Lec 3

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Lec 3
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2015-04-16 08:53:05
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  1. Why do we measure physiological function?
    to understand how size and shape varies with development
  2. What does the electrophysiological assessment allow?
    the assessment of the electrophysiological function of the retina and visual system
  3. Major types of electrophysiological assessments (3)
    • EOG
    • ERG
    • VEP
  4. What types of ERG are there?
    • bright flash ERG
    • pattern ERG
  5. What types of VEP are there?
    • flash VEP
    • pattern VEP
    • special VEP
  6. What is the standing potential in an EOG?
    • corneo-fundal potential
    • difference in electrical potential between the front and back of the eye
  7. How is the potential affected in the EOG?
    • potential is mainly from RPE
    • changes in response to background levels of retinal illumination
  8. What happens to the potential in an EOG when the retinal illumination is changed?
    • switch into darkness: potential decreases for 8-10 mins
    • subsequent retinal illumination: potential falls over 60-75s then slow but large rise over 7-14mins
  9. What is the fast oscillation in an EOG?
    the potential decrease over 60-75secs after subsequent illumination after darkness
  10. What is the light response in an EOG?
    the slow but larger rise in potential over 7-14 mins following the fast oscillation phase
  11. How is the EOG conducted?
    • electrodes are placed at inner and outer canthi
    • eyes move horizontally L and R
    • potentials measured will change in time to produce a square wave
  12. The square wave is _______
    a fixed proportion of the standing potential
  13. If the medial canthi is more -ve, then then difference is ____________ in the potential in an EOG
    -ve
  14. When the px is looking straight ahead, the potential is _______ in an EOG
    0
  15. The square wave shows the __________ between the ______________
    • potential difference
    • cornea and retina
  16. The square wave shows the _________ size of the data and not the _________ size
    • relative
    • absolute
  17. How is EOG recorded?
    • as the Arden ratio
    • max light amp/min amp in dark
  18. What ratio is normal in the EOG?
    >1.8
  19. Why use an EOG?
    • to monitor eye mvts as it's able to get a direct measurement off the RPE
    • to measure PR and RPE development
  20. The max light amp represents what function in an EOG?
    PR
  21. The min light amp in the dark for EOG represents?
    RPE function
  22. What is the standardised ERG?
    an electrophysiological test for retinal function
  23. ISCEV protocols are named according to the _______ and the ________
    • state of adaptation
    • stimulus
  24. What is the stimulus measured in in an ERG?
    flash strength in cd.s.m^-2
  25. Can ERGs be compared world wide and why?
    • yes 
    • ISCEV wrote a standardised basic clinical ERG protocol
  26. What are the ISCEV standard ERG protocols? (6)
    • Dark adapted 0.01 ERG
    • Dark adapted 3 ERG
    • Dark adapted 10 ERG
    • Dark adapted oscillatory potentials
    • Light adapted 3 ERG
    • Light adapted 30 Hz flicker ERG
  27. What does the dark adapted 0.01 ERG test?
    Rod-driven response of ON bipolar cells
  28. What does the dark adapted 3 ERG test?
    • the combined responses from PR and bipolar cells of both rod and cone systems
    • rod dominated
  29. What does the dark adapted 10 ERG test?
    combined response with enhanced a-waves which reflect PR function
  30. What does the dark adapted oscillatory potentials test?
    responses mainly from amacrine cells
  31. What does the light-adapted 3 ERG test?
    • responses from the cone PR
    • a waves from the cone PR and OFF-bipolar cells
    • b waves from the ON and OFF cone bipolar cells
