1
Q
what is an ERG ?
A
is a recording of changes in the resting potential in the retina when stimulated with a brief flash of light
2
Q
what does an ERG show ?
A
it can show breaks or weaknesses in the retinal circuitry which indicates retinal disease
3
Q
how does an ERG record changes in the resting potential in the retina ?
A
it does this by measuring changes in the electric current flowing through the eye after a light stimulus
4
Q
what is electric current ?
A
- potential difference in volts (V) between two points is needed to induce a current
- the potential difference and resistance of the tissue determines the current size
5
Q
what is evoked potentials ?
A
- you do something to manipulate current flow and record what happens
- transient responses that occur in response to a stimulus
- voltage changes within each cell are tiny but cells in the visual pathway are highly ordered
- can measure the summed electrical response from all the retinal cells
6
Q
how are evoked potential from the retina measured ?
A
- evoked potentials from the retina are measured using the electoretinogram ( ERG)
- electrode at the cornea and on the temple detects current flow through the eye
- posterior - anterior - external - back of eye
- displayed as changes in potential difference
7
Q
how are transient evoked potentials measured ?
A
- transient evoked potentials are measured by an electrode on the cornea and temple
8
Q
what are the three types of electrode used ?
A
- reference
- earth/ground
- active
9
Q
what does the exact waveform depend on ?
A
depends on
- stimulus wavelength
- stimulus intensity
- stimulus duration
- retinal adaption ( scotopic/mesopic/ photopic )
. these parameters can be modified to investigate different aspects of visual function
10
Q
what are different forms of electrodes to get an ERG recording ?
A
. electrode that sits perfectly around the cornea
. silver impregnated cotton thread which you lie in the lower fornix
. J shaped gold plated foil - you hook into lower fornix
. electrode into lower eyelid and temple
11
Q
what are the methods of light stimulation for ERGs?
A
- several methods for stimulating the eyes
. strobe lamps - can vary intensity and duration of light
. ganzfeld
12
Q
what does light stimulus consist of ?
A
- the light stimulus consists of flashes of light for 2s.
- thus each flash of light is shorter than the integration time of photoreceptors
13
Q
what is the transmission of nerve impulses in retinal receptors and neurons mediated by ?
A
- the transmission of nerve impulses in retinal receptors and neurons is mediated by changes in electrical potential across the cell membrane, which results in discharge (changes in potassium ions inside and outside the cell )
14
Q
what happens in most nerves ?
A
- in most nerves, action potentials generate the discharge and there is a baseline level of continuous discharge. hence changes in frequency or rate of discharge in the nerve, an increase normally representing an increase in activity and a decrease being inhibition
15
Q
what are the only retinal cells that generate action potential ?
A
- retinal ganglion cells generate action potential
- bipolar, photoreceptors theses generate only graded potentials, and are either positive or negative discharges
16
Q
what is signal initiation ?
A
- the electrical response in the retina is initiated by phototransduction
- normal cell -ve inside cell, +ive outside . maintained by Na+ and K+ ion on either side
- when stimulated, there is a gradual depolarization ( inside becoming more +ve). due to influx of Na+ through opened ion channels
17
Q
what happens in photoreceptors ?
A
- hyperpolarisation
- in the dark, ‘resting conditions’ the outer segment is depolarized as its Na+ channels are open
18
Q
what can ERG detect ?
A
- an ERG detects these small electrical potential generated through the retina
19
Q
how do ERG recordings look like ?
A
- they have specific, repeatable waveforms with each peak/trough giving info about different groups of retinal cells
- can be rod dominated or cone dominated
20
Q
what is the a- wave in full field/global ERG ?
A
- sometimes called the late receptor potential is the first negative wave. this reflects the hyper polarisation of the photoreceptors (rods and/or cones )
21
Q
what is the b-wave in full field/global ERG?
A
- the large positive wave. this reflects the state of the inner retina; the Muller cells and ON-bipolar cells
- two theories
1. the Muller cell hypothesis
ON-bipolar leaks k+ changing the Muller cell’s membrane potential
1.ON-bipolar cell hypothesis
ON bipolars are directly responsible for the b-wave
22
Q
what is c- wave in full field/global ERG?
A
RPE
23
Q
what is d-wave in full field/ global ERG?
A
- off-bipolar cells of the retina
light turned off at this point
24
Q
when is RPE response rarely seen ?
A
- the RPE response is rarely seen in a cone dominated response ( photopic)
25
what is oscillatory potentials ?
- you get three OP1, OP2 , OP3
- these are fluctuations in the potential difference by activity from amacrine cells
- inner retinal input to b-wave
. background mesopic
- represents the high frequency component (100-300Hz) of the b-wave
- superimposed on the rising edge when a bright stimulus is used
26
how can you use full-field/global ERG to look at ganglion cells ?
- negative wave
- immediately follows the photopic b-wave
- best recorded as red flash ( stimulate cones ) against a blue background ( suppresses rods)
- PhNR probably reflects ganglion cell activity
27
what information can you get from ROD dominated ERG ?
- information about photoreceptors , bipolars and RPE
28
what information can you get from CONE dominated ERG ?
- photoreceptors , bipolar cells and ganglion cells
29
what information can you get from mesopic ERG , bright flash ?
oscillatory potentials
30
what is the ERG waveform interpretation ?
