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The study showed that the SFI is a feasible test of middle ear function in neonates. The SFI results revealed two regions of resonance with the lower resonance (287 Hz) possibly related to the movements of the outer ear canal wall and higher resonance (1236 Hz) related to the resonance of the middle ear. The normative data developed in this study will be useful in evaluating outer and middle ear function in neonates.

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The high sensitivity of mammalian hearing is achieved by amplification of the motion of the cochlear partition. This cochlear amplification is thought to be generated by the elongation and contraction of outer hair cells (OHCs) in response to acoustical stimulation. This motility is made possible by a membrane protein embedded in the lateral membrane of OHCs. Although a fructose transporter, GLUT-5, was initially proposed to be this protein, a later study identified the gene of the motor protein distributed throughout the OHC plasma membrane. This protein has been named Bprestin.^However, although previous morphological studies by electron microscopy and atomic force microscopy (AFM) found the lateral wall of OHCs to be covered with 10-nm particles, believed to be motor proteins, it is unknown whether such particles consist only of prestin or are a complex of GLUT-5 and prestin molecules. To determine if the 10-nm particles are indeed constituted only of prestin, plasma membranes of prestin-transfected and untransfected Chinese hamster ovary (CHO) cells, which do not express GLUT-5, were observed by AFM. First, the cells attached to a substrate were sonicated so that only the plasma membrane remained on the substrate. The cytoplasmic face of the cell was observed by the tapping mode of the AFM in liquid. As a result, particle-like structures were recognized on the plasma membranes of both the prestintransfected and untransfected CHO cells. Comparison of the difference in the frequency distribution of these structures between those two cells showed approximately 75% of the particle-like structures with a diameter of 8-12 nm in the prestin-transfected CHO cells to be possibly constituted only by prestin molecules. Our data suggest that the densely packed 10-nm particles observed on the OHC lateral wall are likely to be constituted only of prestin molecules.