POSITION, GUIDANCE, AND MAPPING IN THE DEVELOPING VISUAL-SYSTEM

Positional identity in the visual system affects the topographic proje ction of the retina onto its central targets. In this review we discus s gradients and positional information in the retina, when and how the y arise, and their functional significance in development. When the ax ons of retinal ganglion cells leave the eye, they navigate through ter ritory in the central nervous system that is rich in positional inform ation. We review studies that explore the navigational cues that the g rowth cones of retinal axons use to orient towards their target and or ganize themselves as they make this journey. Finally, these axons arri ve at their central targets and make a precise topographic map of visu al space that is crucial for adaptive visual behavior. In the last sec tion of this review, we examine the topographic cues in the tectum, wh at they are, when, and how they arise, and how retinal axons respond t o them. We also touch on the role of neural activity in the refinement of this topography. (C) 1993 John Wiley & Sons, Inc.