Low-frequency pathway in the barn owl's auditory brainstem.

The cytology of the nucleus magnocellularis and the nucleus laminaris in the barn owl, as well as the axonal pathways connecting them, were studied. The interest was focussed on those regions of both nuclei coding the low-frequency end of the tonotopic spectrum (below approximately 2 kHz) because many previous reports on a variety of bird species had indicated significant differences to higher frequencies, both in morphology and physiology. Standard light- and electron microscopy, as well as immunocytochemistry and tract-tracing techniques, were used. The nucleus magnocellularis contains a distinct stellate cell type in the low-frequency region, in addition to neurons classified as a small version of the principal cell. In the nucleus laminaris, two cell types were characterized as distinct to the low-frequency region: stellate neurons with long, smooth dendrites, and multipolar neurons with thick, spiny dendrites. The low-frequency projections from the nucleus magnocellularis showed two terminal fields in the nucleus laminaris: one containing a rough tonotopic representation and a second one where all low-frequency projections converged. In addition, the anatomical basis for delay lines, which are known to play an important role in the coding of interaural time differences at higher frequencies, was not observed. The morphological differences observed at low frequencies in both nuclei, compared to the well-studied higher-frequency regions, may reflect inherent limitations to the accuracy in the processing of interaural phase disparities at low frequencies.