Segregation of efferent projections to different turns of the guinea pig cochlea.

Localized intracochlear injections of the fluorescent retrograde label diamidino yellow were used to investigate the organization of efferent projections from the brainstem to different turns of the cochlea, in the guinea pig. It was found that the location of small neurones within the lateral superior olive ipsilateral to the injection varied in a systematic manner when injections proceeded from base to apex of the cochlea. In addition, a cruder form of cochleotopic organization was present in that most of the large medial system efferent neurones were labelled only after injection into the 3 most basal turns of the cochlea. The decline of medial system efferent innervation proceeding from base to apex was most striking for the contralateral efferent neurones. The details of base to apex innervation density were different for the different nuclei of origin of the medial system, implying the existence of complex subsystems within the medial neurone population.