Neural and Perceptual Mechanisms for Coding Frequency Modulation

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Frequency modulation (FM) provides critical information in both speech and music, meaning that accurate neural encoding and perception of FM is essential for human communication. Similarly, binaural information helps us navigate complex everyday environments by allowing us to localize sounds and segregate spatially separated sources. Both FM and binaural perception worsen with age in a manner that cannot be accounted for by hearing loss, suggesting some degree of aging effects are driven by changes in the auditory brain. In addition, people with mild cognitive impairment (MCI) and Alzheimer’s Disease (AD) exhibit poor binaural processing and speech-in-noise perception. The present proposal will test the hypothesis that some of the challenges experienced by people with MCI and AD when navigating complex auditory scenes can be accounted for by underlying degradation in auditory encoding, beyond what is due to memory impairment and general cognitive decline. Exacerbated aging effects in the auditory brain in people with AD and related dementias have been shown to be concomitant with difficulties in perception. MCI is the prodromal phase of AD, and an opportune stage for investigating early neural and perceptual markers of AD. There is a critical need for psychophysical experiments in MCI patients that carefully control for memory and task demands, especially given the long-term potential for understanding the neural mechanisms of AD and preventing the progression of further decline. The proposed research will be the first characterization of monaural and binaural FM and amplitude modulation (AM) perception in MCI and will test whether perception is differentially impacted for stimuli with and without temporal fine structure (TFS) cues. We will specifically test people who have been diagnosed with MCI due to AD with AD biomarkers as well as age- and audiometrically matched controls. Aim 1 will assess modulation perception using diotic stimuli where a single ear can be used to perform the task optimally. We will test whether MCI is associated with an FM-specific deficit beyond what can be explained by age, hearing loss, and task demands. We will use a novel, AM stimulus that simulates the effects of FM perception, but without the presence of TFS cues, to assess the role of monaural TFS versus envelope (ENV) coding in MCI. Aim 2 will further probe the central auditory system by testing whether binaural impairments previously found in MCI are driven by binaural processing of TFS or ENV, or by higher-level constraints. The results will provide the first rigorous test of auditory encoding in MCI that targets specific auditory mechanisms (ENV vs. TFS processing) and compares measures that rely on one versus two ears. Results will provide the preliminary data and foundation for larger-scale studies on how auditory neural coding and perception change through early and more advanced stages of AD, and is a key opportunity for the PI to expand her research scope to the interaction of auditory perception and cognition. Findings will also contribute to the search for an early, affordable, and accessible perceptual marker for AD.