We do not yet know exactly when the human cochlea or the processes that regulate cochlear function become fully functional and mature in babies. Institute Scientist, Caroline Abdala, from the Division of Communication and Auditory Neuroscience, studies cochlear function in newborns by recording inner ear responses called distortion product otoacoustic emissions, or DPOAEs. These responses are present in the normally functioning ear and are recorded easily and non-invasively using a sensitive microphone that captures acoustic distortion coming from the ear. This distortion is produced by the vigorous movement of the hair cells as they work to amplify the waves created along the basilar membrane by sound.

Past experiments in Dr. Abdala’s lab have established that newborn cochlear responses are not adult-like at birth. DPOAEs show differences in tuning (how the ear processes frequency or pitch) and in their amplitude (size or magnitude). The source or origin of these differences is not well understood and may include immaturies in the outer and middle portion of the ear (in contrast to the cochlea). The lab is currently measuring DPOAEs in newborns using an innovative technique applied in only a handful of labs. No other laboratory in the country has access to the newborn ear within 48 hours of birth to make these unique and informative measurements.

A DPOAE is actually made up of two elements that come from different parts of the cochlea and represent distinct functional properties. Dr. Abdala’s lab is currently measuring DPOAEs with very fine resolution, allowing these two uniquely informative components to be separated with innovative signal processing techniques so that they can be evaluated individually. Because each DPOAE component seems to reflect a distinct aspect of cochlear function, it may become possible to describe maturation of the ear with more specificity and detail than ever before. Initial results show that one of these elements, the “reflection” component, is relatively large in babies (vs. adults). This suggests the cochlear amplifier, a process that enhances ear tuning and sensitivity, is robust and highly functional at birth.

Another exciting area of work involves regulation of cochlear function. The nerve fibers that connect the brainstem to the cochlea are called “MOC” fibers. These fibers inhibit or put the “brakes” on cochlear hair cells, preventing them from moving efficiently when very loud sound is presented to the ear, thereby protecting the ear from potentially damaging sounds. The MOC reflex also helps a listener distinguish between the message (speech, for example) and intrusive background noise. This is of great importance for babies and young children learning language and speech, sometimes in noisy classrooms.

Having a more detailed and precise map of normal cochlear development will improve the accuracy of guidelines for neonatal hearing screening and assessment. This will aid in distinguishing what is pathology/disease and what is normal immaturity. When we fully understand the contribution of various parts of the ear to hearing, we will be better able to simulate the normal hearing process when we design hearing aids or other auditory prostheses, such as the cochlear implant.