Implicit vs. Explicit Memory in Animals and Humans Lecture
Key Memory Systems
- Explicit / Declarative Memory
- Mediated by the Medial Temporal Lobe (MTL) Memory System.
- Core structures: hippocampus, rhinal cortex (entorhinal, perirhinal, parahippocampal cortex).
- Supports one-trial, flexible, conscious learning of facts & events.
- Implicit / Non-declarative Memory
- Multiple, distributed neural circuits; no single hub.
- Includes skills, habits, priming, classical conditioning, emotional learning, unconscious decision biases, etc.
- Early work (e.g., Kandel’s sea-slug studies, 1968) showed classical conditioning can be implemented by different micro-circuits depending on the learning context.
Lesion Methodology (“an lesion”)
- In non-human studies, “lesion” = surgical ablation or destruction of a brain area, typically the hippocampus.
- Purpose: determine whether a task requires that structure.
- Animal models used here: rats and monkeys (Macaca).
Experimental Tasks Used to Probe Memory Systems
Pattern Discrimination (PD)
- Setup: Subject chooses between two simple visual patterns (e.g., an X vs L); one is always rewarded.
- Human performance: one-trial learning via explicit memory — hippocampal & rhinal dependent.
- Monkey performance:
- Requires hundreds of trials; gradual stimulus–response habit learning.
- Survives hippocampal lesions ⇒ relies on implicit systems (basal ganglia, striatum) rather than MTL.
- Significance: Demonstrates that the same behavioral task can tap different neural systems across species.
Delayed Non-Matching (or Matching) to Sample (DNMS / DMS)
- Trial structure:
- Single sample object presented; subject moves it, receives reward.
- Delay inserted (opaque screen up).
- Two objects appear: the original sample plus a novel object.
- DNMS rule: subject must choose the non-matching (novel) object to get reward.
- Critical variables: delay length (seconds→minutes), novelty, object–location shuffling.
- Findings:
- Both humans & monkeys need intact hippocampus → MTL lesion impairs performance.
- Hence DNMS demands declarative / relational memory, not pure habit learning.
- Illustrates evolutionary continuity of hippocampal function.
Concurrent Discrimination (CD)
- Design:
- 8 independent object pairs shown in pseudo-random order within a session.
- Within each pair, one object is always correct (rewarded).
- 40 trials per day, repeated across days.
- Performance profiles:
- Humans: master mapping rapidly; can flexibly list “all 8 correct objects” ⇒ explicit knowledge.
- Monkeys: learn slowly, by habit; cannot flexibly rearrange or verbally report knowledge.
- MTL lesion effect: No deficit in monkeys (habit based); large deficit in humans (explicit based).
- Key paradox: Same hippocampal structure supports task in humans but not in monkeys, prompting evolutionary analysis.
Comparative Conclusions (Squire et al., 2006 review)
- Examined data across rats, monkeys, humans.
- Core claim: “Evolutionary continuity” — hippocampus performs comparable relational/declarative computations in all mammals but is recruited by a broader task set in humans.
- Some tasks (e.g., DNMS) recruit hippocampus in both rats and primates; others (e.g., CD) only in humans because primates solve them via non-declarative routines.
Implicit Memory: Neural Landscape (Schacter & colleagues, 2014)
- Virtually any cognitive process can run unconsciously.
- Examples & probable circuits (subset):
- Unconscious error detection → anterior cingulate & insula.
- Priming of key-press motor responses → motor cortex, cerebellum.
- Subliminal threat processing → amygdala & superior colliculus.
- Quote: “Virtually any brain process can operate without awareness.”
Amygdala – Dual-Route Emotional Processing
- Quick-and-Dirty Route (Low Road)
- Path: sensory thalamus → amygdala.
- Fast, coarse, unconscious; triggers reflexive defensive actions (e.g., braking before awareness).
- Slow-but-Accurate Route (High Road)
- Path: sensory thalamus → sensory cortex → amygdala.
- Adds cortical analysis, still precedes full conscious awareness but carries richer information.
- Despite adjacency to hippocampus, amygdala is not part of the MTL memory system; it modulates both implicit (fear conditioning) and explicit (emotional tagging) learning.
Practical & Philosophical Implications
- Clinical: Understanding species-specific reliance on hippocampus guides animal models of amnesia.
- Ethical: Lesion studies highlight balance between scientific gain and animal welfare.
- Conceptual: Reminds us that identical behaviors need not reflect identical cognitive mechanisms (multiple realizability).
Key Quantitative Facts & Parameters
- Lesion = hippocampal ablation in rats/monkeys.
- Pattern Discrimination: “hundreds” of acquisition trials in monkeys.
- Concurrent Discrimination: 8 object pairs, 16 total objects, 40 trials/day.
- Historical milestones: sea-slug work 1968; comparative hippocampal review 2006; implicit-process compendium 2014.
Summary
- Explicit memory: single integrated MTL circuit; conserved across mammals.
- Implicit memory: heterogeneous, circuit-specific, often subcortical; encompasses everything from motor habits to unconscious number processing.
- Behavioral tasks (PD, DNMS, CD) reveal which species & which brain systems are engaged, clarifying when hippocampus is essential and when habits suffice.
- Amygdala exemplifies implicit processing via dual thalamo-cortico routes, mediating both reflexive and nuanced emotional responses.
- Overall evidence supports evolutionary continuity: same hippocampal computations, diversified deployment in humans.