The challenge of testing a young infant's vision is that they cannot verbally communicate what they can and cannot see. There are many cases, such as one with Chris where a child may have sight yet not appear to have any visual response to stimuli. In these cases it is crucial to determine what is happening in the child's brain and eyes at a more direct physiological level. The VEP and ERG are both such tests that yield important data about what is happening at the physiological level and are thus especially important in the diagnosis of visual impairments in very young children.