We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally localized Wannier functions (MLWF)are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors (NNs), and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms. (C) 2001 Elsevier Science B.V. All rights reserved.