Critical role of the hippocampus in memory for sequences of events

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Critical role of the hippocampus in memory for
sequences of events

• Norbert J. Fortin, Kara L. Agster and Howard B
  Eichenbaum
• Nature Neuroscience 2002

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Memory stores
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Episodic Memory

• One of the two branched of declarative memory
• Pertains to memory of events, times, places
• Key players seem to be the hippocampus and the PFC

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Hippocampus

• Name derived from Greek to mean seahorse
• Located in the medial temporal lobe
• Part of the limbic system
• Seems to play a role in episodic memory

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Introduction

• Hippocampal (Hpc) function tied to the recall of
  personal experiences
• Traditional research emphasizes its role in
  spatial memory
• Distinction between spatial and temporal cues
• Little evidence proving a role in memory of
  sequential events

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Question

• Does the hippocampus play a main role in the
  memory for the sequential order of events,
  specifically and independently of other memories?
• Compared sequential order tasks with prior
  occurrence tasks

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Methods

• Used distinguishable spices to identify different
  pots
• Rats had to dig in the correct pots to obtain a
  reward
• Sequences One pot at a time was shown
• Probe Two pots placed side by side
• Order Correct which was presented first
• Recognition Correct which was not presented

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Methods Continued
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Training

• Dig for reward
• Sequential order task
• Operation (sham or lesion)
• Sequential order task
• Recognition task

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Operation

• Dura was pierced with a syringe at 12 sited
  bilaterally
• Electrodes made radiofrequency lesions
• Shams did not have electrodes lowered
• 55-77 loss in Hpc volume
• More specific than neurotoxic lesions

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Results Sequential Order

• Lesion rats performed at near-chance levels for
  almost all probes

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Results Sequential Order

• In lag 3 (greatest temporal situation) lesion
  rats performed better than chance

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Results - Recognition

• Control and lesion rats performed equally on all
  probes
• Recency effects provided better performances in
  both groups

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Results Difficulty?

• No differences across groups for recognition
• Significant differences in sequential order tasks

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Discussion

• Good performance in recognition task not due to
  the task being too easy
• Hpc lesions left an intact capacity to recognize
  odours
• Rats couldnt, however, remember the correct
  order
• Hippocampus is essential to remember the order of
  sequences of events

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Discussion Continued

• No dependence on spatial cues
• Past research indicated a Hpc role for
  recognition
• No previous research has outlined a Hpc role for
  sequence independent of space
• Unique architecture may contribute
• Local context neurons and cognitive maps
• Synaptic modification, sparse connections and
  recurrent activation

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Conclusions

• Hippocampus is necessary for memory of sequences
  of events
• It is not necessary for recognition

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Things to ponder

• Specific nucleus for sequence memory?
• Behavioural significance?
• Few, if any, cases of amnesiacs with only
  temporal deficits

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Thank you