Quantitative estimate of bilaterally projecting medial olivocochlear neurones in the guinea pig brainstem.

Bilateral intracochlear injections of the fluorescent retrograde neuronal tracers fast blue and diamidino yellow were used to identify efferent neurones in the guinea pig brainstem which project to the inner ear on both sides. Bilaterally projecting neurones, detected by their double-labelled appearance in fluorescence microscopy, were found in all the nuclei of origin of the large neurone extra-LSO system of olivocochlear neurones. No double-labelled neurones were found in the small neurone LSO system. When precautions were taken to maximize their capture, these bilaterally projecting neurones comprised 5.4% of the total extra-LSO system neurones projecting to any one cochlea. On the basis of these quantitative data, it was suggested that the bilaterally projecting neurones may correspond to the small group of binaurally activated cochlear efferents identified in physiological studies.