Cytoarchitecture of the human superior olivary complex: Nuclei of the trapezoid body and posterior tier

The superior olivary complex (SOC) is a cluster of nuclei situated in the caudal brainstem tegmentum that forms an essential component of the auditory pathway. The SOC includes two principal nuclei, the medial and lateral superior olives (MSO and LSO respectively), that have clear roles in sound source localization. Surrounding the principal nuclei are a number of periolivary nuclei (PON) that vary significantly between mammalian species but function in multiple aspects of hearing. Although the PON have been studied in numerous laboratory animals, these nuclei have not been delineated in human. The major goal of this study is to, based on myeloarchitecture, location, neuronal morphology and cytoarchitecture, define the PON within the human SOC and provide estimates of neuronal number within these nuclei. Results from the study of twelve human brainstems provide evidence for six morphologically distinct cell groups: three within the trapezoid body and three along the posterior aspect of the SOC. Based on the analysis of human tissue stained for myelin, Nissl substance, or impregnated with silver, the human PON appear largely homologous to the PON described in other low-frequency hearing animals.