The electroretinogram (ERG) is a test developed in the late 19th century to measure the retina’s response to light. The first electroretinograph machines from the 1870s required wires and electrodes to be placed directly on a patient’s eyes, giving them a scary cyborg-like appearance. The test became clinically useful in the mid-20th century and made use of improved, less intimidating devices to diagnose retinal diseases.