Auditory structure and function in the bird middle ear: an evaluation by SEM and capacitive probe.

The anatomic features of the middle ear in five avian species were identified in the scanning electron microscope. Various aspects of the conductive apparatus were quantitatively measured in a number of specimens from each species. These included the tympanic membrane, columella footplate and oval window area; the length of the columella, extra-stapedius and Platner's ligament: and the angular relations between the columella and the tympanic membrane, extra-stapedius and footplate. The velocity vs frequency response curve, measured from the tip of the concave tympanic membrane, and corrected to a constant stimulus level of 100 dB SPL, was obtained for the neonatal chick and parakeet for frequencies between 0.2 and 10.0 kHz with a capacitive probe. In both species this curve resembled a bandpass filter whose best frequency was in the range of 1.5-3.0 kHz. The low and high frequency roll-off was 6 and 16 dB per octave, respectively. Displacement of the TM in the chick was measured at several frequencies between 70 and 120 dB SPL and was found to be linear. The results of displacement measured from the tip of the tympanum and from a location 1.0 mm more central on the drum membrane revealed a large difference in displacement over all frequencies. A comparison between the shape of the audibility curve and the tympanic membrane velocity function in the parakeet revealed that both curves were nearly the same for the mid-range frequency region. These findings add to our understanding of middle-ear function in the avian ear.