Postnatal Maturation of Human Cochlear Function and Sources of Immaturity

Principal Investigator:  Carolina Abdala, Ph.D.

Research Staff: 

Sandy Oba, M.S., Research Audiologist
Ellen Ma, M.D., Laboratory Mgr
Ping Luo, M.S.E.E., Systems Engineer
Leslie Visser-Dumont, M.S.,  Research Audiologist

Consultant:  Douglas Keefe, Ph.D. Boystown National Training and Research Hospital
 

Experiment #1 DPOAE (2f1-f2) Suppression in Three-month-old Infants: Evidence for Postnatal Maturation of Human Cochlear Function?  (J. Acoust. Soc. Amer., 116, (6), 3572-3580).

The complete timeline for maturation of human cochlear function has not been defined. DPOAE-based measures of cochlear function have shown non-adultlike responses from premature and term-born neonates at high f2 frequencies, however,  older infants have not been included in these studies.  In this experiment,  previously collected DPOAE ipsilateral suppression data from premature neonates was combined with new data collected from normal-hearing adults, term-born neonates and 3-month-old infants to further examine the time course for the maturation of cochlear function. DPOAE suppression tuning curves (STC) and suppression growth patterns were measured in all subjects groups using moderate primary tone levels, at f2= 6000 Hz with an f2/f1 of 1.2 DPOAE STC width, slope and tip features were measured as well as suppression growth.  Results indicate that term-born neonates,  as well as 3-month-old infants,  have non-adultlike STC width, slope on the low-frequency flank and tip level. However, the two infant groups were not significantly different from one another.  Additionally, suppression growth patterns for low-frequency suppressor tones show a clear developmental progression.  The youngest premature neonates  show shallow, compressive growth of suppression for the lowest 4 suppressor frequencies; term-born neonates and 1-month old infants show shallow growth for these same tones but appear to be moving toward adult slope values; and suppression growth from 3-month-olds is adult-like in all ways except for the lowest-frequency suppressor tone.  These results suggest a high-frequency postnatal immaturity in cochlear function as measured by DPOAE suppression.  Although presumed to be cochlear in origin, these results may have been influenced by non-cochlear factors, such as middle ear immaturity.

 

Experiment #2: Effects of Aspirin on DPOAE Suppression in Human Adults: comparison with neonatal data (In Press J. Acoust. Soc. Am)

One of the distortion product otoacoustic emission (DPOAE) paradigms used to study cochlear function is DPOAE (2f1-f2) ipsilateral suppression.  Newborns do not have adult-like DPOAE suppression.  At 6000 Hz, infants show excessively narrow DPOAE suppression tuning and shallow growth of suppression for low-frequency suppressor tones.  The source of this immaturity is not known but the outer hair cell (OHC) is one possible locus.  In the present study, DPOAE suppression was measured at f2 = 1500 and 6000 Hz from two groups with impaired OHC function in an attempt to model the observed immaturity in neonates: adults with aspirin-induced OHC dysfunction and subjects with sensorineural hearing loss (SNHL).  Their DPOAE suppression results were compared to those obtained from a group of term newborns to address whether infant DPOAE suppression resembles suppression from individuals with known OHC dysfunction.   Results indicate that aspirin systematically alters DPOAE suppression in adults at f2 = 6000 Hz, but not 1500 Hz.  However, neither aspirin-induced OHC dysfunction nor naturally occurring SNHL produces “neonatal-like” DPOAE suppression at either test frequency. This finding does not support the hypothesis that non-adultlike DPOAE suppression characterizing newborns can be explained by minor impairments or alterations of OHC function.

 

Experiment #3: Development of Human Cochlear Function: A Longitudinal Study through 6 months of age. (ongoing).

This is the primary study of this project. The objective of this project is to define the timecourse during which functional maturation of cochlea occurs in human infants. Two groups of  newborns (term-born and prematurely born but at “term-like” conceptional age) are being tested at f2 = 1500 and 6000 Hz with the DPOAE suppression paradigm. They are tested at birth, and 3, 4, 5, and 6 months of age.  It is hoped that this study will elucidate the actual time period during which DPOAE suppression tuning and growth become adult-like for both low and high-frequency stimuli.  Additionally, results of this experiment will help determine if neonates born prematurely develop cochlear function during the same time course as do term-born infants or whether they show a delay in maturation.

Thus far, we have screened 46 babies for subject participation.  Of these, 37 babies were enrolled in the study resulting in a 80% success rate for recruitment.  21 of these are active subjects currently being tested at appropriate age intervals (8 premature and 13 term). Six subjects have completed the entire 6 month protocol. The results of the study are not available at present since there has been limited data collected.  It is hoped to be complete by summer 2006.

Experiment #4:  The Influence of Middle Ear Maturation on DPOAE Suppression Characteristics (ongoing)

This experiment investigates another potential source for non-adultlike DPOAE suppression observed in human infants.  In this study, middle ear function is measured in infants during a series of longitudinal sessions at birth, 6 weeks, 3, 4, 5 and 6 months of age.  Because the stimulus for generating the DPOAE must pass through the middle ear to stimulate the cochlea and the DPOAE response must travel (through reverse transmission) back through the middle ear to be detected in the ear canal, the status of the middle ear can greatly influence DPOAE results. Power-based measures such as otoreflectance provide important information about middle ear function (Keefe et al., 1993; 1994) and can explain variations in how the middle ear receives, absorbs and transmits sound energy.  In infants, reflectance as a function of frequency, changes with increasing age.  However, it is not known whether changes in reflectance co-vary with changes that have been consistently noted in DPOAE suppression over the first 3 months of life.  This experiment will analyze the co-variance of these two measures over 6 months and elucidate whether changes/maturation of middle ear function can explain or account for developmental changes in  DPOAE suppression.   21 subjects are currently enrolled in this study.