Cooperativity Between Hair Cells And Role Of Noise In The Spontaneous Generation Of Sounds By The Inner Ear

This grant will support research that will contribute new knowledge regarding how sounds are generated by the inner ear in the absence of any external sound. These are called spontaneous otoacoustic emissions. Humans with normal hearing can detect faint sounds due to the active feedback from hair cells within the inner ear. Spontaneous otoacoustic emissions are a direct consequence of this feedback. These emissions are commonly measured in humans and other species, but remain poorly understood. This project focuses on fundamental research that provides needed knowledge about how the ear operates and generates spontaneous emissions. Hearing impairment, which affects millions of Americans, is often caused by loss of hair cell function. By improving understanding of the ear, this project will ultimately help to better diagnose and treat hearing loss, which will benefit US society. This project involves multiple disciplines including biophysics, biomechanics, computational modeling, acoustics, and nonlinear dynamics. The project will help broaden participation of underrepresented groups in research, positively impact engineering education, and sensitize students to the risks and consequences of noise overexposure. Spontaneous otoacoustic emissions (SOAEs) provide important clues about how the inner ear operates because it implies that the ear is an active system. However, two seemingly irreconcilable theories currently exist for SOAE generation, the standing wave (SW) model and the coupled limit-cycle oscillator (CLCO) model. This project aims to clarify the biophysical mechanisms of SOAE generation by evaluating the ability of these theories to predict the key characteristics of experimental SOAE recordings. The central hypothesis of this project is that the two theories predict significantly different nonlinear dynamics when the effect of external stimuli and noise on SOAEs is considered. To test this hypothesis, the research team will implement a series of models of varying complexity for the SW and CLCO theories. These models will be used to investigate (1) the generation mechanisms of SOAEs; (2) the effect of external tones and noise (both internal and external) on the nonlinear dynamics of SOAEs; (3) the functional consequences of noise and SOAE generation on the detection of low-level sounds. Comparison of model predictions to experimental SOAE measurements will be used to assess the validity of the models, constrain these models, and identify the best model(s). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.