The problem of obtaining reproducible quantitative data of the olivocochlear pathway as exemplified in the guinea pig.

We have tried to quantify the number of efferent neurons comprising the olivocochlear bundle in the guinea pig by using the retrogradely transported fluorescent dye, true blue. The dye was injected into numerous regions of the cochlea for various distances. The data from 14 complete experiments have been evaluated. The majority of the labelled cells were found ipsilaterally, with the lateral superior olive containing 59.2% and showing cochleotopicity. Of the 28.9% found contralaterally, the nucleus of the ventral lateral lemniscus had 16.9%, and the trapezoid body nucleus 8.0%, differing significantly from the other nuclei of the superior olivary complex. Although the distribution of the labelling was consistent, there was great variability in actual numbers. Calculations based on cell density showed no differences along the length of the basilar membrane, and resulted in a total of around 2500 efferent neurones. This figure agrees closely with the maximum counts found by other workers. However, the cause of the individual variability remains unanswered.