Distortion product otoacoustic emission phase and component analysis in human newborns

Abstract

Apical distortion product otoacoustic emissions (DPOAEs) are comprised of at least two components, as evidenced by the interference pattern of alternating maxima and minima known as fine structure. DPOAE fine structure is produced by the shifting phase relationship in the ear canal, between the generator and characteristic frequency (CF) component of the response. Each component arises from a different cochlear region and, according to theory, reflects a distinct generation mechanism. The analysis of DPOAE components and phase in newborns may provide a window into targeted aspects of cochlear physiology during development. 2f(1)-f(2) DPOAE fine structure was recorded from 15 adults and 14 newborns using a swept-tone technique. DPOAE group delay, as well as magnitude and phase of each component, was compared between age groups. Results show narrower fine structure spacing, a longer group delay (steeper phase gradient) in low frequencies, and a stronger relative contribution from the CF component in newborns. The prolonged group delay for low-frequency DPOAEs could indicate immature basilar membrane motion in the apex of the cochlea and warrants further investigation. The enhanced contribution from the CF component may have implications for clinical practice as well as for theories of cochlear maturation.

Figure 1

DPOAE level as a function of DPOAE frequency for 15 adults and 14 newborns. Level was averaged into 500-Hz-wide frequency bands. The frequency displayed represents the upper limit of this band. Error bars=±1 SD.

Figure 2

[(a)–(d)] DPOAE fine structure (thick line), phase (thin line), and noise floor (gray line) from two newborn and two adult subjects. The range of values is the same in each graph, though absolute values vary. Note that DPOAE phase is referenced to the right vertical axis.

Figure 3

Mean DPOAE fine structure features from 15 adults and 14 newborns: (a) frequency spacing between fine structure periods and (b) depth of fine structure periods. Data were averaged into 500-Hz-wide frequency bands. The frequency displayed represents the upper limit of this band. Error bars=±1 SD.

Figure 4

Mean DPOAE group delay (negative of the slope of the phase) as a function of DPOAE frequency for 15 adults and 14 newborns. Data were averaged into 500-Hz-wide frequency bands. The frequency displayed represents the upper limit of this band. Error bars=±1 SD.

Figure 5

[(a) and (b)] DPOAE level as a function of frequency for each component and age group separately. The smaller inset graph shows the level difference between components (generator level subtracted from CF level) for each age group. Data were averaged into 500-Hz-wide frequency bands. The frequency displayed represents the upper limit of these bands. Error bars=±1 SD.

Figure 6

DPOAE phase for the generator (gray) and CF component (black) as a function of DPOAE frequency for both age groups. Lines represent phase from one adult subject, whereas each newborn subject has two such traces displayed. The mean slope of phase values, calculated with a linear regression function, are shown for each age and component.