Biological basis of learning and memory

Question 1

Case study - HM

Answer 1

• Suffered intractable epilepsy
  1953 – underwent a bilateral
  medial temporal lobectomy
  Removal of: Most of hippocampi, amygdale, adjacent limbic cortex
• Successfully treated the epileptic seizures but HM developed severe memory deficits
• STM intact - holdy fairly normal convo and digit span test scored 6 (within 7 +/- 2 range)
• LTM impaired - intact memory for events in distant past (childhood), poor ability to recall events within last 2y (retrograde amnesia), failure to make new memories (anterograde amnesia)
• Implicit LTM remained intact
  Mirror-Drawing test- Improved
  performance seen over 3 days
  (indicates good retention), but no
  conscious recollection of having
  performed it before (Milner, 1965)

3 major advances in understanding…

Finding – MTLs play a vital role in specific memory functions
Consequence - renewed efforts to relate individual brain structures to specific mnemonic processes

Finding – Some forms of LTM affected by bilateral MT lobectomy, but STM remained intact
Consequence – supported theory of different modes of storage for STM and LTM.

Finding – performance improved on some tasks without conscious recollection
Consequence – Two parallel distinct categories of LTM developed - Explicit (conscious) and Implicit (unconscious)

Question 2

MTL Amnesia

Answer 2

• Intact STM and implicit LTM
  Inference: MTL not involved in STM / implicit memory
• Intact retrieval of distant explicit (“declarative”) memories
  Inference: MTL is not the site of permanent storage for declarative long term memories
• Failure to lay down new long-term declarative memories
  Inference: MTL is involved in formation of long term declarative memories (consolidation: STM  LTM)
• Failure to recall events in lead up to trauma
  Inference: MTL mediated consolidation process is gradual – these lost memories not fully consolidated before trauma?

Question 3

Declarative Memory Consolidation

Answer 3

Scoville and Milner (1957)
- Hippocampus plays a key role in formation of long term declarative memories
- Hippocampus involved in initial consolidation (STM->LTM)
- Consolidation is a gradual process and if disrupted (e.g. By MTL damage), memories are lost
- Hippocampus only temporary storage area for the LTM. When fully consolidated, these declarative LTMs are permanently stored in the more stable neocortex
- More recent evidence suggests cortical sites are also important for consolidation of declarative memories. Neocortex still considered main storage area for consolidated memories

Question 4

Delayed non-matching to sample test (DNMTS)

Answer 4

• Lesion studies with monkeys suggest other parts of the MTL also involved in some types of declarative memory
• Series of studies conducted assessing the performance of monkeys on the delayed non-matching to sample test (DNMTS)
• To be successful, the monkey must form an explicit LTM of the sample object, so the novel object can be identified and selected.
• Using the DNMTS task allows us to examine the monkey’s performance on this task after lesioning specific brain areas (e.g. Parahippocampal regions).
• Infer that hippocampus works in tandem with surrounding cortical areas to form explicit long-term memories for visual objects

Question 5

Hippocampus and Spatial Memory

Answer 5

Hippocampus key brain structure involved in memory for spatial locations

Animal evidence
- Lesions to hippocampi of rats disrupts performance on Morris water maze (spatial memory task) (Morris et al. 1992)
- Food-caching birds require remarkable spatial memories to remember the location of their food caches in their territory (vital for survival)
- Sherry & Vaccarino (1989) – cross-species comparisons revealed these birds have larger hippocampuses than non-food-caching species

• Maguire et al. (2000) - London taxi drivers

Question 6

Emotional memory

Answer 6

• Amygdala (MTL structure) provides the hippocampus and other areas involved in memory with additional information about the emotional content of the memory which enhances consolidation and strengthens the memory (McGaugh, 2000)
• Flashbulb memories – vividly remembered highly detailed, ‘snapshot’ of the moment and circumstances in which a piece of surprising and consequential (or emotionally arousing) news was heard
• Greater amygdala activation at
  retrieval correlates with an enhanced “feeling of remembering” of emotional scenes, even when accuracy of detail is not enhanced (Sharot et al. 2004)

Question 7

Amnesia of Korsakoff’s Syndrome

Answer 7

• Consequence of a thiamine deficiency (vitamin B1)
• Associated with heavy alcohol consumption over long period of time

Characteristics:
Anterograde amnesia for explicit episodic memories, Severe retrograde amnesia with disease progression, Sensory and motor problems, Extreme confusion, personality changes

Associated with damage to diffuse brain regions including:
Medial diencephalon in particular mediodorsal nuclei of the thalamus and medial hypothalamus
Hippocampus, Cerebellum, Neocortex (including frontal lobe)

Question 8

Other areas involved in memory

Answer 8

Striatum (implicit memory)
- Stores memories for consistent stimulus-response relationships (habit formation).
- Impaired in individuals with Parkinson’s Disease (associated with damage to the basal ganglia)

Cerebellum (implicit memory)
- Procedural (motor) memories
- Impaired in Korsakoff’s Syndrome)

Prefrontal Cortex
- Working memory and memory for temporal order of events
- Impaired by prefrontal damage

Question 9

Neural mechanisms of learning and memory

Answer 9

• Hebb (1949) – learning and memory represented in neural circuits (“cell assemblies”) throughout the brain
• During learning, interconnected cells are co-activated and ‘reverberate’ which leads to a strengthening of these connections
• Activation/strengthening of the networks underlies our ability to lay down memories
• Length of reverberatory activity linked to formation of STM or LTM
• Reverberation lasting several hours ->creation of STM
• Prolonged reverberation in the neural circuit leads to more permanent structural changes ->creation of LTM.

Question 10

Support for Hebbian Theory - Long Term Potentiation (LTP)

Answer 10

• LTP is a change in the strength of synapses within a circuit as result of concurrent firing in the pre- and post-synaptic neurons.
• “Cells that fire together,
  wire together”

Evidence for LTP as a neural correlate of L&M:
- LTP most commonly seen, and has most prominent effect in the hippocampus (brain region involved in consolidation of memory)
- Drugs that impact on L&M also effect LTP (e.g. In rats drugs blocking LTP impairs performance on the Morris water maze (Morris et al., 1989)