Patient presents with an iron foreign body in the eye. Which of the following is the best investigation to monitor his vision?

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 (fined 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 (photoreceptors). B-wave. It is a large positive wave which is generated by Muller's cells, but represents the activity of the bipolar cells . C-wave. It is also a positive wave representing metabolic activity of pigment epithelium (seen only in dark adapted eye). Both scotopic and photopic responses can be elicited in ERG. Foveal ERG can provide information about the macula. Components of normal electroretinogram (ERG) Arden ratio is calculated in electrooculography and is not useful to monitor vision. VEP represents mainly macular response and not the entire retina and is thus not useful. Dark adaptometry measures the dark adaptation time and not useful to monitor vision.