Occult macular dystrophy (OCMD) is characterized by progressive decline of visual acuity in both eyes, associated with a normal fundus and normal fluorescein angiography. Patients have normal full-field electroretinograms (ERGs) but severely reduced focal macular ERGs, as recorded by conventional techniques using small stimuli under background illumination. OCMD patients are believed to have localized retinal dysfunction distal to the ganglion cells in the central retina (summary by Piao et al., 2000).