Convergence of topographic projections to the inferior colliculus from the auditory subcollicular nuclei

Light and electron microscope studies on experimental material were undertaken in order to identify the composition of sets of specific subcollicular afferents converging to distinct groups of neurons in the inferior colliculus and the morphological characteristics of their parent cells. Microinjections of the retrograde tracer HRP placed in gride-like distribution into the CNIC revealed topographical order and quantitative differences in the convergence of axon projections from specific regions of the VCN, DCN, MSO, LSO, VNLL and DNLL. Pilot studies indicated very low number of labelled neurons and without discernible topographic arrangement in the auditory brain-stem nuclei, when the microinjections were in or largely in the cortical or external nuclei of the inferior colliculus. Neurons occupying the ventromedial quadrant of CNIC receive converging contralateral input projections from the dorsal part of the DCN, dorsomedial half of the LSO, and DNLL as well as ipsilateral connections from the ventral part of the VNLL, DNLL and the dorsal half of the MSO. The set of converging afferents to neurons in the ventrolateral quadrant of CNIC arises from the contralateral part of the VCN, the DNLL and from the ipsilateral ventral part of the VNLL, dorsal part of the MSO and the dorsolateral half of the LSO. Projections to cells of the dorsomedial and dorsolateral quadrants are mirror images of those to the ventral ones across the transverse axis of the CNIC.