MCB 161 Lec 7 Retinal Processing II

  1. Why is motion detection essential?
    For survival
  2. Which cells are direction selective?
    Retinal ganglion cells
  3. What does it mean to be direction selective?
    RGC's firing depends on the direction of the moving object that passes the eye
  4. What are the preferred directions of ON-OFF direction selective cells?
    • Superior (up)
    • Inferior (down)
    • Nasal
    • Temporal
  5. What are the hypothetical circuit models for motion detection?
    • Cross-correlation
    • Inhibitory "veto" model
  6. What does motion detection require?
  7. Why is asymmetry required for motion detection?
    Without asymmetry, you get the same response from all directions
  8. What is the Δτ on the motion detection models?
    The delay (longer) line
  9. What's the point of the delay line?
    Allows summation at optimal time (in preferred direction)
  10. 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
  11. What's another way of saying delay?
  12. In a simple model cross-correlation model, what is the preferred direction if the delay line is on the left?
    From left to right
  13. What is a likely reason for the inhibition in the inhibitory model?
    Probably an inhibitory interneuron
  14. Why does the "and not" gate exist?
    Because of summation and subtraction
  15. Why does direction matter in an inhibitory model?
    Inhibition subtracts from excitation
  16. In an inhibitory circuit model, what is the preferred direction if the delay line is inhibitory and on the left?
    From right to left
  17. 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??
  18. What are the different ways to test the models of direction selectivity?
    • Pharmacology
    • Recordings
    • Cell ablations
    • Anatomy
  19. How does "pharmacology" test models for direction selectivity?
    GABA receptor blockade on inhibitory synapses eliminates direction selectivity
  20. 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
  21. 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
  22. How does "recording" test models of direction selectivity?
    Directly measuring temporal relationship of excitation and inhibition (using a voltage clamp)
  23. What does a recording show when an object goes in the preferred direction?
    • There are higher peaks of excitation than inhibition
    • Excitation preceded inhibition
  24. What does a recording show when an object goes in the null direction?
    • There are higher peaks of inhibition than excitation
    • Inhibition precedes excitation
  25. 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
  26. What model was the recordings consistent with?
    Model that inhibition should precede excitation
  27. What neuron is the essential source of direction inhibition on RGCs?
    Starburst amacrine cells (SACs)
  28. What are some characteristics of SACs?
    • Don't fire APs
    • Have axons and dendrites in the same areas
    • Releases from dendrites too
  29. What makes the SAC inhibitory?
    Dendrites release GABA onto ganglion cells
  30. How do "ablations" test models of direction selectivity?
    Immunotoxin ablation of SACs eliminates DS
  31. 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
  32. Define ablation
    Removal of body tissue
  33. What can you see [in the firing] once you kill the amacrine cells with a toxin?
    • There's no selectivity
    • Mostly radially symmetric
  34. What does the radial symmetry [once the amacrine cells are killed off] mean?
    Shows that the cells are at least necessary
  35. Remember, what do we need to get direction selectivity?
  36. 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
  37. 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
  38. How do you test models of direction selectivity with "anatomy"?
    Probing anatomical basis of DS with correlated functional imaging and electron microscopy reconstruction
  39. 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
  40. What do the circles and colors in functional calcium imaging of the retina mean?
    • Circles = direction selectivity
    • Colors = preferred direction
  41. 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
  42. Image Upload 1wut
    • 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)
  43. 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
  44. What are the different kinds of motion detection in the retina?
    • Approaching motion
    • Global motion
    • Differential motion
  45. What's approaching motion?
    Something coming at you
  46. What's global motion?
    • Optic flow
    • Moving through space
    • Side to side?
  47. What's differential motion?
    • Figure vs. background
    • Ex. guy running past trees
    • Use background as reference?
    • Local firing rate is higher than global
  48. What are local features of the retina?
    • Brightness (luminance)
    • Contrast
    • Color
    • Size?
    • Motion
  49. How do retinal circuits compute brightness/luminance?
    • ON,OFF pathways
    • Cone vs. Rod pathways
  50. How do retinal circuits compute contrast?
    Horizontal cells
  51. How do retinal circuits compute color?
    L-, M-, S-cone pathways
  52. Where are the multiple, parallel output channels?
    Optic nerve
  53. What kind of computed info is projected to the visual cortex?
    Spacial contrast and color (fine and course scale)
  54. What kind of computed info is projected to the brainstem, cerebellum?
    • Local motion
    • Global motion
  55. What does the visual cortex turn the spatial contrast and color into?
    Form and object perception
  56. What does the brainstem, cerebellum turn local motion into?
    • Pursuit eye movements
    • Motion perception
  57. What does the brainstem, cerebellum turn global motion into?
    • Flow field motion
    • Stabilizing eye movements
  58. What kind of computed info is projected to the brainstem, hypothalamus?
    Global luminance
  59. What does the brainstem, hypothalamus turn the global luminance into?
    • Pupillary light reflex
    • Circadian rhythms
Card Set
MCB 161 Lec 7 Retinal Processing II
MCB 161 Lec 7 Retinal Processing II