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