The long-range goal of this project is to contribute to the understanding of the physical bases of signal processing in the vertebrate ear. During the proposed project period, the work will address three questions: (1) How is high-dynamic order achieved in the tuning structures of lower vertebrates? (2) Is high-dynamic order a universal property of acoustic tuning structures and low-dynamic order a universal property of equilibrium tuning structures? (3) What other linear and nonlinear signal-processing properties distinguish acoustic and equilibrium sensors? Answers to these questions should provide deeper understandings of the sense of hearing and the equilibrium senses, which in turn should provide integrative perspectives for the studies of micromechanical and molecular devices involved in hearing and equilibrium sensing. Question (1) win be addressed with a combined experimental and modeling study aimed at one of the most thoroughly studied hair-cell systems-- the sacculus of the American bullfrog. Questions (2) and (3) will be addressed with an intensive comparative physiological study of afferent neurons of four nonhomologous acoustic sensors (frog lagena, sacculus, amphibian papilla, and gerbil cochlea) and two equilibrium sensors (frog lagena and utricle). The study will focus on the following issues: (a) linear tuning dynamics; is the dynamic order high or low; does the temperature dependence imply involvement of molecular kinetics (b) adaptation; is it linear (e.g., akin to differentiation) or nonlinear (e.g., akin to automatic gain control); what does it imply about underlying mechanisms? (c) response to heartbeat: is the sensor so sensitive that it responds to the animal's own blood flow? (d) suppression: does the sensor exhibit single-tone or two-tone suppression, (e) temporal resolution: does the sensor respond to subtle temporal events; does its temporal resolution improve with increased ambient noise level? Hypotheses to be tested: High dynamic order, nonlinear adaptation, responsiveness to heartbeat, suppression, responsiveness to subtle temporal events, and improvement of temporal resolution with increasing noise level all are features of acoustic sensors not shared by equilibrium sensors; when, in the course of evolution, a sensor or pan of a sensor has shifted from equilibrium sense to acoustic sense, or vice versa the shift must have involved the addition or removal of a large array of such features.