Intensity Function of the Cochlea and Cortex

Abstract

Amplitude changes of the cochlear potential due to increases in sound intensity were compared in the same animals with amplitude changes in the evoked potential from the cortex. As is well known, the cochlear potential is linear over a 30–50 dB range. Above this linear region, the cochlear potential plateaus, and then it actually declines when the sound intensity is further increased. The cortical-evoked potential behaves in a similar manner. The amplitude of the cortical-evoked potential increases monotonically with increases in sound intensity as long as the response of the cochlea is linear. When the cochlear potential departs from linearity, the increases in the evoked potential also tend to plateau. At the point the cochlear potential decreases in response to an increase in sound, the evoked potential from the cortex also decreases. This decrease in the amplitude of the evoked potential stresses the need for careful measurement of the sound intensity and/or the cochlear potential. In several studies, units have been observed to be inhibited by loud tones. True neural inhibition can be differentiated from overdriving the cochlea only if the proper measurements are made.