Fondements de la notion de période sensible du développement visuel

In higher mammals, impaired visual experience can lead to amblyopia. This occurs only during a period called « sensitive period » or « critical period » of visual development. The limits of the critical period vary, depending on the clinical or the neurophysiological experiments that define it. This chapter reviews the basis of the notion of critical period in the visual development. The neurophysiological notion of critical period is based on several experimental facts. Mature properties of visual neurons are acquired during life, until an age which marks the end of the critical period. Beyond a particular time, impaired visual experience would be without any consequence on these properties. In case of restored visual experience, the reversibility of the impairment of visual properties is possible only before a specific moment that characterises the end of the critical period. The clinical notion of critical period is based on a particular chronology of the development of visual properties. It is also noticed that impaired visual experience leads to different consequences, depending on the age at which it occurs. Furthermore, the probability of success of the treatment of amblyopia decreases with time. Correlations between neurophysiological experiments and clinical experience are not easy. The term of critical period addresses several notions, depending on the visual properties which are studied both neurophysiologically and clinically. There are not one, but many critical periods of visual development. Better understanding these notions should result in an optimisation of the therapy of amblyopia, adapted to the precise chronology of visual development and to the different functions of vision.