Anatomic evidence of a three-dimensional mosaic pattern of tonotopic organization in the ventral complex of the lateral lemniscus in cat

The ventral complex of the lateral lemniscus (VCLL, i.e., the ventral and i ntermediate nuclei) is composed of cells embedded in the fibers of the late ral lemniscus. These cells are involved in the processing of monaural infor mation and receive input from the collaterals of the fibers ascending to th e inferior colliculus. Whereas tonotopic organization is a feature of all o ther nuclei of the auditory system, this functional principle is debated in the VCLL. We have made focal injections of the tracer biotinylated dextran amine into different frequency band representations of the inferior collic ulus in cat. Retrogradely labeled cells and terminal fibers (collaterals of efferent local axons and other ascending lemniscal fibers) were found in t he ipsilateral VCLL. The spatial distribution of the labeling was analyzed using three-dimensional (3-D) reconstruction and computer graphical visuali zation techniques. A complex topographic organization was found. In all cas es, labeled fibers and cells were distributed in multiple clusters througho ut the dorsoventral extent of the VCLL. The shape, size, and location of th e labeled clusters suggest an interdigitation of clusters assigned to diffe rent frequency-band representations. But an overall mediolateral distributi on gradient was observed, with high frequencies represented medially and lo wer frequencies progressively more laterally. We conclude that the clusters may represent discontinuous frequency-band co mpartments as a counterpart to the continuous laminar compartments in the r emaining auditory nuclei. The 3-D orderly mosaic pattern indicates that the VCLL preserves the spectral decomposition originated in the cochlea in a w ay that facilitates across-frequency integration.