 By contrast, fundus perimetry, produces reliable results even in patients with unstable or eccentric fixation., or advance macular degeneration.When central vision is compromised, as in the case of macular scotoma, patients develop an eccentric or extra-foveal vision, normally with unstable fixation.The retinal area used by eccentric viewers to substitute the foveal vision is known as the Preferred Retinal Locus (PRL) In Microperimetry systems, the fundus (eye) is imaged in real time, while an eye tracker compensates eye movements during stimuli projection, allowing correct matching between expected and projected stimulus position onto the retina.