The spatial representation of frequency in the rat dorsal cochlear nucleus and inferior colliculus

The spatial distribution of neural activity produced by tones was assessed in the rat dorsal cochlear nucleus (DCN) and inferior colliculus (IC), using the 2-deoxyglucose (2-DG) technique. Eight pure tones, spanning the range of reported single unit characteristic frequencies in the rat, were presented at 40 dB above behavioral threshold. The relationship between frequency of stimulation and location of neural activity within each nucleus was evaluated quantitatively. Based on the 2-DG uptake pattern across animals, a tonotopic axis in the transverse plane was defined for each nucleus. This axis transected the centers of regions of evoked 2-DG uptake for each frequency. There was an orderly relationship between stimulus frequency and the location of evoked neural activity along the axis. Each pure tone stimulus activated an approximately equal proportion of this axis, for all frequencies tested, in both the DCN and IC. This suggests the existence of equal 'spatial bandwidths, in rat central auditory structures, across its entire frequency range. Equal spatial bandwidths could facilitate signal analysis strategies which require interaction between neurons with closely-related CFs. In the horizontal plane, however, the proportion of stimulated tissue was not equal across frequency. High-frequency (greater than 8 kHz) tones produced increased neural activity along a much greater extent of the anterior-to-posterior axis of the IC than did low-frequency tones.(ABSTRACT TRUNCATED AT 250 WORDS)