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Why do we measure physiological function?
to understand how size and shape varies with development
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What does the electrophysiological assessment allow?
the assessment of the electrophysiological function of the retina and visual system
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Major types of electrophysiological assessments (3)
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What types of ERG are there?
- bright flash ERG
- pattern ERG
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What types of VEP are there?
- flash VEP
- pattern VEP
- special VEP
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What is the standing potential in an EOG?
- corneo-fundal potential
- difference in electrical potential between the front and back of the eye
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How is the potential affected in the EOG?
- potential is mainly from RPE
- changes in response to background levels of retinal illumination
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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
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What is the fast oscillation in an EOG?
the potential decrease over 60-75secs after subsequent illumination after darkness
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What is the light response in an EOG?
the slow but larger rise in potential over 7-14 mins following the fast oscillation phase
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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
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The square wave is _______
a fixed proportion of the standing potential
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If the medial canthi is more -ve, then then difference is ____________ in the potential in an EOG
-ve
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When the px is looking straight ahead, the potential is _______ in an EOG
0
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The square wave shows the __________ between the ______________
- potential difference
- cornea and retina
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The square wave shows the _________ size of the data and not the _________ size
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How is EOG recorded?
- as the Arden ratio
- max light amp/min amp in dark
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What ratio is normal in the EOG?
>1.8
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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
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The max light amp represents what function in an EOG?
PR
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The min light amp in the dark for EOG represents?
RPE function
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What is the standardised ERG?
an electrophysiological test for retinal function
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ISCEV protocols are named according to the _______ and the ________
- state of adaptation
- stimulus
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What is the stimulus measured in in an ERG?
flash strength in cd.s.m^-2
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Can ERGs be compared world wide and why?
- yes
- ISCEV wrote a standardised basic clinical ERG protocol
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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
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What does the dark adapted 0.01 ERG test?
Rod-driven response of ON bipolar cells
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What does the dark adapted 3 ERG test?
- the combined responses from PR and bipolar cells of both rod and cone systems
- rod dominated
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What does the dark adapted 10 ERG test?
combined response with enhanced a-waves which reflect PR function
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What does the dark adapted oscillatory potentials test?
responses mainly from amacrine cells
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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
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What does the 30Hz flicker ERG test?
sensitive cone-pathway-driven response
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What is an ERG?
a mass electrical response of the retina to light stimulation
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What does the standard ERG produce?
- a wave
- b wave
- oscillatory potentials
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What does the non-standard ERG produce?
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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
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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
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What are the quantifiable components of the ERG?
- amplitude
- peak latency = implicit time = from when the flash was presented
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How is retinal function evaluated?
disorder of the retina shows up as decreased amp/change in the implicit time
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What response dominates in an ERG following a white flash?
- massed rod response
- 120mil rods vs 6-7mil cones
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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
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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
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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
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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
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What is glycine?
an inhibitory transmitter in the retina associated with amacrine cells
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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
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What types of ERG are there? (6)
- CL electrodes
- conductive fibres
- conductive foils
- conjunctival wire loops
- corneal wicks
- skin electrodes
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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)
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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
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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
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What is the gold foil electrode?
- gold foil hooks onto lower lid
- one of the easiest to apply
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What is the skin electrode?
- a commercial system
- sticks to the front of the skin
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What are the cotton wick and burian speculum type electrodes?
the original designs which keeps the lids apart
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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
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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
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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
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Benefit of skin electrodes
may be used when eye-contact with electrode is not possible
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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
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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
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If ERG and EOG are normal but VEP isn't then the problem lies in which part of the pathway?
after the retina
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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
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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
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Where do we place the electrodes in VEP?
- V1
- Cz motor cortex
- Fz frontal cortex
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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
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What do we connect V1, CZ and FZ to?
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What is a stimulus that can be used to measure the VA with VEP?
checkerboard pattern reversal
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What is the criteria for choosing a stimulus for VEP?
- part of visual system to be stimulated
- a quickly reversing stimulus stimulates the cones
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What are the features of a stimulus with pattern reversal? (2)
- a fixation spot
- black and white can be reversed
- variable size
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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
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When is it noticed that the first signal is received in a normal pattern-reversal VEP?
75 microseconds
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When do we get a pattern reversal VEP?
everytime the stimulus reverses
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What is the pattern onset offset stimulus?
the stimulus appears then disappears
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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
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We get the ______ VEP if the ________ stimulus is used
- pattern
- pattern onset offset
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What points are seen in the response of a pattern VEP? Is this response both onset and offset or only onset?
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What should we record about the VEP?
- at least 2 diff recordings
- everything about the stimulus
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As spatial frequency __________, VEP amplitude _______ and allows us to assess ______ and not __________
- increases
- decreases
- how well V1 is at processing the visual stimuli
- perception
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Problem with VEP (3)
- ambiguity
- unable to distinguish points with 2 recording overlaps
- brain is plastic: individual VEP is different
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Alternative methods to VEP (2)
- Steady state VEP
- Objective methods
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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)
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Why is the individual response variable in VEPs? (2)
- brains are noisy: if noise is removed, nice signal achieved
- brains are chaotic: nonlinear interpretation
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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
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Advantage of sinusoidal waveform
Understandable according to linear mathematics --> Fourier analysis --> one amplitude is given
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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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