Visual System Development, Plasticity

Abstract

In research on visual system development, as eisewhere, the concept of plasticity has meant different things to different people at different times, and it is unlikely that the evolution of the meaning of the term is over. Any attempt at definition must thus take into account the plasticity of “plasticity.” In speaking broadly of 20th Century neurobiology, it is useful to distinguish three roughly successive periods. During the earliest phase, a major concern of investigators was with the issue of whether the adaptive Organization of the visual system reflects predesign based on genetic information, as opposed to processes of adaptive shaping based on functional effectiveness. Plasticity was used to characterize the latter alternative and was understood to mean “the capacity to adjust functional Organization based on sensory experience.” In subsequent years, attention shifted to documenting the existence of orderly patterns of anatomical connections in the visual pathways and to the hypothesis that these resulted from a refined capacity of outgrowing axons to recognize and distinguish between target neurons based on cytochemical affinities. During this period, plasticity became more specifically linked to problems of anatomical Connectivity but largely lost its reference to the broader problem of accounting for functionally appropriate Organization. The term was generally used to refer to any phenomenon suggesting a capacity to make a pattern of connections other than the “normal” one, hence challenging the chemoaffinity hypothesis.