Loss of vision in people with glaucoma is due to a progressive loss of retinal ganglion cells — the cells that transmit visual information from the retina to the brain through the optic nerve. Doctors use a visual field test, also known as perimetry, to detect the resulting functional vision loss. However researchers now know that at least 50% of retinal ganglion cells are lost before vision problems are identified on standard visual field tests.
During a computerized visual field test, when you see a flash of light appear, you press a button. The most commonly used test displays white lights on a white background, but some researchers have demonstrated that short-wavelength automated perimetry (SWAP), which uses blue lights on a yellow background, may detect defects from glaucoma earlier.
Another method of checking peripheral vision is electroencephalography (EEG). Special sensors are placed on your scalp to record your brain's response to visual stimuli, such as flashing lights. Another test, frequency doubling technology (FDT) perimetry, is based on an optical illusion that can occur when viewing patterns of black and white parallel bars. Some research suggests that this illusion arises from retinal ganglion cells, which have a high tendency to get damaged in glaucoma.
Source: Johns Hopins Health Alerts 8/6/10