Tonotopic maps obtained from the surface of the dorsal cochlear nucleus of the hamster and rat.

Tonotopic organization was mapped over the surface of the dorsal cochlear nucleus (DCN) of the Syrian golden hamster and albino rat. The purpose of this study was to describe comparative similarities and differences in fine map features that exist between these two species, and to differentiate features which show a high degree of constancy from those which show significant variations across individuals of the same species. In general, the tonotopic organization seen in both species was characterized by a mediolateral gradient in which high CFs were located medially and low CFs laterally. Maps within each species displayed a high degree of constancy both in the slopes of the gradient as well as in the preferred rostrocaudal orientation of isofrequency contours. However, between species significant differences were seen in the slope of the CF gradient. In the rat, CFs declined toward the lateral extremity at a rate which was nearly twice that seen in the hamster, despite the fact that there were no apparent differences in the width of the DCN in these two species. The precise configuration of areas subtending selected frequency ranges also showed considerable individual variation and defined a 'microstructure' of tonotopic organization that was unique for each animal. The implications of these findings on concepts of DCN development and modes of innervation by the auditory nerve are discussed.