THE 3-DIMENSIONAL FREQUENCY ORGANIZATION OF THE INFERIOR COLLICULUS OF THE CAT - A 2-DEOXYGLUCOSE STUDY

The 3-dimensional (3-D) functional organization of the cat's inferior colliculus (IC) was examined using the 2-deoxyglucose method. Animals were dichotically stimulated with pure tone stimuli at an intensity of 80 dB SPL. Autoradiographic sections from these animals, cut in the t hree standard planes, were serially reconstructed to reveal the 3-D to pography of the isofrequency sheets of labelling. In all 3-D reconstru ctions, the isofrequency sheets extend rostrocaudally through the IC w ith the rostral aspect of the sheet being situated more ventral than i ts caudal aspect. In the mediolateral dimension, sheets are angled at between 40 degrees and 60 degrees to the horizontal, running from a do rsomedial to a ventrolateral position. The low-frequency sheets (0.5 a nd 2 kHz) are dorsolaterally convex and situated in the dorsolateral r egion of the IC. The 4 and 10 kHz isofrequency sheets have a helical s tructure and are situated in the mid-region of the IC. The high-freque ncy sheets (20 and 30 kHz) are dorsolaterally concaved and situated in the ventromedial region of the IC. The topography of these isofrequen cy sheets generally agree with, and extended our knowledge of, the ton otopic organization of the IC as derived from electrophysiological stu dies. The functional organization revealed by the 2-deoxyglucose metho d only partially correlated with the neural laminae in the anatomical models of the IC proposed by Rockel and Jones [J. Comp. Neurol. 147 (1 973) 11-60] and Oliver and Merest [J. Comp. Neurol. 222 (1984) 237-264 ]. It is therefore concluded that the neural laminar organization of t he IC may not be a necessary substrate for the tonotopic organization seen the IC.