Neural bases of masked amplitude-modulation perception - The proposed project uses behavioral and neurophysiological experiments in an animal model to explore mechanisms underlying the “modulation filterbank” of human perceptual studies. Envelope fluctuations, called amplitude modulation (AM), are a critical acoustic feature of sound necessary for robust speech perception. Human listeners show diminished AM sensitivity in competing envelope fluctuations of similar frequency, known as modulation masking. Modulation masking is not explainable by classic power-spectrum models, but instead indicates a “modulation filterbank” processing strategy that separates concurrent sounds (e.g., AM targets from noise) that have different AM frequencies. The modulation filterbank is an exciting theoretical advancement because in addition to explaining modulation masking, the model can predict differences in speech-in-noise perception across fluctuating and multi-talker noise sources. Physiological mechanisms of the modulation filterbank are crucial to understand given high relevance to real-world hearing, but remain poorly understood due in part to limited development of animal models. The proposed research will significantly advance our knowledge of the modulation filterbank through operant-conditioning experiments and neural recordings in actively behaving budgerigars. Budgerigars, a parakeet species, are selected based on prior studies showing human-like behavioral sensitivity to many simple and complex sounds. Furthermore, budgerigars and other birds show largely conserved auditory processing mechanisms with mammals from the auditory nerve to the forebrain processing level. Aim 1 establishes the budgerigar as a new animal model of the modulation filterbank using behavioral modulation-masking experiments conducted with operant- conditioning procedures. Preliminary results demonstrate first-ever evidence of the perceptual modulation filterbank in a nonhuman species. Aim 2 quantifies neural mechanisms of modulation-masked behavioral AM sensitivity using extracellular neural recordings from the midbrain and forebrain in behaving animals. Preliminary results support the hypothesis that midbrain average-rate responses explain behavioral band-pass modulation masking, the hallmark of the modulation filterbank, at mid-to-high target AM frequencies. Recordings are made at two processing levels to assess transformation of the modulation filterbank code along the ascending pathway. Aim 3 extends our analysis of the modulation filterbank concept to a real-world listening task in fluctuating noise using “comodulation masking release” (CMR) experiments. CMR is the improvement of tone-in-noise sensitivity observed in fluctuating noise, selected based on recent theoretical studies implicating the modulation filterbank as a key potential mechanism. Aim 3 experiments empirically test a modulation-filterbank mechanism for CMR using behavioral and neurophysiological experiments for the first time in the same species. The proposed project significantly advances our understanding of the modulation filterbank and the role of this exciting model for real-world hearing in noise.
