Schmidt hippocampus 3 Flashcards
What led to the discovery of Jennifer Aniston cells?
Patients with pharmacologically-resistant epilepsy had an implantation of electrodes to detect the focus of the seizure onset
Targeted areas in the medial temporal lobe including the hippocampus and presented large numbers of visual stimuli whilst recording neuronal responses
Found that neurons responded with increase in firing rate to pictures of Jennifer Aniston and not as much with pictures of other celebrities
What are the features of the Jennifer Aniston cells?
It was invariant to the specific picture type and more related to the abstract concept so even the written name of the person could evoke a response
Been debated before with Grandmother cells
And could see similar neuronal responses for other things such as Sydney Opera house
What are the implications for place cells from Jennifer Aniston cells?
Place cell properties may apply to this, suggesting a generalisation of hippocampal function from place cells
It may also have relations with episodic memory
What is episodic memory?
According to Tulving (1972): information about temporally-dated episodes, autobiographic events and tempero-spatial relations among these events.
Part of declarative memory and requires consciousness
What did Clayton introduce?
The concept of ‘episodic-like memory’
What is episodic-like memory?
Refers to the behavioural criteria of episodic memory without requiring conscious experience
Relevant for animal studies as they may lack consciousness
What are the 3 key criteria for this retrospective aspect of episodic-like cognition?
- Content: a what-when-where memory of a single past experience
- Structure: what-when-where components are bound together to discriminate overlapping memories e.g. in the sample place but different events at different times
- Flexible deployment: use the information to generalise across episodes e.g. learn from past experiences
What is the network of place cells like within CA3?
Can be connected with each other with different strengths, usually a one-way projection from neuron to neuron.
If the connection is strong the neuron projected to will be affected more by an action potential from the projecting neuron
The relation between different spatial locations may be stored in the connections between place cells
What other connections could be made in the CA3?
In the CA3 of humans, relations between more abstract concepts, e.g. persons, may be stored
What is synaptic plasticity?
Changes in synaptic strengths, possibly the neural correlate of learning?
Outline spike timing dependent plasticity
- Neuron 1 has an excitatory connection to neuron 2
- If neuron 1 fires briefly BEFORE neuron 2, it strengthens the connection from neuron 1 to neuron 2
- If neuron 1 fires briefly AFTER neuron 2, then it will weaken the connection from neuron 1 to neuron 2
What does spike timing dependent plasticity relate to?
Correlation coding
How do we organise the timings of action potentials?
Through looking at oscillations, as the relative timing of APs in 2 connected neurons may be important for learning
What are oscillations?
They reflect behavioural states and can be measured through EEG or local field potentials using electrodes
What do oscillations reflect?
A temporal organisation of large networks of neurons
May provide a temporal reference similar to a clock - thereby the timing of APs of different neurons can be investigated
Which oscillations are looked at in relation to hippocampus spikes?
Theta oscillations, which are around 8Hz, important for organising the timing of spikes
What is a spike phase?
The phase tells us what the relation is between spikes and the oscillation
Can be seen that spikes don’t occur at random times relative to the oscillations
Give an example of phase locking
Spikes occur at a preferred phase e.g. always occurring at the peak, which indicates that the neuron activity is modulated by the oscillation
Can visualise this in a histogram of spike phases (X axis = 0-360^, Y axis = No. of spikes)
What happens when there is more variability in phase locking?
There will be a broader distribution on the histogram
What can be said about phase locking in hippocampal neurons?
Different types of neurons are active at different theta phases e.g. pyramidal cells have different phase locking patterns to PV basket cells
What is phase precession?
From cycle to cycle the spike phase decreases
Can use this phenomenon to understand how the spike times are organised in the hippocampus
This systematic precession happens in the place cells of the hippocampus relative to theta oscillations
Spike sequences that reflect behavioural sequences
What is an example of phase precession research in animals?
An animal is at one side of a linear track with food at the other end.
As the animal crosses the track, there will be multiple cells in the hippocampus that have this place field and will become more active at these points, thus producing respective spike trains
Looking at the produced spike trains in relation to the theta oscillations can see the space advancement and phase precession
What can be found when looking at multiple place cells in phase precession?
You can see the organisation between (for example) place cell A, B and C, with A firing just before B and B firing just before C
If looking just at A and C can see the greater distance in place fields reflected in the spike train organisation
Important because as the animal walks along the linear track it will experience a sequence of place cells also represented in phase precession within each theta cycle
What can these sequences of phase precession be used for?
Storing spatial-temporal relations between place cells, relating back to spike-time dependent plasticity and strengthening the connection from A to B and B to C
What does this phase precession also relate back to?
Episodic memory, through storing this sequence into a network using spike timing dependent plasticity
May be a simple form of an episode for an animal, same principle may apply for more abstract concepts in humans
What can the place cell sequences be described as?
The sequences are compressed from a behavioural timescale (seconds) to a synaptic timescale (milliseconds)
