Isofrequency labelling revealed by a combined [14C]-2-deoxyglucose, electrophysiological, and horseradish peroxidase study of the inferior colliculus of the cat.

Tonotopic sequences of single units were recorded in the central nucleus (ICC) of the inferior colliculus of cats. The sites of units with a particular characteristic frequency (CF) were marked histologically. After injection with [14C]-2-deoxyglucose (2-DG), each animal was stimulated with tone bursts, which corresponded to the CFs of the selected units. The position of a selected unit was found to correspond with the position of a band of discrete 2-DG labelling. In some animals, multiple penetrations revealed that more than one unit with the same CF was located on the same band, indicating that the 2-DG bands represent isofrequency contours. Single units were recorded in the pars lateralis (LV) of the medial geniculate body of cats and areas of consistent CF were found, which contained the same frequencies as some of those employed with 2-DG. Horseradish peroxidase (HRP) was ejected into these selected areas and labelled cells were identified in ICC after retrograde transport. The labelled cells were found to lie in a band which had a similar orientation to the 2-DG bands produced by the same frequencies. Both sets of data indicate that ICC is organized into three-dimensional isofrequency sheets, the orientation of which do not match the concentric model of frequency organization inferred from some Golgi studies. The isofrequency sheets slope from medial to lateral throughout ICC with high-frequency contours found more medially and caudally and low-frequency contours more laterally and rostrally. The organization of the isofrequency contours is largely in agreement with a recent Golgi model of the laminations in ICC put forward by Oliver and Morest ('84). The 2-DG contours do not agree with the orientation of laminae in the lateral part of ICC as described by this model. The 2-DG contours also extend further into the dorsal cortex than previous degeneration evidence would suggest. The functional isofrequency contours are often quite narrow and could represent two or three overlapping cellular laminae. The results show clearly that there is only one tonotopic organization extending throughout ICC and the dorsal cortex.