The following electroretinograms (ERG) belongs to a patient who has poor vision in the left eye: OD OS a. What structures (see ERG of the right eye) are responsible for:  i) the a-wave ii) the b-wave? b. Suggest a common cause for the poor vision in the left eye     which would also account for the left ERG appearance.

Answer to a
a-wave results from the hyperpolarization of the photoreceptor. 
b-wave is generated by the Muller cells 
(c-wave not shown here is generated by pigment epithelium)
 

Answer to b
Left central retinal artery occlusion.
The left ERG shows absent b-wave giving the
ERG a negative deflection.
The appearance suggests extensive damage of
the Muller's cells with preservation of the 
photoreceptor. Central retinal artery occlusion is
a common cause. The photoreceptors are preserved 
because they derive their blood supply from the choroidal circulation.
 

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