Lecture 3a- mfERG (multifocal ERG) Flashcards
What is the mfERG
The mfERG is a recording of responses from multiple discrete areas of retina
What is the mfERG used for
It is primarily used to assess spatial variations in cone function such as discrete retinal lesions involving an area too small to affect the ERG (e.g. enlarged blind spot)
How does the mfERG work
It employs a structured stimulus consisting of multiple elements which stimulate many areas of the retina simultaneously (Figure 3.1). Each element (hexagon) flashes following a pattern of ‘ons’ and ‘offs’ determined by a maximum length or ‘M’-sequence e.g. 000100110101111 (this may be described as ‘pseudorandom’).
What is the pattern of the mfERG stimulus called
contrast reversing stimulus
How is a minature ERG given
individual responses are deconvolved from the mass response
What does the m sequence ensure
it ensures that for each element all other responses are noise and averaging out
this is done in the same way as signal averaging. 1st and higher order responses are possible
What is needed to account for spatial variation in cone density throughout the retina
Scaling
Which cones are at the maximum density at the fovea
L and M cones
-they thin out towards the periphery
What cant be found in the fovea
Rods or S cones
As elements increase in size and eccentricity what happens
approximately equal sized responses occur
What type of electrode is used to record mfERG
DTL thread electrodes as they dont interfere with vision
Why are the pupils dilated for the mfERG
for consistent and repeatable retinal illuminance
What is less critical compared to the PERG
focus and constrast
What is critical for the mfERG
A + S
alignment and steady fixation
How are the amplitudes of the main positive peak represented
a surface plot to aid visualisation, although the
trace arrays contain much more information
