17.8 Forgetting, Interference, Memory Distortion & Long-Term Potentiation

17.8 Forgetting, Interference, Memory Distortion & Long-Term Potentiation

Forgetting, Interference & Memory Distortion

The Ebbinghaus Forgetting Curve

Hermann Ebbinghaus conducted self-experiments with nonsense syllables (e.g., DAX, BOK), demonstrating that memory decay follows an exponential decay function: the rate of forgetting is steepest immediately following learning (losing ~50% within the first hour), after which forgetting levels off into a stable, durable asymptote.

Interference Theory

Forgetting is driven primarily not by passive temporal decay, but by competition between overlapping memories:

• Proactive Interference: Previously learned, older information interferes with the encoding or retrieval of newer information. Example: Accidentally writing your old address or typing your previous password after moving or updating credentials.
• Retroactive Interference: Newly learned, recent information interferes with the retrieval of older, previously stored information. Example: Memorizing your new phone number causes you to forget your childhood landline number.

Proactive:   [OLD Memories] ──(Disrupts Retrieval)──> [NEW Memories]
Retroactive: [OLD Memories] <──(Disrupts Retrieval)── [NEW Memories]

Amnesia and Neuropathology

• Anterograde Amnesia: Inability to consolidate new explicit, declarative long-term memories following neurological trauma (e.g., bilateral surgical resection of the medial temporal lobes in Patient H.M.). Crucial Insight: Patients with pure anterograde amnesia retain intact working memory and can acquire new implicit, procedural motor skills (e.g., mirror-tracing tasks) at normal rates, despite possessing zero conscious memory of ever performing the task.
• Retrograde Amnesia: Inability to retrieve explicit memories that were formed prior to the neurological insult. Often temporally graded, following Ribot's Law: older memories are more resistant to disruption than recent memories because older memories have undergone decades of neocortical consolidation.
• Korsakoff's Syndrome: Severe memory deficit caused by thiamine (vitamin $B_1$) deficiency, classically associated with chronic alcoholism and malnutrition. Pathologically damages the mammillary bodies of the hypothalamus and the mediodorsal thalamic nuclei. Characterized by severe anterograde and retrograde amnesia combined with confabulation—the spontaneous fabrication of vivid, false autobiographical narratives without conscious intent to deceive.
• Alzheimer's Disease: Progressive neurodegenerative dementia characterized by extracellular amyloid-$\beta$ plaques and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. Begins in the entorhinal cortex and hippocampus before sweeping through the neocortex. Marked by profound loss of cholinergic neurons originating from the nucleus basalis of Meynert.

Reconstructive Memory & Distortions

Memory is not a video recording; it is a reconstructive process subject to systematic error:

• Misinformation Effect: Elizabeth Loftus demonstrated that providing post-event misleading information alters eyewitness recollections. Subjects who watched a car crash and were asked "How fast were the cars going when they smashed into each other?" estimated significantly higher speeds and falsely remembered seeing broken glass compared to subjects asked with the word hit.
• Source Monitoring Errors: Confusion regarding the true origin or source of a memory (e.g., recalling that your physician gave you medical advice when you actually read it on an unverified social media forum).
• Cryptomnesia: An unconscious source monitoring failure where an individual produces an idea or melody, genuinely believing it is their own original creative insight, when they are actually recalling a forgotten prior encounter.
• Flashbulb Memories: Exceptionally vivid, detailed, emotionally charged memories associated with learning of a shocking or tragic public event (e.g., 9/11). Despite the intense subjective confidence held by individuals, empirical studies prove that flashbulb memories decay and accumulate distortions at rates comparable to mundane episodic memories.

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The Neurobiology of Memory: Long-Term Potentiation (LTP)

Donald Hebb famously postulated in 1949: "Neurons that fire together, wire together." The physical, cellular substrate of this synaptic strengthening is Long-Term Potentiation (LTP), first characterized by Bliss and Lømo in the hippocampal perforant path and Schaffer collateral CA3-to-CA1 synapses.

The Molecular Mechanics of LTP Induction

LTP at excitatory glutamatergic synapses requires the cooperative action of two ionotropic glutamate receptor subtypes on the postsynaptic dendritic spine:

1. AMPA Receptors: Ligand-gated cation channels permeable to $Na^+$ and $K^+$.
2. NMDA Receptors: Ligand-gated and voltage-dependent cation channels permeable to $Na^+$, $K^+$, and critically, calcium ($Ca^{2+}$).

Step-by-Step Induction Cascade

1. Basal Neurotransmission: Under low-frequency stimulation, presynaptic action potentials release glutamate into the synaptic cleft. Glutamate binds to both AMPA and NMDA receptors. Sodium flows in through AMPA receptors, producing a modest excitatory postsynaptic potential (EPSP). However, the pore of the NMDA receptor is physically plugged by an extracellular magnesium ion ($Mg^{2+}$) held in place by the negative resting membrane potential (-70 mV). Calcium cannot enter.
2. High-Frequency Stimulation (Tetanus): High-frequency presynaptic firing drives massive, sustained glutamate release. Intense, repetitive activation of AMPA receptors allows an overwhelming influx of $Na^+$, driving strong, prolonged depolarization of the postsynaptic dendritic spine (to approximately -30 mV).
3. Expulsion of the $Mg^{2+}$ Block: The positive electrical charge inside the depolarized dendritic spine electrostatically repels the positively charged $Mg^{2+}$ ion, expelling it outward from the channel pore of the NMDA receptor.
4. Calcium Influx: With the $Mg^{2+}$ block removed and glutamate bound, the NMDA channel opens, allowing a massive influx of $Ca^{2+}$ into the postsynaptic spine.
5. Enzyme Activation: The rise in intracellular $Ca^{2+}$ acts as a critical second messenger, binding to calmodulin to activate Calcium/Calmodulin-dependent Protein Kinase II (CaMKII) and Protein Kinase C (PKC).
6. Early LTP Expression: Activated CaMKII carries out two distinct actions:
   • Phosphorylates existing AMPA receptors, increasing their single-channel open probability and single-channel ionic conductance.
   • Drives exocytosis of intracellular vesicles: Promotes trafficking and insertion of dormant AMPA receptors from endosomal pools directly into the postsynaptic density membrane.
   • Net Outcome: The postsynaptic membrane now displays significantly greater AMPA receptor density. Subsequent baseline glutamate release yields a dramatically larger EPSP.
7. Late LTP & Structural Remodeling: Sustained $Ca^{2+}$ signaling activates adenylyl cyclase $\rightarrow$ cyclic AMP (cAMP) $\rightarrow$ Protein Kinase A (PKA) $\rightarrow$ phosphorylation of the nuclear transcription factor CREB (cAMP Response Element-Binding Protein). CREB stimulates transcription of immediate early genes, directing the synthesis of structural proteins that enlarge the dendritic spine, split synapses, and stimulate the release of retrograde messengers (e.g., nitric oxide, NO) that enhance presynaptic glutamate exocytosis.