Mind, Brain, and Memory – Lecture on the Medial Temporal Lobe Memory System

Review of Classic Amnesia Cases

• Henry Molaison (H.M.) – 1953 surgery
  • Bilateral removal of the hippocampus (plus surrounding medial-temporal tissue) to treat epilepsy.
  • Result: anterograde amnesia for explicit (declarative) memories; implicit (non-declarative) memory spared.
  • Could learn new motor or conditioned skills (e.g., playing an accordion, making a latte) but could not consciously recall learning them.
• Clive Wearing – viral encephalitis
  • Progressive destruction of the hippocampus and adjacent structures.
  • Preserved: language, previously consolidated semantic knowledge, emotional recognition of wife.
  • Lost: continuous formation of new episodic memories; lives in a perpetual "now".

Taxonomy of Memory (Unit 2 Refresher)

• Long-Term Memory (LTM)
  • Explicit / Declarative ➜ conscious, hippocampally mediated.
  • Implicit / Non-Declarative ➜ outside awareness, not hippocampally mediated (procedures, habits, priming, classical & operant conditioning).
• Working / Short-Term Memory
  • Capacity ≈ 7\pm2 chunks; duration ≈ 30\text{ s} without rehearsal.
  • Supported by prefrontal & parietal circuits, not by hippocampus.

Key Diagnostic Q&A From Lecture

• "Could H.M. encode any new LTM?" ➜ Yes (implicit systems intact).
• "Is repeating a name over and over declarative memory?" ➜ No (it is working-memory rehearsal).

Neurodevelopmental Background

• Neurogenesis (new neurons):
  • Peaks \text{last trimester before birth}; slows post-natally but persists in dentate gyrus of hippocampus, especially with physical fitness & learning.
• Synaptogenesis (new synapses):
  • Peaks at \text{~2 yrs}; followed by dendritic pruning until \text{~20 yrs}.

Medial Temporal Lobe Memory System (MTLMS)

• Two major components:
  • Hippocampus (plus dentate gyrus, CA1–CA4 sub-fields)
  • Rhinal Cortex (triad of peri-, para-hippocampal, and entorhinal cortices)

Convergent vs. Super-Convergent Zones

• Convergent Zone: many cortical "highways" (visual, auditory, somatosensory) project into a focal hub.
• Super-Convergent Zone: where multiple convergent zones themselves converge.
  • In MTLMS ➜ hippocampus is the super-convergent zone; each rhinal sub-area is an individual convergent zone.

Rhinal Cortex Sub-Areas & Functions

1. Parahippocampal Cortex (PHC)
   • Encodes & recognizes environmental scenes (rooms, landscapes, campus layouts).
   • Evolutionary role: rapid identification of safe vs. dangerous locales.
2. Perirhinal Cortex (PRC)
   • Handles visual recognition memory for objects & faces in general ("That is a chair", "That is a raccoon").
   • Distinct from FFA, which specifies individual faces; PRC answers "Is this a face at all?".
3. Entorhinal Cortex (ERC)
   • Main cortical gateway to hippocampus; contains grid & place cells.
   • Critical for spatial maps, navigation, and memory consolidation (shifts fragile traces toward stability during sleep).

The Hippocampus as an "Indexer"

• Acts like a library card-catalog: stores pointers to distributed cortical features active during an event.
• Retrieval ➜ hippocampus re-activates original cortical patterns ("mental time travel").
• Damage removes the pointer system for new memories but old memories survive if fully linked cortico-cortically.

Nomadic (Migrating) Memory Concept

• Over time the hippocampal node in a given memory circuit becomes unnecessary.
  • Newly encoded memory ➜ hippocampus essential.
  • Months–years later ➜ cortical regions interconnect directly; memory becomes hippocampus-independent.
• Explains why amnesics retain remote childhood memories yet cannot lay down new episodic traces.
• Reinforces Ribot’s Law: earliest-formed memories are most resistant to disruption.

Example Walk-Through: Reactivating Yesterday’s Event

• Cue prompts hippocampus to poll ERC → PHC → PRC → sensory cortices.
• Successive retrievals modify the pattern (memory is reconstructive, not a verbatim replay).
• Most daily experiences are never reactivated and therefore fade ("What did you eat exactly 7 days ago?").

Clinical & Ethical Notes

• 1950s surgical lesions (H.M.) illustrate progress—and risks—in psychosurgery.
• Awareness of implicit learning capacity shapes rehab programs (teach procedures without stressing explicit recall).

Connections to Earlier Course Topics

• Face Fusiform Area (FFA) & prosopagnosia (face-blindness) highlight specialized visual convergent zones.
• Unit 2’s discussion of conditioning & habit learning aligns with spared abilities in MTL-lesion patients.
• Neuroplastic mechanisms (synaptogenesis, dendritic pruning) set the stage for dynamic memory circuitry.

Practical Implications & Study Tips

• Memory consolidation demands sleep; schedule review sessions within 24 h and again after 48–72 h.
• Use spatial/contextual cues (PHC & ERC friendly) when studying—e.g., consistent study locations or mental mind-palaces.
• For rote facts, engage PRC by pairing visuals with terminology.
• Recognize that retrieval changes memory; self-test but also protect key details from distortion by checking sources.

Quick Glossary

• Anterograde Amnesia: inability to form new explicit memories.
• Implicit Memory: skills & conditioning outside conscious awareness.
• Explicit Memory: consciously accessible facts & events.
• Consolidation: gradual stabilization of memory traces (largely during sleep).
• Grid/Place Cells: ERC & hippocampal neurons that fire for specific spatial coordinates.
• Ribot’s Law: gradient of retrograde amnesia sparing oldest memories.