coronal brain sections across three planes:

    • Section A: Resection of the amygdala and entorhinal cortex.

    • Section B: Resection of the anterior hippocampus, collateral sulcus, and entorhinal cortex.

    • Section C: Resection of the remaining mid-hippocampal formation.

  • Preserved Cognitive Functions: Normal intellectual capacity, preserved perceptual processing, unimpaired short-term working memory (could hold normal conversations and repeat digit spans via rehearsal), and intact remote episodic/semantic memories from early childhood.

  • Profound Impairments: Severe, total anterograde amnesia for declarative information; inability to transfer short-term memories into permanent long-term memory stores.

  • Key Insights from Patient H.M.:

    • The hippocampus is NOT the permanent storage site for remote long-term memories.

    • The hippocampus is NOT required for the retrieval of well-established remote long-term memories.

    • The hippocampus is NOT the anatomical substrate of short-term memory.

    • The hippocampus IS essential for memory consolidation—converting labile short-term neural activity into permanent structural and biochemical changes (via LTP-mediated synaptic remodeling).

• Temporal Gradients in Retrograde Amnesia:

  • Ribot's Law: Retrograde memory loss is temporally graded: older memories are more resistant to disruption, whereas recent memories formed just before brain damage are vulnerable.

  • Empirical Findings in Hippocampal Lesion Patients (Reber & Squire, 1998):

    • Recall accuracy tested across decades prior to amnesia onset:

    • Remote Memories ($25\text{--}30\,\text{years}$ before damage): Hippocampal amnesia patients score comparably to healthy controls ($\sim 45\text{--}50\%$ correct).

    • Recent Memories ($0\text{--}5\,\text{years}$ before damage): Severe impairment in hippocampal lesion patients ($\sim 30\text{--}35\%$ correct) compared to healthy control participants ($\sim 50\text{--}65\%$ correct).

    • This temporal gradient confirms systems consolidation: over time, memories undergo slow reorganization in neocortical networks, eventually becoming independent of the hippocampal formation.