Abstract:
Ascending output of nucleus ovoidalis (Ov) to Field L is purely ipsilateral for all birds, as far as is known, and since there are no connections between auditory areas in separate hemispheres, what one hemisphere ‘understands’ about the origin of sounds in space is entirely dependent on the nature of the input from the midbrain nucleus mesencephalicus lateralis pars dorsalis (MLd) to the thalamic Ov. The present study aimed to characterise, in a preliminary way, the proportion, density, organisation and termination of MLd cells with ipsilateral only, contralateral only and bilateral projections to Ov in the zebra finch by retrogradely labelling MLd neurons by way of injections of different fluorescent tracers into each Ov. Results show a predominance of ipsilateral projections of MLd to Ov and identify for the first time MLd neurons that project purely contralaterally, and cells that project bilaterally via branched axons. Also, cells with ipsilateral only projections are generally located in caudal regions of MLd and tend to terminate laterally within Ov, while cells with contralateral only projections are generally located in rostral MLd and tend to terminate medially in Ov. Cells with bilateral projections are found throughout MLd with a greater density mid-MLd and terminate throughout Ov, but generally do so homotopically. Similar results were also found for dorsal nucleus of the lateral lemniscus (DLL) and ventral nucleus of the lateral lemniscus (VLL) projections to Ov. There was no correlation between major subdivisions of MLd (inner, MLd.I and outer, MLd.O) and the distribution of cells with ipsilateral only, contralateral only or bilateral projections to Ov. Finally, the trajectories of MLd and lateral lemniscus axons to Ov were charted and shown in detail for the first time. These data and those from other species, will aid in understanding the neural circuitry underlying binaural localisation processing in the brain as a whole.
