Projections from the primary auditory cortex onto the dorsal cortex of the inferior colliculus in albino rats.
The topography of connections from the primary auditory cortex (Te1) onto the dorsal cortex (DC) of the inferior colliculus (IC) was studied using the anterograde neural tracers Phaseolus vulgaris (PHA-L) and Biotinylated dextran amine (BDA). Injections in different restricted portions of the ventral Te1 showed labelled axonal sheets in the ipsilateral DC with extensions in the central nucleus and the external cortex. Also contralateral labelled axons were found with a patchy appearance. The pattern of labelled axonal laminae is arranged in an orderly fashion, so that a correlation exists between the injection field (antero-to caudoventral injections) and the pattern of labelled axonal sheets (from a ventromedial to a dorsolateral arrangement) in the IC. However, variations from this pattern were observed. Also only the injections made in rostral and middle parts of Te1 V are orderly arranged in the territory of the DC, while the Te1 V caudoventral projection partially overlaps those from antero and mid-central regions. A topographical arrangement in the fibre projection from the medial and dorsal zones of Te1 (Te1 D) exists in the neuropil of the IC. The bands from Te1 D overlapped with the bands from Te1 V but in different order. This indicates that the projection of the mid-dorsal zone of Te1 is not topographically arranged in the DC like Te1 V. Based on the results we have obtained, there is strong evidence that the projection from Te1 V to the DC is topographically organized in a rostrocaudal sequence which gives definite terminal fields. However, each of these fields (medial, central and external) receives a principal bulk of primary auditory cortical fibres and a supplementary one. This implies that territories of the DC which contain neurons responsive to low frequencies receive a projection from Te1 fields with neurons which are responsive to high or to low frequencies.