Acoustic chiasm: efferent projections of the lateral superior olive.

The efferent connections of the cat's lateral superior olive (LSO) were examined first with kainic acid-induced anterograde degeneration and tritiated leucine autoradiography and then by systematic repetition of HRP and fluorescent dye retrograde tract-tracing techniques. The results show that virtually all LSO cells have axons ascending either contralaterally or ipsilaterally to high pontine and midbrain levels of the brainstem. Most terminate in the ventrolateral division of either the ipsilateral or contralateral central nucleus of the inferior colliculus, some terminate in the ipsilateral or contralateral dorsal nucleus of the lateral lemniscus, and a small number terminate in the ipsilateral intermediate nucleus of the lateral lemniscus. Only a small proportion (less than 5%) of LSO cells project to both sides via axon collaterals. The ipsilateral, contralateral, and bilateral projections arise from three overlapping subpopulations of cells within LSO: Those projecting ipsilaterally are concentrated in its lateral limb; those projecting contralaterally are concentrated in its medial limb; the few projecting bilaterally are thinly scattered throughout. Therefore, a lateral-medial gradient is present across LSO based on the laterality of its cell's efferent targets. This gradient parallels LSO's tonotopic gradient: The higher the characteristic frequency of an LSO cell, the more likely it is to project contralaterally. This arrangement of LSO's ascending projections, with most of its lateral cells projecting ipsilaterally and most of its medial cells projecting contralaterally, is similar to the arrangement of the optic chiasm in animals with overlapping eye-fields. Its presence seems to provide an anatomical basis for some recent electrophysiological and behavioral reports of chiasm-like properties of the superior olivary complex.