Representation of the cochlea within the inferior colliculus of the cat

Abstract

The representation of the cochlear partition within the 3 dimensions of the inferior colliculus of the cat was studied by determining the best frequency of neurons isolated in series of parallel electrode penetrations. Cats were anesthetized with sodium pentobarbital. There was a systematic representation of the cochlea within the pericentral nucleus of the inferior colliculus. The boundary between the pericentral nucleus and underlying central nucleus could be sharply drawn on physiological bases. There was a highly ordered representation of the cochlea across the 3 dimensions of the central nucleus. Neurons within the dorsomedial nucleus and external nucleus of the inferior colliculus were not driven by simple tonal stimulation. The change in best frequency as a function of depth in the colliculus was the same for parallel penetrations entering over almost the entire dorsal surface of the central nucleus. This indicated that a given frequency-band was represented across a laminar figure completely sectioning the nucleus; and that laminae were flat except along the extreme margins of the nucleus. Definition of ‘isofrequency contours’ from reconstruction of rows of parallel penetrations confirmed these conclusions. Frequency-band laminae were tilted down rostrally, especially in the middle and lateral aspects of the central nucleus. They were also tilted down laterally. Orientation of isofrequency contours paralleled that of laminae described within anatomical studies of the central nucleus 29. The disk-shaped regions representing lowest frequency octaves were not as thick nor as extensive as those representing middle and high frequency octaves. However, when best frequency values were converted to corresponding cochlear place, cochlear place was observed to be a nearly linear function of colliculus depth for the representation of the apical half of the cochlear partition. There was a somewhat larger representation, across colliculus depth, of the basal half of the cochlea. The change in best frequency with change of depth in the central nucleus was stepwise. This suggested that small sectors of the basilar membrane of approximately equal length were represented across individual anatomically defined cellular laminae within the nucleus.