Predictive coding and stochastic resonance as fundamental principles of auditory phantom perception

Mechanistic insight is achieved only when experiments are employed to test formal or computational models. Furthermore, in analogy to lesion studies, phantom perception may serve as a vehicle to understand the fundamental processing principles underlying healthy auditory perception. With a special focus on tinnitus—as the prime example of auditory phantom perception—we review recent work at the intersection of artificial intelligence, psychology and neuroscience. In particular, we discuss why everyone with tinnitus suffers from (at least hidden) hearing loss, but not everyone with hearing loss suffers from tinnitus. We argue that intrinsic neural noise is generated and amplified along the auditory pathway as a compensatory mechanism to restore normal hearing based on adaptive stochastic resonance. The neural noise increase can then be misinterpreted as auditory input and perceived as tinnitus. This mechanism can be formalized in the Bayesian brain framework, where the percept (posterior) assimilates a prior prediction (brain’s expectations) and likelihood (bottom-up neural signal). A higher mean and lower variance (i.e. enhanced precision) of the likelihood shifts the posterior, evincing a misinterpretation of sensory evidence, which may be further confounded by plastic changes in the brain that underwrite prior predictions. Hence, two fundamental processing principles provide the most explanatory power for the emergence of auditory phantom perceptions: predictive coding as a top-down and adaptive stochastic resonance as a complementary bottom-up mechanism. We conclude that both principles also play a crucial role in healthy auditory perception. Finally, in the context of neuroscience-inspired artificial intelligence, both processing principles may serve to improve contemporary machine learning techniques

Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

The current trend for multipurpose rooms requires enhanced acoustic treatments capable to meet ever more demanding specifications in terms of performance, compactness and versatility. The reason is the variety of activities to be hosted and the corresponding requirements in terms of acoustic quality which may be very different and even conflicting. In any process to improve listening comfort, the treatment of low-frequency sound is a major concern. The problem stems from the proven ineffectiveness of passive soundproofing solutions of the state of the art, or from their bulkiness that may be prohibitive. This thesis focuses on the analysis, design, realization and characterization of tunable electroacoustic resonators intended to specifically address this issue. This concept deals with loudspeakers, the acoustic impedance of which can be easily adjusted in a controlled fashion. Creating an electroacoustic resonator out of a loudspeaker is the result of an interdisciplinary effort. Such a challenging task combines conceptual tools, models, and applied solutions, drawing from the fields of audio engineering, control theory, and electrical engineering, both in the analog and digital domains. A unifying theory is introduced, covering different strategies from passive electrical shunt to active control of acoustic impedance in a single formalism. This research shows that achieving a desired acoustic impedance at the transducer diaphragm is equivalent to the implementation of a specific functional relationship between the electrical current and voltage across the transducer terminals, and vice versa. From a design perspective, the specific electrical load is tailored by using an internal model of the transducer. The result is an innovative model-based synthesis methodology where the active control of acoustic impedance is reformulated as an electrical impedance synthesis, thus removing the use of sensor. This concept opens new opportunities to improve listening spaces by providing efficient acoustic absorption at low frequencies. Experiments clearly show the benefits of the proposed methodology in a field where there is currently no competitive solution. It is believed that the technological advances resulting from the coupling of a loudspeaker with a synthetic load should pave the way to innovative techniques in noise control and, hopefully, stimulate research in related areas