Memory Systems Flashcards
What is the MEC?
Meidal Entorhinal cortex. Space and movement related firing.
Get “grid cells” which fire in a hexigonal lattice pattern and “headwind direction cells (where you’re facing”
and “border cells”
What are grid cells?
RFs independent of local cues. Persist in the dark but will degrade.
Contentless spatial coordination system that receives dorsal stream inputs about space/self/somatosensory and vsitubular input.
Many grid cells have adjacent receptive fields so that every point in space is represented by an overlapping “grid”
Important for moving in the space.
What are border cells?
fire along walls and stay consistent when environment is stretched. Creating a new wall is consistent with firing field.
What is the LEC?
Lateral entorhinal cortex. Responsible for object-oriented firing properties.
“object cell” fires when object is present in a box
“object trace cell” fires where object was located but removed - (in rats for 10 days persisting)
what does damage to LEC and MEC do?
LEC: impairs object memory. Rats’s don’t explore new objects when placed in environment.
MEC: impairs spatial contextual memory. Rats don’t explore new space when room is turned black (same objects).
In both experiements the rats habituate over time.
The LEC and MEC do talk to each other so the paths aren’t totally distinct and you see some overlapping deficits. MOSTLY converge in hippocampus tho.
What are receptive fields in the hippocampus?
place cells and place fields.
Hippocampus has a spatial map which is anatomically organized in a given environment but is not understood. No ability to predict a neurons place field in X based on its field in enviornment Y. No organization.
What is the spatial map and how has it been tried to be studied?
Rats navigating a restricted track, see the beginnings of a map, each cell is specific as to where it fires.
This spatial map is also a cognitve map (goals, experience etc.) Hippocampal representations are rich in detail and allow for adaptive behavioral responses. - things that aren’t just spatial.
What are the 2 ways place fields can be altered?
Global remapping: two different environments completely, same box. One spatial map dissapeasrs and another appears. Some stop/start/fire at different rates. Two totally different maps for the two different places.
Suggests independent neuronal populations for different places.
- Rate remapping: same place, different box. change in local stimuli but main environment stays the same. Place cells stay in same feilds but firing rates change by a lot. Heavily dependent on objects. Same field, but different firing. Different rates of expression to represent different experiences.
What happens to rate remapping when you lesion the LEC? What does this indicate the LEC does?
In a normal animal, when you change the testing chamber, the LEC firing lessens because cues have changed, but still in the same room. In and LEC lesioned animal, the firing continues the same way.
The LEC makes info distinct stimlui (but doesn’t have to be visual, can be reward motivation, where the rat started)
How does summation in the hippocampus work?
Gets inputs from LEC (odor, specific object related neurons)
Gets inputs from MEC (border and grid cell neurons)
Post synaptic cell will fire if it reaches threshold, minimum input needed for summation.
How does firing rate and information coding look at different levels of processing
Early: linear relationship between firing rate and stim intensity.
Higher level: in association cortex like hippocampus, firing rate encodes info, but not in a linear way. Pattern of activity represents specific information. Distinct patterns = distinct experiences.
Doesn’t mean ‘more of a stimulus’
How is imagination studied in the hippocampus?
place cells depict possible future paths. 100s of electrodes implanted to see places in an open field.
“imaging something you haven’t done before by the things you’ve experienced in life”
allow navigation of the whole field, and then you get map which you can use.
Can see Trajectories of possible place cells. Suggests rat is imagining where it will go, then it shows rat taking that path !
What does flexible spatial learning rely on? what happens if this is damaged?
the hippocampus of course! experiment: rats released from the same start point in a maze, can learn where the platform is (take a bit longer but still learn) - possibly due to muscle memory or even habits. however, hippocampal lesioned rats (fornix) when released from a novel start point cannot figure out where the platform is even when the control rats can.
Suggests the hippocampus creates a map that control rats can use but lesioned rats cannot.
IN HUMANS:
taxi drivers require hippocampus to navigate. Change structure of their brain actually. When asked to drive routes he did okay for the A roads (easy main roads) but had extreme difficulty on non-a roads.
