Volume 45(1-4):23-42, 2001 Acta Biologica Szegediensis http://www.sci.u-szeged.hu/ABS REVIEW ARTICLE 1 Department of Physiology, 3 Department of Ophthalmology, University of Szeged, Szeged, Hungary, 2 Laboratory of Vision Research, Rutgers University, NJ, USA Normal and abnormal development of visual functions in children Petra Kozma 1,2 *, Ilona Kovács 3 , György Benedek 1 ABSTRACT The human visual system goes through substantial changes during the first few months of postnatal life. The development of visual functions and structures occurs at different times and different rates. It has been a generally held belief that the development of visual functions and their critical period come to an end early in life. Most of the developmental data confirm this theory, although the findings sometimes are contradictory. Thus, our knowledge concerning visual development does not seem to be complete. The determination of exact timing of the different visual functions is relevant in children since a proved extended maturational timeframe can promote the trial of enhancement of visual abilities at a later age, up to puberty or beyond. There have already been suggestions for an extended developmental time span for some of the visual functions. Here we review the most relevant data with reference to the normal development of the eye, visual functions and visual pathways found in the literature and provide further evidence for the maturation and plasticity of visual functions after the age of 5 years. Acta Biol Szeged 45(1-4):23-42 (2001) KEY WORDS visual development critical period contour-integration amblyopia plasticity Accepted December 5, 2000 *Corresponding author. E-mail: pkozma@matavnet.hu 23 The need for understanding visual development has received increasing attention in the last four decades (Fiorentini 1984) since it is a system where work at behavioral, anatomical and physiological levels can be correlated to lead to study the basic mechanisms involved (Daw 1994). Until the early 1960s little was known about the anatom- ical and functional properties of the visual system of the newborn and of its subsequent development during infancy. A new impulse to the investigation of visual development was given by the classical, pioneering single cell studies of Hubel and Wiesel on cats who received the Nobel Prize in Medicine in appreciation of their work in 1981 (Hubel and Wiesel 1963a, b). Hubel and Wiesel deprived one or both eyes of visual impulses from birth in cats. They found more significant structural and morphological changes of the visual pathways during closure of one eye than during the closure of both eyes (Wiesel and Hubel 1965). On the other hand, closure had no effect on the eyes and on the cells in the visual cortex of adult cats. Accordingly, Hubel and Wiesel were the first who described and termed the “critical period” of vision. The critical period is a definite period of time, early in life during which the visual system is plastic and is susceptible to environmental influence as well as to abnormal visual experience (Hubel and Wiesel 1970; Barlow 1975; Wiesel 1982; Daw 1994). If stimulus deprivation occurs during this period, visual development will be impaired. Since the work of Hubel and Wiesel there has been an explosion of morpho- logical, electrophysiological and behavioral experiments that studied how postnatal visual development occurs and wheth- er and how it could be affected by early visual deprivation or by manipulations of the visual environment (Fiorentini 1984). Thus, the visual system has become the model for the understanding of plasticity (Daw 1994). The study of vision in human infants has progressed considerably in parallel with the animal studies (Fiorentini 1984). It was shown that critical periods also exist in the visual development of humans. The clearest demonstration of this similarity, came out in studies on kittens and young monkeys with experimentally induced strabismus based on the pioneering work of Wiesel and Hubel (1963a, b). The human visual system is immature at birth both anatomically and functionally and goes through substantial changes especially during the first few months of postnatal life (Atkinson 1984; Fiorentini 1984; Garey 1984). The development of the visual cortex occurs in a hierarchical order. The critical period seems to vary in onset and duration between different brain regions and even between layers of an individual cortical area. Lower levels of the visual system and deeper layers of the cortex mature earlier compared to the higher and more superficial ones (Conel 1939-1967; Harwerth et al. 1986; Daw 1994). Different functions may emerge at different times and develop at different rates (Levi