PROJECT SUMMARY According to the Centers for Disease Control, hearing loss is the third most common chronic physical condition in the US, surpassing diabetes, and cancer in prevalence. Most cases of hearing loss occur due to damage to the cochlear amplifier. The term "cochlear amplifier" refers to the chain of processes that amplify low-level vibra- tions within the cochlea, thereby enhancing the sensitivity and dynamic range of hearing. While many details of the amplifier's mechanisms remain unclear, it is well-established that electromotility, the voltage-activated length changes of cochlear outer hair cells (OHCs), plays a crucial role. The OHC-driven amplification is vulnerable to common factors such as excessive noise, ototoxic drugs, aging, and congenital defects. Various insults can affect distinct stages of the amplification process, inhibiting electromotility or reducing OHC stimulation, leading to sensory hearing loss. However, current diagnostic tests, such as recordings of otoacoustic emissions (OAEs), have limited ability to provide precise information on the site of damage within the cochlea and its functional consequences. My research aims to address this gap by developing a diagnostic test of cochlear amplification. We propose that recordings of cochlear microphonics (CM), which represent the summed electrical fields of stimulated OHCs, can assess the functional state of local cochlear amplification and identify the site of damage within the amplifier. The proposed test combines the place-specific properties of cochlear two-tone suppression with recent findings on intracochlear motions that reveal broad regions of excitation and suppression in OHC vibrations. We hypothesize that the suppression of CM responses is controlled by local gain changes in cochlear motions; hence, we named it gain-sensitive CM (gCM). Thus, the gCM test should be capable of detecting coch- lear regions with dysfunctional OHCs and evaluating the dynamic range of the amplifier at a specific cochlear location. Furthermore, we predict that gCM, when measured in low- vs. high-intensity regimes, will exhibit differ- ential sensitivity to cochlear amplification loss caused solely by dysfunctional electromotility vs. by disruptions in the processes driving it. Currently, no other hearing test can achieve this level of diagnostic precision, which is essential for developing and testing individualized treatment options. In the short term, we aim to validate the gCM in mice by comparing it to direct, albeit invasive, measures of cochlear gain in the organ of Corti vibrations as well as to more established OAE tests. To evaluate the hypothesized site- and place-specificity of gCM, we will use both healthy and hearing-impaired mice where damage is either limited to a specific stage of the ampli- fication process or to a specific cochlear region. The results of this study have the potential to revolutionize the treatment of sensory hearing loss by providing an objective...