Cellular & sleep consolidation

• Memory consolidation


  Offline neural changes that lead to memory stabilization, enhancement, and integration with pre-existing knowledge
• Levels of memory consolidation


  • Cellular (synaptic)
  • System-level (whole brain)
• Mueller & Pilzecker


  1900
• New memories


  • Initially fragile, but soon resistant to interference
• Experiment by Mueller & Pilzecker


  1. Ss learnt a list of paired-associate syllables (AB) and were tested in cued recall (using the first syllable, A)

  2. Interpolating another list (CD) impaired memory of the first list (AB)

  3. Found temporal gradient, whereby the closer in time the interfering list to the target list, the stronger its amnestic effect
• Memories require time to consolidate
• Retroactive interference compromises the integrity of recently formed—but not yet consolidated— memories
• Interference is 'nonspecific'

  • i.e. the interfering material does not have to be similar to the target material
• The "clay sculpture" metaphor


  Memory initially at its finest, but fragile; over time, it becomes resistant to interference (i.e., it shows less potential for damage)
• Shape of the forgetting function


  The clay metaphor, i.e., the idea that memories become more and more resistant to interference fits well with forgetting curves
• The rate with which we forget is not constant; this would be exactly the property of memoryless systems
• Ebbinghaus (1885): the forgetting function = we forget less and less as time goes by
• Jost (1897): If two memories have equal strength but different ages, the older trace will decay at a slower rate
• This continuous reduction in the forgetting rate is a sign of consolidation
• Cellular consolidation


  • Occurs at the neuron level
  • Takes place during the first hours after initial memory formation in the hippocampus
  • Fits well with the idea of a trace-hardening physiological process put forward as far as Mueller and Pilzecker
  • Corresponds to the discovery of long-term potentiation (Bliss & Lømo, 1973)
• Long-term potentiation (Bliss & Lømo, 1973)


  Long-lasting enhancement of synaptic efficacy induced by a tetanus (short burst of high-frequency stimulation) to the presynaptic neuron
• If subsequent encoding (mental exertion) interferes with memory consolidation


  Factors blocking new encoding should promote memory stabilization
• Factors blocking new encoding


  • Alcohol (Bruce & Pihl, 1997; Carlyle, Dumay, et al., 2017)
  • Benzodiazepines (anxiolytic drug; Weingartner et al. 1995)
  • Slow-wave (non-REM) sleep (Yaroush et al. 1971)
• Systems consolidation


  • Temporally graded retrograde amnesia: impairment of memories formed before surgery, stronger for young than old memories
  • Declarative memories = independent of hippocampus and more dependent on neocortex
• HM's bilateral medial temporal lobe (MTL) resection

  • Anterograde amnesia: inability to form new declarative memories (the 'what')
  • Temporally graded retrograde amnesia
• Bayley et al. (2006; Manns et al. 2003): 6 amnesic patients with damage limited to the hippocampal region showed a temporal gradient of semantic memory - behaviour
• Smith & Squire (2009): 160 questions on news events over 30 years showed the same temporal gradient over 24 hours
• Richness of the memorized materials may determine how fast systems consolidation occurs
• Significance of shift of activation from the medial-temporal lobe to the neocortex

  • Fragments of declarative memories stored in neocortex from the outset (e.g., sensory and semantic areas) as they travel through it
  • Hippocampus acts as a relay station and binds these fragments together
  • Over time, cortico-cortical associations develop, such that these memories become independent of the hippocampus
• Complementary learning systems


  • Fast learning systemthat holds info temporarily (medial temporal lobe)
  • A slow learning system that serves as a long-term store (i.e., the neocortex)
  • Info initially stored in hippocampus = fed back to neocortex (via 'neural replay'), so that pre-existing knowledge can accommodate new info
  • The hippocampus = the internal sparring partner of the neocortex
  • Both systems encode new learning - reactivation/redistribution has to occur offline