ERG is an eye test used to detect abnormal function of the retina due to inherited or acquired diseases such as retinitis pigmentosa and plaquenil toxicity. During the test, an electrode is placed on the cornea (at the front of the eye) to measure the electrical responses to light of the cells that sense light in the retina at the back of the eye. An ERG can also be useful in determining if retinal surgery or other types of ocular surgery such as cataract extraction might be useful. It is the only test that can detect plaquenil toxicity before significant vision loss has occurred.