Neurobiology of Memory

Neurobiology of Memory

Infantile Amnesia

  • 0-2 years: Very little is explicitly recounted.
    • Specific types of memory take longer to develop.
    • Other forms of memory occur; infants can learn and retain information, but explicit/autobiographical memories are less likely to be remembered.
  • 2-5 years: Some memories can be recounted.
    • Emotional memories are amplified via the amygdala.
    • Memory systems are developing but still need assistance.
    • First memories often have an emotional component, boosted through valence.
  • Changes result from:
    • Maturation of learning structures.
    • Establishment of neural connections.
  • Retention and consolidation depend on the rehearsal of information.
  • Circuits aren’t mature enough to rehearse memory, leading to a lack of consolidation or retention.

Anterograde Amnesia

  • Inability to form new declarative memories (memories you can verbalize).
  • Damage to the rehearsal circuit:
    • Hippocampus: Profound anterograde amnesia; crucial for new memories.
    • Basal Forebrain
    • Amygdala
    • Thalamus
  • Consolidation deficit, but old memories can be retrieved.
  • Adults can form new memories, but damage to the rehearsal circuit impairs the ability to form new memories.
  • Example: Dory from Nemo; Retrieval of old memories is possible, but there's difficulty creating new memories.
  • Case of Clive Wearing:
    • Experiences a continuous present moment with no past or future.
    • Memory lasts for only 2-3 minutes.
    • Situational and working memory fades.
    • Unable to perceive the passage of time.
    • Emotional response to his wife suggests emotional memories may originate from a different area.

Retrograde Amnesia

  • Inability to recall information before the onset of the amnesic event.
  • Often results from brain injury.
  • Can span minutes to years and be temporary or permanent.
  • Memories nearest the event are most affected/lost.
  • Often accompanied by anterograde amnesia.
  • In pure retrograde amnesia, long-term information cannot be retrieved.
  • Retrieval cues may help unless memories are completely gone.
  • Example: Brain injury during a boxing match, where the person forgets the past 3 rounds.
  • Association Cortex (permanent) is affected, and memories from this area cannot be retrieved.

Diseases of Memory – Alzheimer’s Disease

  • Dr. Alzheimer described the disease.
  • Impaired memory (initial symptom).
  • Disorientation.
  • Emotional flux.
  • Worsening of symptoms.
  • Neuropathologic characteristics:
    • Atrophy in the brain
    • Cerebral arterial sclerosis (hardening of brain tissue).
    • Histological abnormalities: Amyloid Plaques & Neurofibrillary Tangles (Intra and extracellular features).
  • Alzheimer’s disease is now the most common neurodegenerative disorder (~4.5 million in the US).
  • 26% of people >85 years old meet the criterion for AD.
  • Age is the number one cause/risk factor.
  • Alzheimer’s selectively degenerates brain regions:
    • Early AD:
      • Hippocampus
      • Basal Forebrain (Acetylcholine)
    • Late AD:
      • Amygdala (Emotional flux)
      • Cortical Areas (loss of long-term memories)

Diseases of Memory – Alzheimer’s Disease - Possible Causes

  • Aging increases mutations.
  • Genes, but only 4-8% is genetic.
  • Brain damage.
  • Inflammation.
  • Current Treatments:
    • Cholinesterase inhibitors – increase Ach.
    • NMDA antagonists – reduce glutamate (Glutamate is important for memory but prevents).
    • Cognitive and physical exercise.
    • Immunotherapies against amyloid plaques.
  • "A Man Who Mistook His Wife for a Hat" – Oliver Sacks

Diseases of Memory – Korsakoff’s Syndrome

  • Sergei Korsakoff (1889)
    • Patients converse normally.
    • Retain old memories.
    • Severe anterograde amnesia.
    • Display wild confabulation (tales).
  • The Korsakoff’s brain:
    • Degeneration of the Mammillary bodies.
    • Lesions of the thalamic nuclei (Thalamus -rehearsal circuit – consolidation issue).
  • Living in a segmented world where every 2-3 minutes their memory resets.
  • Alzheimer’s patients have retrograde amnesia, but Korsakoff’s patients don’t, which is why they come up with situations to try to make sense of their new circumstances.
  • Korsakoff’s syndrome results from thiamine (vitamin B1) deficiency.
  • KS is often seen in alcoholics:
    • Alcohol replaces calories, leading to less B1.
    • High proof alcohol reduces B1 absorption.
  • Prevention in alcoholics:
    • Australia is adding thiamine to their alcohol.
    • Adding thiamine to bread.
  • One other way to get KS – in the ER
    • Severely malnourished adults without KS are given glucose without thiamine, causing accute KS in the patient. The glucose is used but without the thiamine, it becomes toxic