Memory Part 1 – Brain Mechanisms, Models & Medial Temporal Lobe

The medial temporal lobe plays a critical role in the formation and retrieval of memories by integrating sensory information and linking it to prior experiences. This integration is essential for creating complex representations of experiences, which are necessary for the development of declarative memory. Furthermore, this area of the brain is particularly associated with processes such as consolidation, where short-term memories are transformed into long-term ones, allowing for effective recall in the future.

Necessary vs. Sufficient (Brain–Function Mapping)

  • Some areas necessary for a function (damage → loss) but not sufficient in isolation

    • Example: Fusiform gyrus critical for face recognition but needs distributed support

  • Many areas are multi-functional

Function vs. Structure

  • Function = behavioural / mental output

  • Structure = physical brain locus producing or contributing to that output

  • Non-isomorphism:

    • One structure can underlie multiple functions

    • One function may require multiple structures or be achieved by alternative networks

Human Memory Taxonomy (Purves Fig.​30.130.1)

  • Declarative (explicit; "available to consciousness")

    • Daily episodes (episodic)

    • Words & meanings (semantic)

    • Historical facts, etc.

  • Non-Declarative (implicit; "generally not available to consciousness")

    • Motor skills (e.g., riding a bike)

    • Cognitive skills & puzzles

    • Classical conditioning

    • Priming effects

Declarative Memory – Research Approaches

  • Psychology: behavioural tasks, recall/recognition tests, remember/know judgments

  • Cognitive Neuroscience: imaging (fMRI, PET), ERPs, lesion studies, subsequent-memory paradigms

Subsequent Memory Effects (SMEs)

  • Method: record encoding-phase brain activity

    • Later separate items into remembered vs. forgotten

    • Encoding differences that predict later recall = "SME"

  • Example study:

    • Bloom et al.​ (20182018) on Tip-of-the-Tongue (TOT) states

    • 3030 participants answered general-knowledge questions, labelled TOT/non-TOT on failures

    • ERP: higher late positivity during feedback for TOT items

    • Surprise retest: TOT items showed enhanced later memory → TOT as a metacognitive cue linked to SME

Declarative Memory Sub-types

  • Episodic: memory for personally experienced events (time, place context) – today’s main focus

  • Semantic: facts & world knowledge, context-free

  • Historical note: Endel Tulving pioneered episodic–semantic distinction & neuropsychological validation

Simple Cortico-Hippocampal Model (Encoding & Storage)

  1. Encoding (low-level)

    • Modality-specific cortical areas process features (e.g., "green", "balloon"); Hebbian LTP strengthens intra-cortical links

  2. Integration

    • Hippocampus stores a summary / index pointing to distributed cortical traces

    • Library metaphor: hippocampus = central catalogue; cortex = books on shelves

  3. Damage dissociations

    • Cortical lesion → loss/degradation of specific features

    • Hippocampal lesion → inability to bind & retrieve; isolated traces intact but unindexed

Consolidation – Why Some Memories Last

  • Definition: progressive stabilisation of an encoded memory

  • Synaptic consolidation (Syn-C)

    • Cellular/molecular

    • Time-scale: minutes → hours

    • Requires protein synthesis, synaptic plasticity, LTP

  • Systems consolidation (Sys-C)

    • Network-level reorganisation

    • Time-scale: days → years

    • Gradual transfer of dependence from hippocampus to distributed neocortex

  • Ribot’s Law (temporal gradient)

    • After brain injury, recent memories lost > remote memories

    • Explained by Sys-C: remote memories already hippocampus-independent

  • Standard Consolidation Theory (SCT) summary

    • Encode → index in HC

    • Syn-C begins

    • Repeated reactivation (sleep, rehearsal) drives Sys-C

    • Remote memory ultimately cortically embedded; no longer HC-dependent

Retrieval Mechanism

  • Cue (internal thought or external stimulus) initiates search

  • For recent memories:

    • Cue → HC index → cortical pattern completion ("C-Syn" stage)

  • For remote memories:

    • Cue → direct cortical activation ("C-Sys") – HC bypassed

Medial Temporal Lobe (MTL) Anatomy

  • Macro-organisation

    • Hippocampus (dentate gyrus, CA3 → CA1, subiculum)

    • Rhinal cortex: entorhinal (ERC, medial) & perirhinal (PRC, lateral)

    • Parahippocampal cortex (PHC)

  • Information flow

    • Cortex → PRC & PHC → ERC → Hippocampus

    • Hippocampus → fornix → anterior thalamus & widespread cortex

  • Canonical trisynaptic loop

    • ERC → DG ("perforant path") → CA3 ("mossy fibres") → CA1 ("Schaffer collaterals") → subiculum → ERC/cortex

Functional Theories of MTL

  • Two-Process Theory (2PT)

    • Hippocampus: slow, relational / associative processing (recollection)

    • Perirhinal cortex: rapid, item-level processing (familiarity)

  • Three-Process Theory (3PT)

    • PRC: object memory

    • PHC: spatial / contextual memory

    • Hippocampus: binding objects + context into coherent episodes

  • Relationship to behavioural dichotomies: recollection vs. familiarity; item vs. context; object vs. spatial