Lack of topography in the ventral nucleus of the lateral lemniscus.

Abstract

In contrast to the ease of finding tonotopicity in other nuclei, both anatomical and electrophysiological methods have failed to demonstrate a clear and simple tonotopic map within the ventral nucleus of the lateral lemniscus (VLL). The present study was undertaken in cat with the hope that methods not used previously in studies of VLL might succeed in demonstrating an orderliness in its exiting fibers (i.e., efferents) or its incoming fibers (i.e., afferents). Since the same organization of ascending frequencies present in the cochlea is maintained in these fibers as well as in all main auditory nuclei, demonstration of a similar organization of frequencies in VLL would be evidence of the cochleo- or tono-topicity of this nucleus. Using triple injection of 3 different fluorescent dyes in inferior colliculus to study efferents, orderly and tonotopic cell-labeling is found in each of the brainstem auditory nuclei, with the notable exception of VLL. Instead, labeling of cell clusters, each cluster containing a small number of cells, is found randomly distributed throughout VLL in all 3 of its spatial dimensions. Using the 2-deoxyglucose (2-DG) method, during stimulation at 6 different frequencies, afferent orderliness, indeed, tonotopicity is found in all major brainstem auditory nuclei, again with the notable exception of VLL. Rather, each frequency evokes 2-DG label throughout VLL. In agreement with the results based on electrophysiological methods, therefore, the anatomical methods used here also yield no evidence of tonotopicity in VLL. Thus, if there is orderliness in VLL's efferents or afferents, it is based on an auditory dimension incommensurate with frequency.