Caroline Abdala, Ph.D.- cabdala@mailhouse.hei.org
House Ear Institute
2100 W. Third St.
Los Angeles, CA 90057
Popular version of paper 2pPP2
Presented Tuesday afternoon June 23, 1998
ICA/ASA '98, Seattle, WA
Inner Ear Development
The human inner ear or cochlea is an incredibly intricate and fine-tuned sensory mechanism. The cochlea receives sound waves from the environment after the waves pass through the more peripheral outer and middle ear; it then transforms the energy into fluid motion which disturbs sensory cells with hairlike projections within the ear. Movement of the *hairs* on these sensory cells trigger activity in hearing nerve fibers that fire electrical messages to the part of the brain responsible for interpreting sound.
The maturation of this hearing process has not been widely studied in humans. We know that cochlear anatomy is grossly mature by 22-26 fetal weeks. The cochlea is adult-like in size and the primary sense organ of the ear, the Organ of Corti is generally organized in an adult-like way by the second trimester of pregnancy. A group of critical sensory cells called the Outer Hair Cells and the nerves that connect to these cells continue to mature into the third trimester. However, evidence indicates that cochlear structure is adult-like by term birth.
A more important question is whether this structurally adult-like organism functions in a mature, adult-like fashion by term birth. Afterall, we are more interested in how the newborn ear works than whether it simply looks like an adult ear. The problem with this second experimental question is that there have been few methods available to non-invasively evaluate function of the human cochlea. The cochlea is encased in very hard bone of the cranium and is grossly inaccessible for study in a living human being. However, in the last 10 years a new response that emanates from the inner ear has been discovered and characterized. This response, termed an otoacoustic emission, provides a non-invasive window into cochlear function.
Otoacoustic Emissions
Otoacoustic emissions are sounds created by the cochlea. This is the most recently discovered aspect of inner ear functioning and it underlies a revolutionary concept; the healthy inner ear not only receives sound from the environment, it can produce sound. We know that these sounds or emissions, come from the specialized sensory cells in the cochlea called the Outer Hair cells. One of the important things about otoacoustic emissions, other than their obvious clinical utility, is that they open up a previously inaccessible area of the human auditory system for scientific exploration. Otoacoustic emissions are present in almost all normal hearing individuals and are simple to measure by placing a sensitive, miniature microphone in the ear canal of any human being. The tiny sounds recorded from the ear canal are then amplified and subjected to advanced computer averaging and processing.
Cochlear Tuning
Tuning is the characteristic of the cochlea that allows the brain to distinguish a low- from a high-pitched sound. A small region of the cochlea will vibrate in response to a high-frequency sound like a flute. A completely separate area of the cochlea will vibrate if a bass drum is played. This exquisite mechanical tuning makes it easy for the brain to interpret frequency by the location of vibration within the cochlea. Tuning is important for the perception of speech since many of the cues we use to distinguish one word from the other are based on the ability to discriminate different frequencies.
In the last 5 years, we have developed new techniques based on the measurement of otoacoustic emissions to assess when and how the human cochlea becomes finely tuned and fully functional. The distortion product (DPOAE) is one type of otoacoustic emission. We have measured DPOAE suppression tuning curves in hundreds of premature neonates at the Infant Auditory Research Laboratory of the House Ear Institute. Tuning curves are the graphs we generate from each newborn to tell us how finely tuned their cochlea is. The youngest neonates are tested at 30 weeks post-conception (although they may have been born at 23 weeks of gestation) and our oldest subjects are normal term neonates.
Thus far, our results show that DPOAE tuning curves are adult-like by term birth. Other DPOAE techniques, describing different aspects of cochlear function, also suggest that the cochlea is functionally mature by term birth. However, cochlear tuning does not look adult-like in pre-term neonates ranging from 31 to 36 weeks conceptional age. Does this mean that the ear of these premature neonates cannot tell two frequencies apart? This is not likely, however, it is probable that the immature cochlea is simply not very precise at this task.As the premature neonate grows and approaches full term status (40 weeks), his/her cochlea begins to code frequency in an adult-like fashion.
Conclusions
There are few laboratories in the world looking at human pre-term neonates to study how and when inner ear function develops but the laboratories that are exploring auditory development, produce results consistent with our findings. This area is rife for future exploration and warrants a more detailed description of cochlear function during the pre-term period. We are currently trying to categorize our premature subjects into groups of very narrow age ranges so we can pinpoint when exactly the cochlea becomes tuned like an adult ear.
These questions about cochlear maturation are critical because babies are surviving at younger and younger premature ages. It is important to know when to expect normal cochlear function from these premature babies for diagnostic purposes. It is also important to know when the cochlea is mature in order to fit one more piece of the puzzle into the overall picture of human sensory maturation and development. Finally, this information may be critical to our development of new and better hearing aids and prostheses. Precise understanding of how the incredibly efficient healthy cochlea functions and matures may give us insight into how better to simulate or compensate for lost hearing.