Page 9 - Volume 12, Issue 2 - Spring 2012

ACOUSTICAL TESTS OF MIDDLE-EAR AND COCHLEAR FUNCTION IN INFANTS AND ADULTS
Douglas H. Keefe
Boys Town National Research Hospital 555 North 30th Street Omaha, Nebraska 68131
“Absorbed sound power can control for ear-canal and middle-ear sources of variability across frequency in physiological, behavioral and clinical measurements of auditory function.”
The peripheral processing of stapes can alter the stiffness of an annu-
sound by the external ear, middle
ear and cochlea precedes the
neural encoding of sound. The external
ear collects sound power that is trans-
mitted and reflected within the ear
canal, absorbed by the middle ear, and
transmitted as a coupled mechanical-
fluid wave motion within the cochlea.
The cochlea analyzes the time-varying
frequency components of the incident
sound and converts them into a spatial-
temporal distribution of neural spikes
in the fibers of the auditory nerve. This
encoded signal is subsequently ana-
lyzed by the brain through detection and classification processes, which enables the human listener to construct a perceptual and cognitive map of the auditory world.
The relative efficiency through which sound is transmit- ted from the external field of the listener to the cochlea varies with age, and thereby constrains the maturation of hearing. An important practical goal of measuring acoustical respons- es within the ear canal is to identify the presence of, and quantify the effects of, any dysfunction in the acoustical and mechanical pathway of sound transmission. Such informa- tion is clinically important in screening and diagnosing hear- ing loss, which would negatively affect speech perception and music perception. Of particular note is the growth in the last couple of decades in universal newborn hearing screening programs to identify hearing loss. This article describes how acoustical measurements in the ear canal in response to sound are used to study and clinically assess hearing in chil- dren and adults.
A brief tour of ear anatomy and physiology
The external ear includes the visible structures on the side of the head such as the irregular concave surface of the pinna and the concha cavity, from which the ear canal entrance leads to the eardrum via a curved canal with a length of about 2.5 cm in the adult. The middle ear includes the eardrum, a space bounding the interior surface of the eardrum called the tympanic cavity, and three small bones known as ossicles that couple displacements of the eardrum to a displacement of what is termed the oval window of the inner ear. The tympanic cavity is vented to the nasal cavity via the Eustachian tube, which acts to equalize air pressure across the eardrum. The ossicular chain includes the malleus, which is connected to the eardrum, incus and stapes, which is attached to the oval window. A set of ligaments support the ossicles within the cavity. A stapedius muscle attached to the
8 Acoustics Today, April 2012
lar ligament that couples the stapes foot- plate to the oval window, which can thereby alter the amount of sound- induced mechanical energy transmitted into the cochlea. From an acoustical perspective in normal hearing, the func- tional role of the middle ear is to trans- mit acoustical sound energy from the external ear into a coupled mechanical- fluid motion within the inner ear, and otherwise overcome the large imped- ance mismatch between air and fluid.
The cochlea, which is part of the inner ear, is a bony spiral shell that con- tains three fluid-filled compartments in which are found the cellular structures of the cochlear partition. The fluids with- in scala vestibuli and scala tympani compartments are con- tinuously joined at the extreme apical end of the cochlear spi- ral. The cochlea is coupled at its base to the tympanic cavity of the middle ear via the oval window, whose motion drives a motion of the nearly incompressible fluid in scala vestibuli, and the round window, which moves in response to a motion of fluid in scala tympani. An inward motion of the oval win- dow displaces the cochlear fluid and generates an outward motion of the round window. A pressure difference results across the flexible cochlear partition, which moves in response to this time-dependent force. This partition includes the basilar membrane on which is placed the organ of Corti. The cellular structures on the organ of Corti are the most mechanically sensitive in the human body, and form a critical part of our ability to hear over a ten-octave range of
frequency and 120-dB range of sound pressure level. Through a remarkable series of experiments in the mid- 20th century using partially intact cochleae of human cadav- ers, Békésy (1989) discovered that sound produced an activa- tion pattern of transverse wave displacement of the basilar membrane that had its maximum at high frequencies close to the base and at low frequencies close to the apex. This fre- quency-to-place (tonotopic) encoding of auditory signals at the cochlear level is extensively replicated throughout the brain structures that process auditory information. Basilar- membrane motion displaces the organ of Corti and induces fluid motion in scala media to displace the cilia of the inner hair cells. This ciliary motion gates an electrical current flow into the inner hair cell, which controls spike generation in afferent fibers of the VIIIth (auditory) nerve at the base of the hair cell. The resulting spike trains are transmitted to the
brainstem.
Basilar membrane motion in responses to pure-tone