The feedback pathway from horizontal cells to cones - A mini review with alook ahead

The feedback pathway from HCs to cones forms the basis of the surround resp onses of the bipolar cells and is essential for the spectral opponency of h orizontal cells. The nature of this feedback pathway is an issue of debate. Three hypothesis are presented in literature: (1) a GABA(A)-ergic feedback pathway; (2) a GABA-independent feedback pathway that modulates the Ca-cur rent in cones; and (3) an electrical feedback pathway. In this review the e vidence for the various pathways will be discussed. The conclusion is that the available evidence favors the hypothesis that feedback modulates the Ca -current in the cones in a GABA independent way. An alternative role of GAB A in the outer plexiform layer is discussed and finally the functional cons equences of the negative feedback pathway from horizontal cells to cones ar e presented. (C) 1999 Elsevier Science Ltd. All rights reserved.