MCB 161 Lec 7 Retinal Processing II
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Why is motion detection essential?
Which cells are direction selective?
Retinal ganglion cells
What does it mean to be direction selective?
RGC's firing depends on the direction of the moving object that passes the eye
What are the preferred directions of ON-OFF direction selective cells?
- Superior (up)
- Inferior (down)
What are the hypothetical circuit models for motion detection?
- Inhibitory "veto" model
What does motion detection require?
Why is asymmetry required for motion detection?
Without asymmetry, you get the same response from all directions
What is the Δτ on the motion detection models?
The delay (longer) line
What's the point of the delay line?
Allows summation at optimal time (in preferred direction)
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
What's another way of saying delay?
In a simple model cross-correlation model, what is the preferred direction if the delay line is on the left?
From left to right
What is a likely reason for the inhibition in the inhibitory model?
Probably an inhibitory interneuron
Why does the "and not" gate exist?
Because of summation and subtraction
Why does direction matter in an inhibitory model?
Inhibition subtracts from excitation
In an inhibitory circuit model, what is the preferred direction if the delay line is inhibitory and on the left?
From right to left
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??
What are the different ways to test the models of direction selectivity?
- Cell ablations
How does "pharmacology" test models for direction selectivity?
GABA receptor blockade on inhibitory synapses eliminates direction selectivity
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
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
How does "recording" test models of direction selectivity?
Directly measuring temporal relationship of excitation and inhibition (using a voltage clamp)
What does a recording show when an object goes in the preferred direction?
- There are higher peaks of excitation than inhibition
- Excitation preceded inhibition
What does a recording show when an object goes in the null direction?
- There are higher peaks of inhibition than excitation
- Inhibition precedes excitation
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
What model was the recordings consistent with?
Model that inhibition should precede excitation
What neuron is the essential source of direction inhibition on RGCs?
Starburst amacrine cells (SACs)
What are some characteristics of SACs?
- Don't fire APs
- Have axons and dendrites in the same areas
- Releases from dendrites too
What makes the SAC inhibitory?
Dendrites release GABA onto ganglion cells
How do "ablations" test models of direction selectivity?
Immunotoxin ablation of SACs eliminates DS
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
Removal of body tissue
What can you see [in the firing] once you kill the amacrine cells with a toxin?
- There's no selectivity
- Mostly radially symmetric
What does the radial symmetry [once the amacrine cells are killed off] mean?
Shows that the cells are at least necessary
Remember, what do we need to get direction selectivity?
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
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
How do you test models of direction selectivity with "anatomy"?
Probing anatomical basis of DS with correlated functional imaging and electron microscopy reconstruction
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
What do the circles and colors in functional calcium imaging of the retina mean?
- Circles = direction selectivity
- Colors = preferred direction
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
- 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)
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
What are the different kinds of motion detection in the retina?
- Approaching motion
- Global motion
- Differential motion
What's approaching motion?
Something coming at you
What's global motion?
- Optic flow
- Moving through space
- Side to side?
What's differential motion?
- Figure vs. background
- Ex. guy running past trees
- Use background as reference?
- Local firing rate is higher than global
What are local features of the retina?
- Brightness (luminance)
How do retinal circuits compute brightness/luminance?
- ON,OFF pathways
- Cone vs. Rod pathways
How do retinal circuits compute contrast?
How do retinal circuits compute color?
L-, M-, S-cone pathways
Where are the multiple, parallel output channels?
What kind of computed info is projected to the visual cortex?
Spacial contrast and color (fine and course scale)
What kind of computed info is projected to the brainstem, cerebellum?
- Local motion
- Global motion
What does the visual cortex turn the spatial contrast and color into?
Form and object perception
What does the brainstem, cerebellum turn local motion into?
- Pursuit eye movements
- Motion perception
What does the brainstem, cerebellum turn global motion into?
- Flow field motion
- Stabilizing eye movements
What kind of computed info is projected to the brainstem, hypothalamus?
What does the brainstem, hypothalamus turn the global luminance into?
- Pupillary light reflex
- Circadian rhythms
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