Projections from the auditory cortex to the superior olivary complex in guinea pigs.

We used anterograde tracing techniques to characterize projections from auditory cortex to the superior olivary complex (SOC) in guinea pigs. Large injections of fluorescent or biotinylated dextrans into the temporal cortex labeled many axons in the SOC. Labeled boutons were most numerous in the ventral nucleus of the trapezoid body, with additional boutons in all other olivary nuclei. The distribution of boutons was similar in the ipsilateral and contralateral SOC; however, the contralateral SOC had markedly fewer axons and boutons. Similar patterns of labeling were also observed following injections confined to primary auditory cortex or the dorsocaudal auditory field. Cortical axons in many of the SOC nuclei share numerous morphological features, suggesting that individual axons may innervate multiple nuclei and have widespread effects. In addition, some nuclei contain axons with branching or termination patterns unique to that nucleus; these axons may represent focused projections with effects limited to individual SOC nuclei. Given the many projections of SOC nuclei, cortico-olivary projections are in a position to modify the activity of many brainstem auditory circuits.