True about electroretinogram is

Electroretinography (ERG) is the record of changes in the resting potential of the eye induced by a flash of light. It is measured in dark-adapted eye with the active electrode (fitted on contact lens) placed on the cornea and the reference electrode attached on the forehead Normal record of ERG consists of the following waves a wave. It is a negative wave possibly arising from the rods and cones. b-wave. It is a large positive wave which is generated by Muller cells but represents the activity of the bipolar cells. c-wave. It is also a positive wave representing the metabolic activity of pigment epithelium. Reference: A K KHURANA COMPREHENSIVE OPHTHALMOLOGY, Edition 4, page-488,489