Effects of middle ear pressure on frequency representation in the central auditory system

Changes in middle ear pressure (MEP) are known to produce an attenuation of sound transmission through the outer and middle ear, but the effects on frequency representation in the auditory system have not previously been studied. This issue is of particular interest because of changes in MEP occurring during episodes of otitis media. We have investigated the effect of changes in MEP on the tuning of neurons in the inferior colliculus (IC) of the gerbil to calibrated tone stimulation of the contralateral, pressurized ear. Both negative and positive non-atmospheric MEP produced an elevation of neural thresholds that was inversely related to IC neuron best frequency (BF). A robust, linear relationship was found between BF at atmospheric MEP (control) and BF at -20 daPa MEP. Higher resolution analysis was performed on a sub-sample of neurons that had particularly stable BFs with repeated, control MEP. For the majority of these neurons, alternation of MEP between control and -20 daPa had no effect on BF. However, a few neurons showed small (up to 5%), significant shifts in BF with -20 daPa MEP. These results are consistent with previous reports of the effects of MEP on spontaneous otoacoustic emissions. We conclude that non-atmospheric MEP acts as a high-pass filter on the input to the cochlea, but does not change the frequency organization of the auditory system to any marked extent.