Three subdivisions of the auditory midbrain in chicks (Gallus gallus) identified by their afferent and commissural projections.

The auditory midbrain is a site of convergence of multiple auditory channels from the brainstem. In birds, two separate ascending channels have been identified, through which time and intensity information is sent to the nucleus mesencephalicus lateralis, pars dorsalis (MLd), the homologue of the central nucleus of the mammalian inferior colliculus. Using in vivo anterograde and retrograde tracing techniques, the current study provides two lines of anatomical evidence supporting the presence of a third ascending channel to the chick MLd. First, three non-overlapping zones of the MLd receive inputs from three distinct cell groups in the caudodorsal brainstem. The projections from the nucleus angularis (NA) and nucleus laminaris (NL) are predominantly contralateral and may correspond to the time and intensity channels. A rostromedial portion of the MLd receives bilateral projections mainly from the regio intermedius, an interposed region of cells lying at a caudal level between the NL and NA, as well as scattered neurons embedded in the 8th nerve tract, and probably a very ventral region of the NA. Second, the bilateral zones of the MLd on two sides of the brain are reciprocally connected and do not interact with other zones of the MLd via commissural connections. In contrast, the NL-recipient zone projects contralaterally upon the NA-recipient zone. The structural separation of the third pathway from the NA and NL projections suggests a third information-processing channel, in parallel with the time and intensity channels. Neurons in the third channel appear to process very low frequency information including infrasound, probably utilizing different mechanisms than that underlying higher frequency processing.