Auditory neural activity evoked by pure-tone stimulation as a function of intensity

The 2-deoxyglucose (2-DG) autoradiographic technique was employed to map activation of the central auditory pathway in the mongolian gerbil during stimulation with a 3.0 kHz tone at several intensities. In most auditory nuclei, the tone produced restricted areas in which 2-DG uptake was markedly higher than that of adjacent tissue, at locations consistent with the known tonotopic organization of the structure. The size of these regions changed relatively little with increasing stimulus intensity from 25 to 65 dB SPL (re 0.0002 dyne/cm2). At higher intensities, evoked uptake spreads into locations which represent frequencies above 3.0 kHz. In the inferior colliculus, relative 2-DG uptake decreased with increasing stimulus intensity in bands on either side of the 3.0 kHz region. These bands of reduced uptake became wider with increasing stimulus intensity from 25 to 85 dB SPL. The optical densities of auditory structures were normalized by the density of non-auditory white matter to derive optical density ratios. In the cochlear nuclei, optical density ratios in the 3.0 kHz region increased monotonically with increasing stimulus intensity, to a plateau at 45 dB SPL. In higher auditory nuclei, relative 2-DG uptake increased to a peak at 45 dB SPL and then declined at higher intensities.