Introduction

Vision in mammals is very poor at birth and develops over a relatively long period (weeks, months, or years, according to the species; Fig. 4.1 A in parallel with the anatomical and functional maturation of the visual system, particularly the visual cortex. If visual experience is altered during this period, called the critical period, dramatic consequences follow both in visual cortical development and in the development of vision. For instance, if during the critical period one eye is deprived of patterned vision, as with unilateral congenital cataract, the great majority of visual cortical neurons stop responding to the deprived eye, being driven only by the normal eye, and vision for the deprived eye develops poorly (amblyopia). There seems to be a close link between visual cortical development, critical period duration, and maturation of some visual functions, as shown in Figure 4.1 A: the closure of the critical period for monocular deprivation roughly coincides with completion of visual acuity development in a number of species, from rat to monkey to human.

Manipulations of sensory experience also have been shown to affect the development of auditory and somatosen-sory systems, leading to the widely accepted assumption that the final stage of development of neural connections in sensory systems is under the irreplaceable control of sensory experience. The studies then converged into what may be the various stages for the fulfillement of the task initiated by sensory experience.

The first link in the chain is likely to be electrical activity, the language into which sensory experience is translated, which is already known to guide nervous circuit rearrangements and synapse formation. Here we reexamine the role of electrical activity, both spontaneous and visually driven, in the "construction" of the visual system by analyzing the interactions between electrical activity and neurotrophins in the completion of visual function development. Neuro-trophins are an essential link in the chain of events leading to maturation of visual connections, a link so necessary that electrical activity in the absence of neurotrophins fails to drive developing visual cortical circuits into their final functional state. For instance, even if visual experience is normal, development of visual function is abnormal if specific neurotrophins are missing. Surprisingly, neurotrophins also seem sufficient to drive the development of some aspects of vision in the absence of visual experience.

In presenting the fascinating story of the role of electrical activity and neurotrophins in the development of the visual system, studies which blossomed in the past 10-15 years, we shall follow an historical criterion, which allows us to introduce the various experimental paradigms and the experimental models used. (For recent reviews, see Berardi and Maffei, 1999; Cellerino and Maffei, 1996; McAllister et al., 1999; Pizzorusso and Maffei, 1996).

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