ERG Flashcards
What is an ERG?
An ERG is a recording of changes in the resting potential in the retina when stimulated with a brief flash of light
Why do we conduct ERGs?
They can show ‘breaks’ or ‘weaknesses’ in the retinal circuitry. Which indicates retinal disease. [It does this by measuring changes in the electric current flowing through the eye after a light stimulus]
What determines the current size of a tissue?
The potential difference and resistance of the tissue determines the current size.
What is potential difference?
The change in voltage between two points
What are ‘evoked’ potentials?
Basically when you do something to induce a response. “Transient responses that occur in response to a stimulus”
Although Evoked potentials are on a cellular level ( i.e. you add a stimulus on a cell by cell level) why can you assume responses are representative of the whole retina and therefore paint a reliable picture?
The retina is incredibly organised and structured not only in terms of cells but in it’s responses too thus any pattern seen in one cell represents cells all over the retina ( assuming all retinal cells are healthy).
Can you measure the summed electrical responses from all retinal cells (or all cells in the visual cortex)?
Yes - This is called a VEP
What are evoked potentials in the retina measured via?
An electroretinogram (ERG)
What is the principle behind how an electroretinogram (ERG) works?
There is an Electrode at the Cornea and on the temple detects current flow through the eye and clauclates the potential difference.
Naturally the eye has a current flowing through it as shown by the blue circuit. The current starts at the retina cuts through the middle of the eye, working its way from posterior to anterior before it follows around back to the retina ( around the side of the eye).
What are the three types of electrode used in ERGs and why?
A reference electrode, an active electrode and an earth/ground electrode.
A reference electrode is used to measure current at the temple (and thus placed at the temple).
An active electrode is used to measure current at the cornea ( and thus placed on the cornea)
Your body has lots of ‘background circuits’ occuring all over your body thus an Earth/ground electrode is place anywhere nearby ( e.g. on forehead) so that when it comes time to interpret waveforms, the background circuits can be filtered out.
What does the waveform recieved from the ERG depend on?
- Stimulus Wavelength
- Stimulus Intensity
- Stimulus Duration
- Retinal Adaption (scotopic/mesopic/photopic) (i.e. the conditions you did it in and thus responses from particular photoreceptors).
[These parameters can be modified to investigate different aspects of visual function]
What may source of lighting may be used to stimulate the eyes in an ERG?
Strobe Lamps
Ganzfield lights
What are graded potentials?
Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none ( like action potentials).
Which cells in the retina generate action potentials and which cells generate graded potentials?
Retinal Gnaglion Cells generate action potentials.
All other cells such as bipolar cells, photoreceptor cells, etc, generate graded potentials. (These are either positive or negative).
What is phototransduction?
The process by which light is converted into electrical impulses in rod cells, cone cells and photosensitive ganglion cells of the retina of the eye,
What is a Full-Field/Global ERG?
It is an ERG taken when you are exposing all of your retina to a light stimulus.
(So response you recieve is from all of your retina all at once)
Describe the following ERG wave
The A wave reflects the hyperpolarisation of the photoreceptors (rods and/or cones). This is sometimes called the ‘late receptor potential’ or first negative wave
The B wave reflects large depolarisation where ON-Bipolar cells are depolarised. These ON-Bipolar cells leak K+ changing the Muller Cell’s membrane potential (as they are nearby). Thus this depoalrisation is caused by ON-bipolar cells and Muller Cells.
The C wave reflects potential chnage in RPE
When you turn the light off you get the D wave which is the repsonse as bipolar cells are turning off.
Is an RPE response typically seen in a cone dominated response?
No- the RPE response is rarely seen in a cone dominated (photopic) response.
e.g. refer to the picture
[Equally D wave is rarely seen in a rod response]
What are oscillatory potentials, what are they due to and what can we learn from them?
Fluctuations on the B wave that can be seen if conditions are perfect to do so. These fluctuations are due to potential differences caused by amacrine (inner retinal origin) activity.
From looking at these fluctuations we can infer information about the state of amacrine cells.
[Conditions to isolate Oscillatory potentials are a mesopic background and exactly the righ bright stimulus]
What is the PhNR and what is it suspected to reflect?
It is the negative wave that follows the B wave.
It is suspected to reflect ganglion cell activity.
Summarise the different waves seen on an ERG and what is responsible for each one.
What principle measurements are taken from an ERG waveform?
What is the difference between a flicker ERG and an ERG?
What are the two types of stimuli you can have for pattern ERG (PERG)?
Can a PERG be used to diffrentiate between differentiate between inner and outer retinal diseases?
Yes
What is the stimulus of a mulitfocal ERG and why is it used?
Describe the waveform output you recieve from a multifocal ERG (mERG).
It is basically exactly the same as a global ERG waveform output except with different names. The N1 wave is our a wave and the P1 wave is our B wave.
(It was designed by programmers not biologists hence the differences in terms).
You get a waveform per an area.
How does a multifocal ERG (mERG) output look?
A graph which represented the entire MACULA with multiple waveforms on it - i..e a topogrpahical representation.
