Visual Development

This book will discuss visual development, leading on to visual deprivation, where the development is disturbed by faults in the optics or motor control of the eye. Visual deprivation is an important and fascinating subject from severalpointsofview:clinical,philosophical,historical,andscienti?c. Many general questions in these areas have been framed over the years in terms of the visual system. This is not surprising, because we are visual animals. If dogs ruled the world, the title of this book would beOlfactoryDevelopment. As it is, vision is our most important sense, and the ?rst that we think of in discussing scienti?c and philosophical questions. A large number of people have personally experienced some form of - sualdeprivation. Anythingthataffectstheimagesontheretinasofyoungch- dren can have lasting effects on the part of the brain that processes visual s- nals. This can occur if one eye is in focus, but the other is not; if vertical lines areinfocus,buthorizontallinesarenot;ifthetwoeyeslookindifferentdir- tions (strabismus); if the lens of one or both eyes is cloudy (cataract); or if the eyeballgrowssomuchthatobjectscannolongerbefocusedontheretina. F- quently these conditions lead to poor vision in one or both eyes as a result of changes that have occurred in the central visual system, even after the images ontheretinasaremadeclearandcoordinated. AftertheGreeks,thisisnamed amblyopia, meaning blunt vision or dull vision. The colloquial term for - blyopia islazyeye. Between 2% and 4% of the population may become - blyopic from visual deprivation. Philosophersbecameinterestedinthesubjectofvisualdeprivationwhen William Molyneux posed his famous question in the late 17th century.