Cochlear microphonic iso-response functions reported here suggest an explanation of frequency-dependent changes in hearing sensitivity during early development. The work is a direct demonstration of developmental changes in the spatial frequency map of the mammalian hearing organ. Intracochlear recordings from the midbasal turn in a series of age-graded gerbils reveal a progressive increase in best frequency, spanning approximately two octaves, from the time of onset of function until adultlike responses are seen. It is, therefore, suggested that ontogenetic changes in the cellular structure of the organ of Corti contribute to an agedependent shift in micromechanical response.
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