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What structure is highly conserved across species?
Neocortical circuits
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What is the cortex composed of?
- A crystalline array of a repeated unit
- The cortical column
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How many cortical layers are there and what are they?
- 4 excitatory cortical layers
- Layers 2/3, 4, 5, 6
- Different in anatomy and connectivity that columns
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What's the physical basis of all neural computation?
Circuits
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How do we understand neural computation?
- Need to understand a wiring diagram
- It's necessary but not sufficient
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Why is a wiring diagram not sufficient to understand neural computation?
- Can't reconstruct thoughts
- Dont have connection strength in simple wiring diagrams
- Don't know the types of neurotransmitters in synapses
- Don't know lots of info and dynamics
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Neurons and synapses are highly ______.
dynamic
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What do we need both of to understand neural computation?
Both anatomy (wiring) and function (dynamics)
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What is functional mapping of circuit activity?
Looking at synapses in real neurons in real circuits
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How do we identify microcircuits?
Record two connected cells in the same layer with two electrodes to measure if there is an action potential and see if there is also a postsynaptic potential
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What kind of recording is used to identify the microcircuits with two electrodes?
"paired electrophysiological recording'
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What has functional mapping provided a 'roadmap' for?
The flow of excitation in neural circuits
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What are the 5 strongest connections btwn layers?
- L4 → L2
- L3 → L3
- L4 → L4
- L5A → L5A
- L3 → L5B
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What are the ascending connections?
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What are the descending connections?
- L2 → L5A
- L3 → L5A
- L3 → L5B
- L4 → L5A
- L4 → L5B
- L4 → L6
- L5A → L5B
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Between which layers do the greatest number of synapses come from?
- Greatest number of synapses in any layer comes from neuron in the same layer
- Talking most to neighbors
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Why are there a lot more synapses in the same layer?
Important for amplifying signals and stuff
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Circuit analysis in the 'age of light'??
Laser scanning photo-stimulation??
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Laser scanning photo-stimulation??
- Can target one cell and stimulate all presynaptic cells connected to it
- Can see most input from neighbors nearby
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Inhibition in the CNS can perform what kind of operations?
Can mediate both subtraction and division
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What are some of the putative functions of inhibition?
- Spike timing
- Spatial control
- Oscillations
- Gain control
- Sharpening feature selectivity (orientation tuning, direction selectivity, etc.)
- Sparse representation
- Decorrelation
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What does gain control do?
- Doesn't transform signals
- Just changes dial of "loudness" of signals
- Matches signals w/ range of hearing for example
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What is decorrelation?
When you don't want all neurons spiking at the same time cuz that'll be redundant
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What is this image showing?
Ultra precise timing of neural firing in trigeminal ganglion (1st order neurons)
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What is this image showing us?
- Very precise timing of neural firing in the thalamus (VPM)
- Thalamus = relay from periphery to cortex
- AP's pretty precise with noisy stimulus
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What should biophysical properties of neurons and synapses do as information is transferred to the cortex?
Degrade temporal fidelity
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What is temporal fidelity?
- The precision of spikes in time
- Ex. Jitter of spikes low if high fidelity
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What are some of the biophysical properties of neurons and synapses that degrade temporal fidelity (take more than a ms)?
- Axon conduction
- Channel gating
- Synaptic release
- Membrane time constant (low pass filter on synaptic potentials)
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Where is there also precise timing?
In the somatosensory cortex
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How do circuits achieve precise firing?
Feed-forward inhibition
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Why is feed-forward inhibition important?
Fundamental and conserved circuit motif across entire CNS
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In the cortex, which follows which? (inhibition/excitation)
- Excitation is always followed by inhibition
- (Inhibition comes after excitation)
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When is there a mix of excitation and inhibition in post synaptic cell?
When in between both reversal potentials of GABA and glutamate
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What's this image telling us?
- Figure 1
- Stimulate excitatory cells
- Each produce EPSP
- But each one by itself not sufficient to spike cell
- But b/c each is about 50%, if two come at exact same time, can spike
Delayed them to see window of operation/opportunity when two inputs summate to get a post syn potential
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What does FFI enforce?
Temporal fidelity
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What is FFI?
Feed-forward inhibition
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What generates brief windows for activation?
Fast somatic disynaptic ihibition
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What is this a basic schematic of?
Lateral inhibition
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What is lateral inhibition?
- Get inhibition from sides onto center
- (similar to horizontal cells in retina but actually fire APs)
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Why have lateral inhibition in cortex?
Increases salience of response?
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What was lateral inhibition tested on?
Somatosensation on cat leg
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Why was lateral inhibition tested on somatosensation on cats?
- Somatosensory map on cortex
- So when you stimulate one part of leg next to another, similar thing happening on cortex
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What happened when a neuron on the leg was stimulated then another neuron next to it was also stimulated?
Second stimulation suppressed the firing of first
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How is lateral inhibition helpful?
- General principle across sensory modalities that may improve the resolution of sensory processing
- Sharpens edge resolution
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What is surround suppression good for?
- Mutual inhibition helps distinguish objects?
- Suppression could help discriminate and judge the size of objects
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