- latency - time interval - between onset of the stimulus and the beginning of a wave response ( normally - 2ms)
- implicit time - the time from the onset of the light stimulus until the maximum a-wave and b-wave response ( normally <0.25 seconds )
- amplitude - trough to peak measurement
31
how can rod and cone activity can be significantly isolated ?
- by manipulating adaptation level and background illumination, flash intensity, colour of the flash and rate of stimulation
32
what happens if you have a photopic ERG but a loss of rods?
- no difference in photopic ERG
33
what happens in scotopic ERG and fewer rods ?
- decrease in amplitude of a-wave and a decrease in amplitude of b-wave
34
what happens in scotopic ERG if cones are affected ?
- scotopic ERG will be fine
35
what happens in photopic ERG if cones are affected ?
- a decrease in a -wave and b- wave amplitude
36
what happens if photoreceptors are fine but Bipolar cells are affected ?
- a-waves is fine but b-wave is reduced
37
what happens when the stimulus is presented at a low frequency ?
- when the stimulus is presented at a low frequency you obtain a transient ERG with specific waveforms
38
what happens when the stimulus is presented at higher temporal frequencies ?
- when the stimulus is presented at higher temporal frequencies ( above 8hz) there's no time for retinal recovery between flashes and you obtain a 'steady state' ERG ( repeating waveform ) . usually 30Hz are used for this ( referred to as a flicker ERG )
39
what is flicker ERG?
- uses the difference in the speed of the rod (slow) and cone(fast) responses to isolate rod and cone driven function in the retina
- flicker cone response: under photopic condition to use a repetitive stimuli
10-30Hz
- rods are incapable of responding
- amplitude measured - trough to crest of each wave, this will decrease as flicker increases
40
how to use pattern ERG for the central retina?
- stimulus is a pattern instead of a flash
- pattern of a checker board or grating
- pattern changes while over all luminance stays the same
- <20% of the retina is stimulated ( central retina )
- contrast sensitivity is measured
41
what are the types of stimulus in pattern ERG?
1. pattern reversal - squares go from black to white to black again
- this is most common stimulus
2. pattern onset-offset- pattern appears and disappears against a luminance matched grey background
42
how does pattern reversal ERG look like ?
- 2 peaks
- labelled p for positive or N for negative with a number denoting implicit timing
- N35, P50, N95
- disease reduces PERG amplitude as implicit times are constant
43
what is clinical use for PERG ( pattern ERG)?
- full field flash can be used to assess peripheral retina
- PERG can be used to assess the central retina
- in macular disease P50 and N95 show a proportional reduction in amplitude
- N95 is also reduced in diseases of the ganglion cells and optic nerve damage (e.g. glaucoma) but P50 is normal in these instances
- thus PERGs can be used to differentiate between inner and outer retinal disease
44
what is multifocal ERG ? (mfERG)
- topographical measurement of retinal activity
- used to assess cone function in central retina
- several retinal areas are stimulated by multiple sequences at the same time
- responses from different regions of the retina are recorded simultaneously
- therefore can detect local responses where as full field ERGs only record a summed response
- clinical disease AMD
45
how to do multifocal ERG?
- stimulus of hexagon arrays
- each hexagon generates a waveform
- gives an assessment of localised cone system function across the posterior pole
46
how to interpret mfERG waveform ?
- N1 includes contributions from the same cells that contribute to the a-wave of the light-adapted, full-field ERG- photoreceptors
-P1 and N2 include contributions from the cells contributing to the light-adapted b-wave and oscillatory potentials
ON bipolar/Muller cell and inner retina
47
how does retinitis pigmentosa look like in mfERG?
- the light gray, dark gray and black squares indicate statistically significant field loss at the5, 1 and 0.5 percent level , respectively
48
what is retinitis pigmentosa ?
- gradual loss of rods
- followed by loss of cones
- significant loss in ERG response
- rods are non functional
- some b-waves visible from cone response
49
what is retinoschisis?
- schisis of retina, separating PRs from inner retina
- splitting of retinal layers
- use scotopic white light to assess rods. A wave fine.
- reduced b wave amplitude - photoisomerisation can occur, but no information passed through retina
- no b-wave
50
what is retinal vascular disease ?
- avascular appearance to retina
i. e. CRAO or CRVO
- an ERG with NO b wave
- occluded ophthalmic artery would elicit no ERG waveforms
- focal lesions will reduce full field ERG proportionally
- CRVO would decrease implicit time of 30Hz flicker to beyond 35ms
OV2023 visual perception
flashcards
Decks in class (20)
# Cards
-
the human brain42
-
brain and its relations34
-
Cerebral Cortex94
-
Cerebellum27
-
essential concepts60
-
retina and fovea specialisations57
-
retinal image processing and receptive fields49
-
Retinal Projections: Parallel processing pathways68
-
Visual representations52
-
primary visual (V1) cortex: functional organization37
-
Extra-striate cortex: functional organisation51
-
Colour perception56
-
how to assess human visual function37
-
electroretinogram50
-
Contrast sensitivity49
-
dark adaptation57
-
limits of vision: image formation48
-
limits of vision: visual acuity39
-
limits of vision: luminance detection28
-
temporal vision: Flicker detection48
