PROJECT SUMMARY/ABSTRACT Hearing loss is the second most common health condition among adults. Untreated hearing loss is associated with an increased risk of depression, anxiety, social isolation, dementia, and all-cause mortality. Despite this, only 14% of people who would benefit from hearing aids use them. The most common reason adults give for not using hearing aids is that hearing aids do not help them in the very situations they struggle to hear in the most—complex listening environments (CLEs). CLEs are places with many different sound sources, like restaurants. Indeed, restaurants are the most difficult listening environments for the majority of adults with hearing loss. Hearing aid technologies that are designed to improve speech perception in noise, such as noise reduction and directional microphones, show some modest benefits when tested in a laboratory environment. In the real world, however, these technologies show no benefits. Hearing aids can therefore demonstrate efficacy (how well they can work in the best possible scenario, i.e., in a laboratory test), but fail to be effective (how well they work in the real world). We propose that listening environment complexity differences between the lab and the real world drives the hearing aid efficacy-effectiveness gap. We further propose that the effect of environment complexity on listening performance is moderated by cognitive ability, and the benefit of hearing aid features on listening performance is moderated by environment complexity. Our aims are designed to test these hypotheses in the laboratory using controlled experimental paradigms, as well as in the real world using field experiments. The proposed study is informed by information theory, which defines complexity as the amount of information in a communication system. Under this theory, complexity can be measured using entropy to quantify how much information is in the system. Our team will apply this framework to understanding communication in real world CLEs. In Aim 1, we characterize the relationship between complexity, cognitive ability, and hearing aid feature efficacy in the lab using a controlled experimental paradigm. In Aim 2, we characterize the relationship between complexity, cognitive ability, and hearing aid feature effectiveness in the real world using smartphones to record real world environments and deliver surveys to participants, as well as experimental hearing aids that record how features process signals in real-time. The findings from this study will: enhance our understanding of how hearing aid users perceive real world CLEs, extend information theory to quantify the complexity of real world CLEs and its effect on listening performance, provide important insight into the factors that underlie the hearing aid efficacy- effectiveness gap, and provide groundwork that may help guide developments to close the efficacy-effectiveness gap through clinical and engineering approaches. The proposed study directly ...