Elizabeth S. Olson, PhD
Shyam M. Khanna, PhD
Shyam Khanna and Elizabeth Olson study the mechanics of the auditory system. Their work is primarily experimental, and uses specialized, custom-built devises for measuring the motion and pressures of auditory structures in the middle and inner ears. Dr. Khanna's pioneering studies used a highly specialized interferometer for measurements of the motion of auditory structures. Dr. Olson introduced a tiny pressure sensor capable of measurements of sound pressure within the cochlea.
APICAL TURN OF THE ORGAN OF CORTI in a guinea pig cochlea, with the laser focused on an inner hair cell. As sound stimulus is delivered to the ear, the motion of individual cells in the Organ of Corti is measured via the Doppler shifted frequency of the reflected laser light.
The Middle Ear
The middle ear has a straightforward, yet challenging function – to funnel sound energy from the large, air-filled environment into the tiny, fluid filled inner ear. The usual text-book description of the middle ear describes it essentially as a lever. This description is now known to be overly simplified. Recent research suggests that the middle ear transmits the sound along the eardrum as a wave. Studies done by Dr. Khanna and Dr. Olson were in part responsible for the emergence of this new view, and ongoing studies continue to probe the basis for this wave-like transmission.
The Inner Ear
The cochlea, the mammalian inner ear, derives its name from its snail-like shape (Fig. 1a). The structure of the cochlea is like a hose divided in two along its length, which coils several times around a central core and is encased in a bony shell. The organ of Corti is the sensory tissue of the cochlea. It is the hose's divider, and as such forms a long coiled strip of sensory tissue. Within the central core are the auditory neurons, which branch outward to make contact with the organ of Corti. Cochlear fluid fills the hose and surrounds the organ of Corti.
Within the organ of Corti are the inner hair cells, which translate the sound-induced motion of their sensory hair bundles to intracellular voltage changes. These intracellular voltage changes excite the auditory neurons and produce the nerve spikes that are the language of the brain. Also within the organ of Corti are the supporting structures: the basilar and tectorial membranes and supporting cells. The outer hair cells serve both as sensors and mechanical effectors in the response to sound stimuli. All these elements, coupled to the cochlear fluid, work in concert to present a mechanical stimulus to the inner hair cells that is sharply tuned in frequency. This tuning is a fundamental property of the frequency selectivity of the auditory system, which is responsible for many of the qualities of our lives – for example, our appreciation of music and our ability to communicate through spoken language.
Measuring Motion in the Organ of Corti
The study of cochlear mechanics has a long history. Our modern understanding is usually traced to the work of von Bekesy (G. von Bekesy, 1960). One of Von Bekesy's most influential contributions was his microscopic measurement of the motion of the organ of Corti. He discovered the tonotopic tuning that exists along the length of the organ of Corti: high frequency sounds caused motion of the organ of Corti at the base (input end) of the cochlea, and low frequency sounds caused motion at the apex of the cochlea, several centimeters down the length of the organ of Corti. Von Bekesy also observed the cochlear traveling wave. This is the wave of motion that ripples down the organ of Corti, carrying a stimulus to its own "best" place. He also made measurements of the stiffness of the organ of Corti. Other scientists showed that the stiffness of the organ of Corti and the mass of the cochlear fluid give a physical basis for the traveling wave and tonotopic tuning. von Bekesy's measurements addressed both the questions – "How does the sensory tissue move – what does it look like? and – "What is the physical basis for such motion?" In our work, this aspect of von Bekesy's approach is a guiding influence.
Von Bekesy's measurements were made at high sound pressure levels and often in cadaver ears. The next big breakthrough in cochlear mechanics came when Rhode (1971) discovered that frequency tuning is much sharper in healthy cochleae than in cadaver cochleae or more generally, in cochleae that have been traumatized. We now know this trauma can be due to overexposure to loud sounds, ototoxic drugs, or inadvertent damage to the delicate cochlea upon exposing and opening it for experimentation. This last form of trauma postponed Rhode's discovery and is still a substantial hurdle in intracochlear experiments.
The first measurements of sharp tuning using optical interferometric methods were by Khanna and Leonard (1982). Today, almost all measurements of basilar membrane motion use interferometric methods. The interferometer developed by Khanna and colleagues is a unique instrument that is capable of resolving the motion of individual cells in the organ of Corti, in vivo. The interferometer is used in conjunction with a slit scanning confocal microscope, capable of visualizing cellular detail at any prescribed depth within the organ of Corti. The results that have been garnered with Khanna's interferometer and microscope are among the most path-breaking results in organ of Corti motion measurements.
Measuring Intracochlear Pressure
In the field of cochlear mechanics much of the emphasis has been on measurements of the motion of the organ of Corti. The work of Elizabeth Olson has taken a different and complementary tack, measuring the intracochlear pressure close to the organ of Corti. Pressure measured at many positions within the cochlea was analyzed to reveal mechanical properties of the cochlea and the organ of Corti. Fig. 1b shows the tip of the pressure sensor, positioned about 40 mm from the organ of Corti's basilar membrane. The fluid pressure was found to contain large spatial variations, a finding that reinforced the need for including three dimensions in cochlear models. The results also showed that the pressure close to the organ of Corti shares many of the interesting properties that were known to exist in the motion of the organ of Corti, which indicates that the tuning of the cochlea is closely coupled to the cochlear traveling wave.
References:
Bekesy, G. von (1960) Experiments in hearing, McGraw Hill, NY. Khanna, S.M. and Leonard, D.G.B. (1986) "Basilar membrane tuning in the cat cochlea. Science 215, 305 ?? 306.
Rhode, W.S. (1971) "Observations on the vibration of the basilar membrane in squirrel monkey using the Mossbauer technique," J. Acoust. Soc. Am. 49, 1218 – 1231. |
