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Why is motion detection essential?
For survival
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Which cells are direction selective?
Retinal ganglion cells
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What does it mean to be direction selective?
RGC's firing depends on the direction of the moving object that passes the eye
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What are the preferred directions of ON-OFF direction selective cells?
- Superior (up)
- Inferior (down)
- Nasal
- Temporal
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What are the hypothetical circuit models for motion detection?
- Cross-correlation
- Inhibitory "veto" model
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What does motion detection require?
Asymmetry
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Why is asymmetry required for motion detection?
Without asymmetry, you get the same response from all directions
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What is the Δτ on the motion detection models?
The delay (longer) line
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What's the point of the delay line?
Allows summation at optimal time (in preferred direction)
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What's the difference between the cross-correlation model and the inhibitory model?
- Cross correlation = both excitatory RGC's
- Inhibitory = powerful inhibition on one RGC
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What's another way of saying delay?
Attenuation
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In a simple model cross-correlation model, what is the preferred direction if the delay line is on the left?
From left to right
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What is a likely reason for the inhibition in the inhibitory model?
Probably an inhibitory interneuron
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Why does the "and not" gate exist?
Because of summation and subtraction
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Why does direction matter in an inhibitory model?
Inhibition subtracts from excitation
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In an inhibitory circuit model, what is the preferred direction if the delay line is inhibitory and on the left?
From right to left
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In an inhibitory circuit model, what is the preferred direction if the delay line is excitatory and on the right?
Is there a preferred direction??
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What are the different ways to test the models of direction selectivity?
- Pharmacology
- Recordings
- Cell ablations
- Anatomy
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How does "pharmacology" test models for direction selectivity?
GABA receptor blockade on inhibitory synapses eliminates direction selectivity
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What are the effects of a GABA receptor blockade?
- Blocks GABA receptors
- Preferred response doesn't change
- But firing in null direction increases a lot
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Why is the pharmacology test consistent with the hypothesis but not able to prove it?
- A lot of GABA in the retina
- So this drug probably blocks a bunch of things, possibly messing up the circuit in other ways
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How does "recording" test models of direction selectivity?
Directly measuring temporal relationship of excitation and inhibition (using a voltage clamp)
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What does a recording show when an object goes in the preferred direction?
- There are higher peaks of excitation than inhibition
- Excitation preceded inhibition
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What does a recording show when an object goes in the null direction?
- There are higher peaks of inhibition than excitation
- Inhibition precedes excitation
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What do the height differences in the peaks of excitation and inhibition say about symmetry?
- There is also strength asymmetry
- Maybe temporal offset isn't enough to cause asymmetry
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What model was the recordings consistent with?
Model that inhibition should precede excitation
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What neuron is the essential source of direction inhibition on RGCs?
Starburst amacrine cells (SACs)
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What are some characteristics of SACs?
- Don't fire APs
- Have axons and dendrites in the same areas
- Releases from dendrites too
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What makes the SAC inhibitory?
Dendrites release GABA onto ganglion cells
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How do "ablations" test models of direction selectivity?
Immunotoxin ablation of SACs eliminates DS
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How does the immunotoxin ablation of SACs eliminate direction selectivity?
- Take antibody that targets receptor
- Bring toxin to amacrine cell and kill all cells required for direction selectivity
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Define ablation
Removal of body tissue
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What can you see [in the firing] once you kill the amacrine cells with a toxin?
- There's no selectivity
- Mostly radially symmetric
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What does the radial symmetry [once the amacrine cells are killed off] mean?
Shows that the cells are at least necessary
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Remember, what do we need to get direction selectivity?
Asymmetry
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Where is the asymmetry in amacrine-to-RGC circuits?
- Amacrine cells preferentially inhibit RGCs on their null side
- Amacrine cells on the null side of the RGC make stronger inhibitory synapses
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What do you have to do to test the strength of inhibition of amacrine cells on RGCs?
- Connect SACs to RGC depending on side
- Count synapses between cells
- Measure the IPSPs from both sides
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How do you test models of direction selectivity with "anatomy"?
Probing anatomical basis of DS with correlated functional imaging and electron microscopy reconstruction
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How do you test direction selectivity with functional calcium imaging?
Take a piece of retina and label all cells with color dyes sensitive to activity
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What do the circles and colors in functional calcium imaging of the retina mean?
- Circles = direction selectivity
- Colors = preferred direction
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What kind of electron microscopy is used to see direction selectivity?
- "serial block face" EM
- Stacked on top of each other to get an image
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- Black - amacrine cell
- Colored dots - amacrine presynaptic terminals
- Dashed lines - RGC dendritic fields
- Color indicates preferred direction of RGC
- See that amacrine cells synapses mostly on the null side of RGC (look at RFs)
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How are the different local visual features computed?
- Retinal circuits compute local visual features by comparing across nearby pixels
- Features are computed by distinct retinal subcircuits, which project the info to distinct brain targets
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What are the different kinds of motion detection in the retina?
- Approaching motion
- Global motion
- Differential motion
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What's approaching motion?
Something coming at you
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What's global motion?
- Optic flow
- Moving through space
- Side to side?
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What's differential motion?
- Figure vs. background
- Ex. guy running past trees
- Use background as reference?
- Local firing rate is higher than global
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What are local features of the retina?
- Brightness (luminance)
- Contrast
- Color
- Size?
- Motion
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How do retinal circuits compute brightness/luminance?
- ON,OFF pathways
- Cone vs. Rod pathways
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How do retinal circuits compute contrast?
Horizontal cells
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How do retinal circuits compute color?
L-, M-, S-cone pathways
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Where are the multiple, parallel output channels?
Optic nerve
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What kind of computed info is projected to the visual cortex?
Spacial contrast and color (fine and course scale)
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What kind of computed info is projected to the brainstem, cerebellum?
- Local motion
- Global motion
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What does the visual cortex turn the spatial contrast and color into?
Form and object perception
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What does the brainstem, cerebellum turn local motion into?
- Pursuit eye movements
- Motion perception
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What does the brainstem, cerebellum turn global motion into?
- Flow field motion
- Stabilizing eye movements
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What kind of computed info is projected to the brainstem, hypothalamus?
Global luminance
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What does the brainstem, hypothalamus turn the global luminance into?
- Pupillary light reflex
- Circadian rhythms
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