lecture 10 Flashcards
where is the hippocampus
in the MTL
underneath are the perirhinal cortex, entorhinal cortex and parahippocampal cortex
why is hippocampus described as a complex topographic structure
essentially impossible to damage only the hippocampus without compromising other surrounding regions
cross section shows two-interlocking C-shaped cortices, dentate gyrus and hippocampus proper (corneau ammonus, CA)
what is the microcircuitry of the hippocampus
input and output from the entorhinal cortex
mossy fibres to CA fields
schafer collaterals connect Ca1 fields
trisynaptic pathway
what are the hippocampal functions
evidence from rodent studies indicate that place cells in the hippocampas act as a cognitive map
produce place fields that contribute to navigation
what do water maze experiments demonstrate about the role of the hippocampus in spatial navigation
learning phase of where the platform is
post learning rats should be able to use visual cues and self-tracking to swim directly to the hidden platform
rats with hippocampal lesions take much longer when the platform is hidden, but not when it isn’t
spatial memory
what is the difference between spatial reference memory and spatial working memory (Olton & Papas, 1979)
radial maze experiment
elevated with 8 arms, all of which rats are able to explore
some arms are baited with food
the exploration of the maze is recorded overhead with a motion tracking camera
arms that are never baited with food recruit reference memory, rats should not enter these arms
arms that are baited with food once recruit working memory, rats should only enter these arms once
rats with fornix transections (major input/output pathway of hippocampus) make more WM errors than controls, but not more RF errors
comparison between constant unchanging spatial information vs changing spatial information
how is spatial and non-spatial information encoded in hippocampal neurons (Wood et al., 1999)
in an odour non-match sample task, animals were presented with an odour which required them to dig in a pot
on the next trial there was another pot in a different position
if match, refrain from digging, if non-match, dig
three-factor design using several parameters
9 different locations, 9 different odours, match/nonmatch trials
Record hippocampal neurons
Lots of neurons encoding spatial attributes
But majority encode non-spatial aspects of the task
hippocampal neurons encode a global record of memory
why has the hippocampus been described as a temporal memory buffer (Rawlins, Feldon, Butt, 1985)
temporal buffer procedure:
Training phase: Y-shaped maze, where animals start in one arm and can choose to go to either the continuously reinforced arm or the partially reinforced arm
test phase: 10s delay introduced between rat entering continuously reinforced arm and delivery of water reward
control rats prefer the 100% rewarded arm despite delay
hippocampal lesioned rats switch choices to the immediate reward arm despite getting rewarded on only 25% of trials
the hippocampus may allow animals to associate temporally discontinguous events in intermediate-term memory
thus extends beyond spatial memory
how do hippocampal lesions impair non-spatial memory for sequences of events in rats (Fortin et al., 2002)
in a non-spatial memory task
rats are presented wtih five different odours in a sequence
Have to dig in odour 1 cup
2 ½ minute delay then next one etc
Tested:
Recognition (presented with novel)
Order test (take any two, present together, rewarded for digging earlier cup)
Hippocampal lesions impaired in order test not recognition
Non-spatial memory impaitred by hippocampal damage
what does neuronal activity in non-human primate hippocampus show
Rats have ‘place cells’ in the hippocampus (O’Keefe and Nadel, 1978) which encode where the animal currently is.
Encode where the rodent is in the room, not where they are looking
Whereas macaques have ‘spatial-view cells’ in their hippocampus (Robertson et al, 1998) which encode where the animal is looking, not where it currently is
Dot: position animal looks in room
Outer dot: cell firing (neuron fires only for wall 3 - irrespective of where the animal is in the room)
Important info for episodic memory
Multiple saccades building up pairwise representation of a scene
Important for building up episodic representations
how do lesions to the hippocampus affect object and scene memory
neurotoxic lesions using Ibotenic acid to the hippocampus target cell bodies but not axons so do not damage the surrounding areas
doesn’t impair object recognition memory or association of objects (which is the role of the perirhinal cortex)
does impair scene learning
what can we learn from fornix transections
provide an indirect means to assess the role of the hippocampus system by interrupting major input and output pathways
although not all of the fibres in the fornix terminate in the hippocampus
what is the effect of fornix transections on scene learning (Gaffan, 1994)
Non-human primate learning an object in scene task
Given a stimulus on a big screen that looks like a computer generated scene with foreground objects
Characters don;t mean anything to the animals and they aren’t verbalising
Monkey has to guess which of the foreground objects is reward in that scene (arbitrary)
For 20 scenes, then cycled through again and again
Concurrent discirmination learning task
Within-day learning
Fornix: impaired but not devastating memory loss (which would be chance)
what is the effect of fornix transections on non spatial recency memory (Charles et al., 2004)
5 visual stimuli insequence
Choice between two: rewarded for picking the most recent one
Control recognition task
After fornix lesion there is impaired order judgement
Good correspondence with rodent models for spatial and non-spatial memory
what did Maguire et al. (1998) find about navigation and human brain activity
Able to specify environment: navigate from A to B virtually
3 different versions of routes: accuracy, inaccurate but successful, lost
Control: same visual input but trail following rather than navigating
Subtraction shows activity in the right hippocampus
The magnitude of activity in hippocampus is correlated with how accurate the navigational route actually was
