spatial learning cells that code for space synaptic plasticity in the hippocampus

1. PART 4: BEHAVIORAL PLASTICITY #24: SPATIAL NAVIGATION IN RATS I • spatial learning • cells that code for space • synaptic plasticity in the hippocampus • experiments that are knockouts • summary

2. PART 4: BEHAVIORAL PLASTICITY #24: SPATIAL NAVIGATION IN RATS I • spatial learning • cells that code for space • synaptic plasticity in the hippocampus • experiments that are knockouts • summary

3. SPATIAL LEARNING • rats... • order Rodentia • family Muridae • subfamily Murinae... all common mice & rats • cosmopolitan • arboreal, terrestrial & even aquatic • nocturnal (generally)

4. SPATIAL LEARNING • two most common species • black house rat... Rattus rattus • brown house rat... Rattus norvegicus • really good motor and navigation skills 

5. SPATIAL LEARNING • navigation by cues... two types, both used  path integration • using memory of familiar environment • missing sensory cues (e.g., visual) • use internally generated ideothetic cues... • vestibular • proprioceptive • optic flow (deprived in this e.g.)  external landmarks • learning the environment • direct sensory input (e.g. visual observation)

6. SPATIAL LEARNING – MAZE LEARNING • navigation by learning in a novel environment • how do animals learn this task? • complex problem • rat used to study • behavioral strategies • neural mechanisms • maze learning...

7. SPATIAL LEARNING – MAZE LEARNING • maze learning among earliest experiments with rats • Tolman (1930s) • cognitive maps for solving spatial problems • internal representations • for current problems • extend to novel problems • also, first guy to study inheritance of behavior

8. SPATIAL LEARNING – MAZE LEARNING • radial arm maze • reward at end of each arm • optimal foraging • how many visits until all 8 food items retrieved? • training • few days... 7/8 • ~ 3 weeks... ~8/8

9. SPATIAL LEARNING – MAZE LEARNING • 2 types of memory in this experiment  working memory used within single trials • which arms have yet to be visited? • experiments to test different possible strategies • visible landmarks • rules... e.g., clockwise • marking visited arms • rats establish map of maze over time

10. SPATIAL LEARNING – MAZE LEARNING • 2 types of memory in this experiment  reference memory used between trials • memory of food items in specific arms • remember positions between trials

11. SPATIAL LEARNING – MAZE LEARNING • Morris water maze - spatial learning • must find hidden platform • relative to visible cues in room • training... trials • test reference memory • rat encodes spatial relationships QUADRANTS

12. SPATIAL LEARNING – MAZE LEARNING • Morris water maze - spatial learning • must find hidden platform • relative to visible cues in room • training... trials • test reference memory • rat encodes spatial relationships QUADRANTS

13. SPATIAL LEARNING – MAZE LEARNING • Morris water maze - cued learning • associates visible platform with reward (escape) QUADRANTS

14. SPATIAL LEARNING – MAZE LEARNING • circular platform maze • ~ terrestrial version of Morris water maze • rats prefer to hide rather than be exposed • 1 hole  access to hidden chamber • test reference memory ~ visible cues in room

15. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • rat brain

16. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • hippocamus

17. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS

18. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • hippocampal formation = hippoc. + associated bits • hippocampus... “sea horse” (Greek) • major connections: entorhinal cortex & fornix

19. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • regular longitudinal internal circuitry (~ MB) pyramidal cells in single layer • similar array in dentate gyrus, associated structure  granule cells in single layer    

20. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • 3 main subdivisions: CA1, CA2 (not shown), CA3 • main pathways;  = trisynaptic circuit  perforant path: entorhinal cortex  dentate gyrus  mossy fibers: granule  CA3 cells  Schaffer collaterals: CA3  CA1 cells  commissural path: through fimbria  CA1 • highly oversimplified • additional circuits • feed-back & -forward ~ rest of brain

21. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • limbic system – humans

22. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • hippocampal lesions – humans, other mammals • not all memories lost • memories of facts or events (explicit, declarative, episodic) impaired or lost • post-lesion memory impaired only • pre-lesion memory intact • procedural, implicit memories ok • important for acquisition & memory of types of new information • spatial information in particular LIMBIC SYSTEM

23. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • effects of hippocampal lesions – rats, radial arm maze • cued learning (informed that arm visited) ok • spatial impaired* • perseveration *

24. SPATIAL LEARNING – ROLE OF THE HIPPOCAMPUS • effects of hippocampal lesions – rats, water maze • spatial impairment  longer circuitous routes • swimming impaired or just enjoy a good swim? • 3-part experiment  spatial task  impaired ability to find platform  cued task  can swim, would rather not  spatial task  still impaired... no memory!

25. CODING SPACE – HIPPOCAMPAL PLACE CELLS • extracellular recordings from hippocampus • in freely moving rats • firing of neurons ~ spatial position in chamber, e.g. B • many neurons ~ spatial position  place field maps • “complex spike cells”... CA1 & CA3 (as it turned out)

26. CODING SPACE – HIPPOCAMPAL PLACE CELLS • cues governing a place cell’s spatial preference • free moving rats, chamber, “permanent” electrodes, camera from above recording position ~ space • place field maps • differences in preference, size & shape of fields, some cells (D) do not behave like place cells (strictly)

27. CODING SPACE – HIPPOCAMPAL PLACE CELLS • pattern not innate... acquired, then ~ stable  weeks • cells can have multiple preferences • preferences shaped by contexts (e.g., rewards) • gradual drift in preference ~ changing context

28. CODING SPACE – HIPPOCAMPAL PLACE CELLS • cells fire ~ cues • move cues, cell fires ~ preferred cue new position • remove cues, cell fires ~ memory of preferred cue position... “deduced reckoning” or ideothetic cues • what are these internal ideothetic cues? • movement related... vestibular & visual • test in striped cylinder; walls & floor rotation separately • showed that animals own movement  place cell firing

29. CODING SPACE – HIPPOCAMPAL PLACE CELLS • what happens in the dark (without visual cues)? • train animal in light, turn out lights • place cell activity remains ~ 30 min after lights out • ~ memory • train animal in light, remove, return to dark chamber • cell established a new place field • train animal in light, remove, return to dark chamber, turn on the lights • retains remapped field determined in dark • preference ~ real time or memory of cue preferences

30. CODING SPACE – HIPPOCAMPAL PLACE CELLS • another cool place cell experiment... • cell readings made during all parts of experiment • rats trains in 4-arm maze ~ visual cues • could find food relative to position of cues in test • next, trained in 4-arm maze ~ visual cues • cues removed before test, had to remember place • when tested correctly, cells fired as before in arm • when tested incorrectly, cells fired incorrectly • suggests that if cell firing  (processing... memory... )  behavior 

31. CODING SPACE – HIPPOCAMPAL PLACE CELLS • place cells encode more than simple space • T-maze, trained (fruit loops) to alternate L & R turns • subset of place cells showed interesting pattern • e.g., activity (sector 3) anticipating right turns only • suggests hippocampal network represents episodic memories, cells are small segments of an episode • link of cells with overlapping episodes  memories

32. CODING SPACE – HIPPOCAMPAL PLACE CELLS • spatial dreaming • large # space cells • only ~ 15% active in any 1 environ. • some silent in one environ., active in others • time- & labor-intensive to get larger picture • device to measure 150 cells at once • population or ensemble code • code predicts rat behavior in maze • many environments & codes • overlapping, not interfering • used to study plasticity...

33. CODING SPACE – HIPPOCAMPAL PLACE CELLS • spatial dreaming • plasticity • strengthening of code  learning • accompanied by reduced inhibitory activity • does code relate to consolidated (permanent) memory • trained rats in spatial task • measured code during • training • sleeping before training • sleeping after training • dreaming replay of events  memory consolidation

34. BREAK