Hippocampus And Spatial Memory Flashcards
What are the two major mechanisms for navigation?
Path integration and cognitive map
What type of cells within the hippocampal formation support path integration?
Direction and grid cells
What type of cells within the hippocampal formation support the cognitive map?
Place cells
What type of animal is a good model for path integration?
Ants
What 3 W’s studied navigation of desert ants?
Wittinger, Wehner and Wolff (2006)
Theoretically, what are the two possible ways an ant could navigate back to the nest?
- Remember the path exactly
2. Use motor system and sense of direction (from polarised light) as well as distance travelled
What is the evidence that ants might use their motor system information, sense of direction and distance travelled to navigate back to the nest?
They take the most direct route back to the nest, so they must have a sense of distance traveled, and be able to compute and integrate this information with direction when navigating back to the nest
What were Wittinger, Wehner and Wolff (2006) interested in finding out?
Whether animals actually count their steps back to the nest
What was the main idea of 3W’s experiment
If you make the legs of an ant shorter or longer you can change the distance of the step and thus observe whether ants actually step count
Describe the methodology of the 3 W’s experiment
- Train ants to navigate from nest to source of food
- Modify length of ants legs (short, normal or stilts)
- Let them navigate back to the nest
What were the main findings of the 3Ws experiment?
Normal ants navigated back to the nest fine
Ants with stunk legs stopped before the nest
Ants with stilts stopped after the nest
What did the 3 W’s find happened when they did the same task on the modified ants the next day?
The ants were able to compensate for their modified legs as they travelled the correct distance to the nest
So what type of information do ants use to navigate? What is this evidence of?
They use their own motor information to navigate. This is evidence of path integration
What is a cognitive map?
Internal representation of the outside world based on sensory information
What are the advantages of path integration?
Animals can navigate between locations by integrating linear and angular self motion
What are the disadvantages of path integration
Longer journeys tend to accumulate errors
What are the advantages of a cognitive map
Highly accurate
What are the disadvantages of a cognitive map
Cannot support navigation between locations
What type of navigation is path integration
Egocentric
What type of navigation is a cognitive map
Allocentric
Why is learning to navigate a new environment ‘unsupervised learning’
The animal is not rewarded for learning the layout of the new environment. There is no explicit learning outcome
Who first believed that there were detailed internal maps within the rat brain, and coined the term ‘cognitive map’
Tolman
How did Tolman show that rats use unsupervised learning
Set up a maze experiment whereby one set of rats is trained to retrieve food from the end of the maze, and the other set of rats was just left to explore the maze.
Of the rewarded rats, deviation from the optimal path decreased with time. Rats were operating at s low error rate near the end of the experiment
On the 11th day of the experiment the un-rewarded rats were given food to retrieve.
These rats found the food as easily as the rats that had been trained to retrieve the food. End up performing slightly better due to greater exploration of the maze
Shows that unrewarded rats are learning about the maze
How did Tolman show that rats possess a cognitive map
Used a maze with a series of blockage manipulations.
Rats firstly given free reign on the maze - eventually learn to take the shortest of three possible routes to the food.
Then put in block A - blocks the shortest route, rats take the shortest detour
Then put in block B - blocks shortest route and shortest detour.
Rats take the longest, but only possible, detour.
Evidence of rats using knowledge of maze to guide behaviour
Where does the rodent hippocampus receive major cortical inout from
The entorhinal cortex
What is the trisynaptic circuit in the rodent hippocampus
EC to DG
DG to CA3
CA3 to CA1
CA1 then projects back to EC
What types of mazes can we use to test for spatial memory in rats?
Radial arm maze - David Olton
Water Maze - Richard Morris
When is reference memory used in the radial arm maze
Used between trials, when the same arm(s) is bated with food each trial.
Reference memory error would be choosing an arm that has never been bated before
When is working memory needed in the radial arm maze
When all arms are bated with food each trial and the animal must remember which arms it has visited.
A working memory error would be visiting the same arm twice
How would you test if the animal has spatial memory of the platform in the Morris water maze
Remove the platform and record the amount of time animal spends swimming in the quadrant where the platform used to be
What are the two version of the water maze and radial arm maze you can employ
Spatial vs. Cued
How would you provide a cue in the Morris water maze
Put a flag on the platform
How would you provide a cue in the Radial arm maze
Signal a visited arm by turning on a light
What is different about cued versions of the radial arm maze and the water maze
They do not require spatial memory
How do animals with a hippocampal lesion perform on cued mazes
Normal performance
How do animals with hippocampal lesion perform on spatial Versions of the mazes. What does this provide good evidence for
Poor performance. Provide good evidence that the hippocampus is the site of the cognitive map
How did O’Keefe show that the hippocampus contains place cells
Used an electrode implant in the rat hippocampus. Recorded the activity of the cells whilst simultaneously monitoring the position of the rat with a camera. Every time a cell fires a; action potential you label that with the position of the animal.
Can make a map of where particular cells have fired
What do place cells do?
Fire action potentials whenever the rat is in the corresponding place field
What is a place field
The area in space where the corresponding place cell fires maximally
What is a hyperdrive
It’s a recording tool with 14 independent tetrodes, each recording 3-6 cells (giving 40-90 simultaneous recordings)
Place cells are non-topographic, what does this mean?
Adjacent place cells in the hippocampus do not have adjacent place fields
What type of cells are place cells presumed to be?
Pyramidal cells. They tend to fire action potentials in complex bursts as the animal moves through a specific location
What are a sufficient number of place cells able to do
Map a given environment
What are place fields resistant to
Rotating the environment
What does evidence from place cells offer strong support for
The involvement of the hippocampus in creating the cognitive map
What do place cells provide a very poor neural basis for? Why is this?
Path integration. This is because they do not provide information about movement direction or distance covered
What cells support path integration?
Head direction cells
Grid cells
Where are head cells located
Presubiculum
When do head cells fire?
In a 90 degree arc around their preferred direction.
Fire to the same direction in different environments.
What is the proposed function of head direction cells?
Orient spatial maps in the hippocampus
Where are Grid cells located
Layer 2 of the Medial entorhinal cortex
How do grid cells fire
In a regular grid-like pattern across an environment. There is constant spacing between peaks in firing rate. Grid field centres are located at the points of an equilateral triangle.
Grid cells are arranged typographically. What does this mean
Adjacent grid cells map adjacent positions.
What happens to the grid when an animal is in a completely novel environment
The grid is initially more spread out but contracts back as the animal becomes more familiar with the environment
What are grid cells good at giving information about? What can they not provide information about
Good at providing information about distance covered in any. Bad at telling the animal where it is in space because the firing is context independent.
What do speed cells provide information about
How fast the animal is travelling
Where are speed cells located in the hippocampus
Medial entorhinal cortex
What does the firing of speed cells correlate with
The running speed of the animal
What is the direct evidence that grid cells play a role in path integration
If grid cells are specifically turned off in mEC (through modulation of NMDA receptors) the animals could not path integrate. Place cell firing was not disrupted however
What do border cells fire in response to
The animal coming up to the edge of an environment (arena experimentally)
What do conjunctive cells combine. What does this make them ideal for
Grid cell firing and head direction cell firing. Makes them ideal for path integrators
Where are conjunctive cells located
Deep layers of EC
