Construction of an auditory space map.

The computation of ITD begins with preservation of the phase angle of spectral components by eighth-nerve fibers and by cells of the nucleus magnocellularis. At the level of the nucleus laminaris, the difference in phase angle of corresponding spectral components in the left and right ears is extracted. At the next level, the central nucleus, neurons are consolidated into tonotopic ensembles in which preferred delta phi and frequency covary, so that only a single ITD coactivates all constituent neurons. Neurons of such ensembles project convergently on space-specific neurons, endowing the space-specific neuron with a selectivity for ITD and, therefore, a selectivity for azimuth. Ensembles representing ITDs corresponding to ipsilateral and contralateral auditory hemi-fields are found in the core and lateral shell of the central nucleus, respectively. The lateral shell receives its representation of the contralateral hemi-field from the opposite central nucleus core. The central nucleus core is innervated by the contralateral nucleus laminaris, which contains a representation of contralateral space. The projection from the lateral shell to the ipsilateral external nucleus forms a map of contralateral space in the external nucleus.