The cochlear place-frequency map of the adult and developing Mongolian gerbil

Gerbils are frequently used in auditory research; however, only limited information on the exact frequency representation in the cochlea is available. Therefore the place-frequency map of the gerbil was determined by iontophoretic application of horseradish peroxidase in physiologically characterized single auditory nerve fibers. Locations of the labeled nerve fibers at the inner hair cells were determined from a '3-dimensional' reconstruction of the cochleae. The map was established for frequencies between 0.3 and 31.9 kHz. As in other mammals, a baso-apical frequency gradient from high to low frequencies was found. The slope of the place-frequency map amounted to 1.5 mm/octave for higher frequencies. Below 4 kHz the slope decreased to a value of 1 mm/octave at 0.5 kHz. A shift in the frequency map occurs during cochlear maturation. The place-frequency map in subadult gerbils (18 days after birth) is shifted by a distance corresponding to an octave, at least for frequencies above 6 kHz.