  32. What does the 30Hz flicker ERG test?
    sensitive cone-pathway-driven response
  33. What is an ERG?
    a mass electrical response of the retina to light stimulation
  34. What does the standard ERG produce?
    • a wave
    • b wave
    • oscillatory potentials
  35. What does the non-standard ERG produce?
    • c waves
    • d waves
  36. What is an a-wave?
    • produced in full field standardised ERG elicited by a bright flash
    • reflects physiological health of the outer retinal layers
    • i.e. PR 
    • earlier response as closer to the electrodes
  37. What is a b-wave?
    • produced in a full field standardised ERG elicited by a bright flash
    • reflects physiological health of the inner retinal layers
    • i.e. ON bipolar and muller cells
  38. What are the quantifiable components of the ERG?
    • amplitude 
    • peak latency = implicit time = from when the flash was presented
  39. How is retinal function evaluated?
    disorder of the retina shows up as decreased amp/change in the implicit time
  40. What response dominates in an ERG following a white flash?
    • massed rod response
    • 120mil rods vs 6-7mil cones
  41. What can be optimised to differentiate between a rod and cone response? (5)
    • adaptation level
    • background illumination
    • flash intensity
    • colour of flash
    • rate of stimulation
  42. What is the preferential stimulation to elicit a rod response? (4)
    • use a slow flash response
    • 510nm light of low intensity 
    • use dim red flashes - rods are 3 log units more sensitive than cones
    • scotopic background conditions
  43. What is the preferential stimulation to elicit a cone response? (3)
    • 30Hz flicker - rods cannot manage 20Hz
    • 560nm light - tennis ball yellow is the peak sensitivity = maximal cone response
    • photopic background conditions - suppresses rod response as rod photopigments should be bleached
  44. What is the preferential stimulation for amacrine cells?
    • oscillatory potentials seen on the ascending b wave
    • filter out low freq responses (>100Hz) to filter out a and b waves
    • glycine inhibits amacrine functon
  45. What is glycine?
    an inhibitory transmitter in the retina associated with amacrine cells
  46. What is the ERG procedure?
    • electrodes contact overlying cornea, bulbar conj or skin of the lower lid
    • the electrode comprise of the +/active input for recording
  47. What types of ERG are there? (6)
    • CL electrodes
    • conductive fibres
    • conductive foils
    • conjunctival wire loops
    • corneal wicks
    • skin electrodes
  48. What is the ERG-Jet CL electrodes?
    • a rigid lens with a thin gold ring of some angstrom on the concave surface
    • eye is anaethetised and dilated (pupil constrictions --> noise)
  49. What is the DTL Plus conductive fibre electrode?
    • 2.5in of silver-impregnated nylon-thread affixed to two self adhesive pads which attach to the inner and outer canthus
    • electrode terminated with 1mm pin attached to one of the pads that needs an extra cable for connection to recording equipment
  50. What is the HK look electrode?
    • ERG recorded with a thin insulated flexible silver wire that avoids direct contact with the cornea
    • teflon insulation provides effective screening from electrical noise and prevents shunting
    • constant size of contact window assures even registration of ERG signlasls
  51. What is the gold foil electrode?
    • gold foil hooks onto lower lid
    • one of the easiest to apply
  52. What is the skin electrode?
    • a commercial system
    • sticks to the front of the skin
  53. What are the cotton wick and burian speculum type electrodes?
    the original designs which keeps the lids apart
  54. Comparatively, which ERG electrodes should be used?
    • burian allen and gold foil measures similarly
    • skin electrode has a much lower amp measured as the signal has to travel through the lower lid
