Unusual mechanical processing of sounds at the apex of the Guinea pig cochlea

One of the tenets of mammalian auditory physiology is that the frequency selectivity at the cochlear base decreases as a function of stimulus level. Changes in frequency selectivity have been shown to be accompanied by changes in response phases as a function of stimulus level. The existence of such nonlinear properties has been revealed by the analysis of either direct or indirect recordings of mechanical vibrations of the cochlea. Direct measurements of cochlear mechanical vibrations, however, have been carried out with success primarily in cochlear regions that are tuned to frequencies >7 kHz, but not in regions sensitive to lower frequencies. In this paper we continue to analyze recently published data from measurements of sound-induced vibrations at four locations near the apex of the intact guinea pig cochlea, in a region encompassing approximately 25% of its total length. Analysis of the responses at all locations reveal level-dependent phase properties that are rather different from those usually reported at the base of the cochlea of laboratory animals such as the chinchilla. Cochlear group delays, for example, increase or remain constant with increasing stimulus. Similarly, frequency selectivity at all the regions increases as a function of stimulus level.