STRUCTURE-FUNCTION RELATIONS IN MIDDLE EARS

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The long-term goal of this work is to understand the relationship the structure of external and middle ears and their function in auditory signal transmission. The approach is to measure acoustic performance is selected species that vary in aural structure and to construct analytical models in which structural components are explicitly represented in networks of mechanical elements. From these measurements and models general rules for structure- function relations are derived. During the current grant period attention was focused on cat and lizard. In cat measurements of the properties of the external and middle ear have determined the performance of these parts of the ear as collectors and transmitters acoustic power to the inner ear. The lizard studies have shown that the usual presumption that sound enters the ear primarily though the ear canal must be modified; accessory sound pathways through large areas of the lizard's skin also couple sound to the middle ear, most effectively at low frequencies. Comparison of the measurements on cat and lizard shows that the tympanic membranes have distinctly different acoustic properties; at high-frequencies the cat TM is resistive whereas the lizard TM is masslike. These results suggest that substantial interspecies differences exist in the processes that couple acoustic stimuli into tympanic- membrane motion. The projects proposed here are focused on these processes and on the structural basis of variations in these processes among different vertebrate classes. Three areas will be examined: 1) The performance of the mammalian external ear, both as a collector of acoustic power and as a source of information about the location of sound sources, will be studied through measurements in species whose external ears have interesting structural similarities and and differences. 2) Accessory pathways to the middle ear will be studied in lizard, chicken, and also in some mammals (pig and sheep) in which the tympanic-membrane structure differs from cat, guinea pig, and human. In pigs and sheep a large structurally-distinct section of the tympanic membrane is flaccid rather than stretched. We will explore the hypothesis that this "pars flaccida" functions as an accessory pathway to the middle ear. 3) The relationship between structure and function of the tympanic membranes of cat, guinea, pig, chinchilla, alligator lizard, and chicken will be studied through measurements of their "mechanical transformer" properties. Since the tympanic membrane plays a key role in producing the transformer-like properties of the middle ear and its structure varies dramatically among vertebrates, specification of the TM's structure-function relations is key. The understanding of sound reception that results from this work will be of practical significant to audiologists in the diagnosis of conductive hearing disorders and to otologists in the surgical correction of abnormalities of the ear.