    • after scaling, max amp, morphology and shape is similar
  55. Gold foil vs Burian Allan electrodes
    • b wave amp similar 
    • a wave amp BA significantly > GF
    • b wave peak latency GF significantly > BA
    • a wave peak latency GF significantly < BA
  56. Skin electrode vs GF and BA
    • skin electrode response is 1/4-1/2 size of GF
    • skin electrode and BA response is similar
    • scaling is similar for all
  57. Benefit of skin electrodes
    may be used when eye-contact with electrode is not possible
  58. Limitation of traditional global/full field ERG is?
    • recording is a massed potential from the whole retina
    • >20% retina affected = abnormal ERG otherwise it is normal
    • normal ERG = legally blind person with MD, enlarged blind spot, other small scotomas
  59. What is multifocal ERG?
    • mfERG adapted mathematical sequences (binary m-sequences)
    • program that can extract hundreds of focal ERGs from single retinal signal
    • small scotomas can be mapped and degree of retinal dysfunction quantified
  60. If ERG and EOG are normal but VEP isn't then the problem lies in which part of the pathway?
    after the retina
  61. What is a VEP?
    • visual evoked potential
    • a plot of the voltage potential difference of the visual cortex over time in response to visual stimulation
  62. What does the VEP test?
    • the function of the visual pathway from the retina to the occipital cortex
    • measures the conduction of the visual pathways from the optic nerve, optic chiasm, and optic radiations to the occipital cortex
  63. Where do we place the electrodes in VEP?
    • V1
    • Cz motor cortex
    • Fz frontal cortex
  64. How do we find where to put the electrodes?
    • find calcarine sulcus: from nasion to the inion (33cm M 30cm F)
    • V1: 10% above (3cm)
    • Cz: halfway ~15cm
    • Fz: 20% forward from Cz
  65. What do we connect V1, CZ and FZ to?
    • - active
    • + reference
    • Earth
  66. What is a stimulus that can be used to measure the VA with VEP?
    checkerboard pattern reversal
  67. What is the criteria for choosing a stimulus for VEP?
    • part of visual system to be stimulated
    • a quickly reversing stimulus stimulates the cones
  68. What are the features of a stimulus with pattern reversal? (2)
    • a fixation spot
    • black and white can be reversed
    • variable size
  69. Describe the normal pattern-reversal VEP
    • baseline: 0 = same level of excitiation = time it takes for signal from retina to get to electrode
    • negative deflection: occipital cortex less excited than motor cortex
    • positive deflection: occipital cortex more excited than motor cortex
  70. When is it noticed that the first signal is received in a normal pattern-reversal VEP?
    75 microseconds
  71. When do we get a pattern reversal VEP?
    everytime the stimulus reverses
  72. What is the pattern onset offset stimulus?
    the stimulus appears then disappears
  73. What do we need to be careful of when using the pattern onset offset stimulus?
    • when the stimulus disappears, only the quality that we are interested in also disappears
    • e.g. luminance of background and colour are the same so that if contrast is normal but poor CV = no response
  74. We get the ______ VEP if the ________ stimulus is used
    • pattern
    • pattern onset offset
  75. What points are seen in the response of a pattern VEP? Is this response both onset and offset or only onset?
    • C1
    • C2
    • C3
    • onset
  76. What should we record about the VEP?
    • at least 2 diff recordings
    • everything about the stimulus
  77. As spatial frequency __________, VEP amplitude _______ and allows us to assess ______ and not __________
    • increases
    • decreases
    • how well V1 is at processing the visual stimuli
    • perception
  78. Problem with VEP (3)
    • ambiguity
    • unable to distinguish points with 2 recording overlaps
    • brain is plastic: individual VEP is different
  79. Alternative methods to VEP (2)
    • Steady state VEP
    • Objective methods
  80. What objective alternatives are there to VEP? (2)
    • T2 circ statistics based on linear maths (everything adds up)
    • Correlation dimension (D2) derived from non linear dynamical analysis (doesn't add up in normal space but does in other space)
  81. Why is the individual response variable in VEPs? (2)
    • brains are noisy: if noise is removed, nice signal achieved
    • brains are chaotic: nonlinear interpretation
  82. If variability is believed to be non linear, how do you force the visual system to be linear?
    by having a very high temporal frequency stimulus --> partial stimulus is summated --> sinusoidal wave
  83. Advantage of sinusoidal waveform
    Understandable according to linear mathematics --> Fourier analysis --> one amplitude is given
  84. What is Fourier analysis?
    it says that any wave form can be broken down into sine and cos wave of different wavelengths and amplitude

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