Role of commissural projections in the representation of bilateral auditory space in the barn owl's inferior colliculus.

The central nucleus of the barn owl's inferior colliculus (ICc) contains a representation of both the ipsilateral and contralateral auditory hemifields. The representation of ipsilateral space is found in the "core" of the ICc, a subdivision defined by the terminal field of nucleus laminaris, the avian analogue of the medial superior olivary nucleus. The representation of contralateral space is found in the lateral portion of the "shell" of the ICc. The shell surrounds the core and is defined by the terminal field of the nucleus angularis, one of the cochlear nuclei. The representation of ipsilateral space in the core of the ICc may be accounted for by the crossed projection from the nucleus laminaris because most of the nucleus laminaris is devoted to a representation of contralateral space. We present evidence to suggest that the representation of contralateral space is due to a commissural projection from the core of one side to the lateral shell of the opposite side. Injection of horseradish peroxidase (HRP) into the lateral portion of the ICc shell produced retrogradely labeled somata in the core of the opposite side. Injection of tritiated proline into the core produced anterograde label confined to the lateral shell, thus confirming the observations made with HRP. Thus, for example, the left ICc core, which contains predominantly a representation of the left hemifield, innervates the right lateral shell, endowing it with a representation of the left, or contralateral hemifield. The representation of contralateral space in the lateral shell is ultimately conveyed to the external nucleus of the inferior colliculus where it contributes the horizontal axis to a two-dimensional map of space.