Topographic organization of fimbria-fornix fibers projecting to the lateral septum of rats: a single and field response analysis.

Single-unit and field potential responses, evoked by stimulation of restricted fimbria-fornix (fi-fx) segments, were recorded over the entire extent of the lateral septal complex (LSC) of rats in order to examine electrophysiologically the topographic organization of fi-fx fibers projecting to the LSC. We confirmed earlier studies showing that fi-fx fibers constitute a bilateral, monosynaptic excitatory projection to the LSC. The projection appeared to be topographically ordered in that medial fi-fx fibers innervated neurons in the mediodorsal part of the LSC whereas fibers coursing through progressively more lateral positions in the fi-fx activated neurons in gradually more lateral and ventral parts of the LSC. In addition to the topographic organization, a marked convergence of the fi-fx fibers on neurons located in the dorsolateral part of the LSC was apparent from both the single-unit and field potential experiments.