Afferents from the hindbrain auditory system to the nuclei of the lateral lemniscus were analyzed by the use of orthograde and retrograde axon-tracing techniques. Three divisions of the nuclei of the lateral lemniscus, a dorsal, an intermediate, and a ventral division are discussed. The dorsal nucleus of the lateral lemniscus is a recipient of afferents from cells located mainly in the superior olivary complex and the contralateral dorsal nucleus of the lateral lemniscus. It receives direct afferents from only a few cells in the cochlear nuclei. In sharp contrast, the ventral nucleus of the lateral lemniscus is the recipient of afferents from many cells in the contralateral ventral cochlear nucleus and from only a few cells in the superior olivary complex. Further, it receives no afferents from cells in the contralateral nuclei of the lateral lemniscus. The intermediate nucleus of the lateral lemniscus receives afferents from some cells in the cochlear nucleus and the superior olivary complex. It is unique among the three nuclei of the lateral lemniscus in that it receives a substantial projection from the medial nucleus of the trapezoid body.

When this series of experiments was begun in 1984, the activity of each lateral superior olive (LSO) in the mammalian hindbrain was known to encode the hemifield of acoustic space containing a sound source. However, the almost random bilaterality of its ascending projections seemed to jumble that identification before reaching the midbrain. At the same time, electrophysiological studies of LSO and its efferent target in the inferior colliculus, along with the strictly contralateral deficits in sound localization resulting from unilateral lesions above the level of the superior olives, indicated that hemifield allegiance was largely maintained (though reversed) at the midbrain. Here we present seven lines of biochemical evidence, some combined with prior ablations, supporting the notion that the anatomical segregation of the ipsilateral and contralateral fibers ascending from the LSO is accompanied by a corresponding segregation of their neurotransmitters: most of the ascending ipsilateral projection is probably glycinergic and, hence, inhibitory in effect, while most of the contralateral projection is probably glutamatergic/aspartergic and, hence, excitatory in effect. Taken together, the inhibitory ipsilateral projections and the excitatory contralateral projections serve to amplify functional contralaterality at the higher levels of the auditory system.

The afferent projections to the lateral superior olive (LSO) were examined with horseradish peroxidase, horseradish peroxidase-wheat germ agglutinin conjugate, 125I-wheat germ agglutinin and tritiated leucine autoradiograhy, anterograde axonal degeneration, and 14C-2-deoxyglucose methods. The pathway to the ipsilateral LSO orginates in the spherical cells in anteroventral cochlear nucleus. Although some of the fibers pass above the lateral nucleus of the trapezoid body, most pass below it and turn at right angles to enter the LSO either directly through its ventral, lateral, or dorsal borders, or through its ventral or dorsal hilus. They end in unpolarized terminal fields throughout the LSO. Most if not all of these fibers are true collaterals of axons continuing across the midline in the trapezoid body. Verifying Held's (1893) finding of a major direct projection from the cochlear nucleus to the contralateral medial nucleus of the trapezoid body (MTB) and Rasmussen's ('46) finding of a major projection from the MTB to the LSO, the present results illustrate that this two-neuron pathway probably supplies all but a very small component of the relatively direct input to the LSO from the contralateral ear. This pathway originates in the globular cells of the ventral cochlear nucleus and relays mostly though not exclusively through the "principal cells" in the more rostral parts of the MTB. It terminates mostly in perisomal endings in unpolarized fields throughout the LSO, though most heavily within the (high frequency) medial and middle limbs and less heavily in the LSO's (low frequency) lateral limb. In addition to this indirect pathway, there is a small direct pathway to the contralateral LSO as suggested by Goldberg and Brown ('69) and Warr ('72, '82). This direct pathway to the contralateral LSO, like the direct ipsilateral pathway, probably originates in the spherical cell region of the ventral cochlear nucleus, crosses the midline in the trapezoid body, and terminates in a small circumscribed area within the LSO's ventromedial (high frequency) area. The 2-deoxyglucose method applied to cats in which the ipsilateral and contralateral pathways have been surgically isolated shows that each of the pathways converging on the LSO is topographically and tonotopically organized with the ipsilateral and the combined contralateral terminations in strict tonotopic register.

The volumes of the ten largest subcortical auditory nuclei were measured individually in a sample of 53 mammals, including 16 Australian and four American marsupials. The nuclear sizes relative to the total of subcortical auditory tissue were normalized and then analyzed individually for statistically reliable deviations. The overall form of the entire system of ten nuclei and two nuclear subsystems (cochlear nuclei, superior olives) were also analyzed for similarities and notable deviations among the animals. The results show that the absolute size of the auditory system varies more than 139-fold among the 53 mammals (with moles the smallest and humans the largest). Log auditory system volume and log brain weight are closely correlated (r = 0.903, p <0.0001). Bats, kangaroo rats, marmosa opossums, and Norway rats have the largest auditory systems relative to their brain size, while humans have the smallest by far. The other primates also have auditory system/brain size ratios smaller than the sample average, suggesting that the condition in humans is one result of an expansion of non-auditory brain parts rather than a reduction of the auditory system over geological time. The relative sizes of the ten nuclei are well ordered, with the inferior colliculus the largest nucleus by far and medial superior olive the smallest. Because the size of the superior olives, collectively, is reliably related to the size of anteroventral cochlear nucleus (r = 0.744, p <0.001), and not to the size of dorsal cochlear nucleus, the interconnectivity of the subcortical auditory system is probably a factor in the size of the nuclei. In its overall form, the subcortical auditory system is highly similar among mammals, with an average correlation across nuclei of 0.923. This high value means that the overall form of the system has been relatively stable over geological time. The animals with least deviation from the average form are ring-tailed possums, bandicoots, and yellow-bellied gliders, all marsupials. Those with the most unusual forms are mice, bats, and kangaroo rats, all placentals.

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.