The cochlea sends signals to the brain by two types of sensory neurons, and is controlled by two neuronal feedback systems. The Liberman lab studies the neurophysiology and neuroanatomy of all four neuronal pathways in normal and damaged ears. Our work on sensory pathways demonstrated functional subgroups of auditory nerve fibers differing in threshold sensitivity and clarified the contributions of each to stimulus coding in the periphery. Our work on efferent feedback demonstrated roles for these pathways in protecting the ear from overstimulation, in minimizing effects of masking noise on signal detection, and in balancing auditory-nerve excitability in the two ears for sound localization based on interaural level differences.

Our animal work on noise damage has clarified the functionally important structural changes to the inner-ear sensory cells underlying reversible vs. irreversible noise-induced hearing loss. More recently, we discovered that there is dramatic noise-induced loss of auditory-nerve fibers, even in cases where the threshold shifts are fully reversible: a phenomenon known as “hidden hearing loss”. This primary neural degeneration has minimal effects on threshold detection, but impairs the discrimination of complex sounds, especially in a noisy environment. Our analysis of human autopsy material has shown that this neural degeneration is also a major feature in the pathology of age-related hearing impairments.

Our lab is now developing therapeutic interventions designed to slow or reverse this noise-include and age-related loss of primary sensory neurons in the inner ear. We have shown that neurotrophins applied to the inner ear after noise damage can regenerate nerve terminals and restore normal neural responses. Ongoing work aims to extend the trauma-treatment interval over which these therapies are effective.