Auditory and non-auditory factors involved in loudness modulation: A model for tinnitus and hyperacusis

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Loudness, the subjective perception of sound level (dB), is one of the major perceptual attributes of sounds. While the general relationship between loudness and sound level is monotonic, the details of the loudness function can vary depending on several auditory and non-auditory factors. For instance, acoustic conditions such as reduction of sensory inputs and sound enrichment can increase and decrease auditory sensitivity, respectively. The auditory system has the capacity to modulate the gain depending on ambient auditory factors. This central gain model, however, is a recent proposition that has not received much empirical support. While it has been suggested that brainstem mechanisms may be involved in central gain modulation, it is unknown whether higher levels of the auditory system may also participate. In PART I (Auditory factors) we will use sound deprivation (earplugs) and enrichment (noise generators) to investigate the influence of the duration and intensity of deprivation/enrichment, right/left sided exposure, and age-related hearing loss on loudness modulation. Groups of young and older participants will wear either earplugs or noise generators for one week, and hearing measures from periphery (e.g., spontaneous otoacoustic emissions) to cortex (e.g., magnetic long latency auditory responses) will be taken before and after this manipulation. ***In PART II (Non-auditory factors) we will investigate the role of stress on loudness modulation. A few recent studies (some of which from our laboratory) have revealed strong associations between stress and loudness disturbances, but no data are available concerning the influence of stress in loudness modulation in normal hearing. We will first examine how psychosocial stress and synthetic cortisol alter loudness, with the expectation that either one can increase sensitivity to sounds. Next, we will examine the combined effect of psychosocial stress/cortisol and sound deprivation/enrichment on loudness, using protocols similar as in PART I. ***Finally, in PART III we propose to modulate cortical excitability by transcranial direct current stimulation, a painless and safe neuromodulation technique. Modulation of cortical excitability is expected to change the probability of hearing phantom sounds, as well as change loudness growth functions, each of which being a relevant model of tinnitus and hyperacusis, respectively. ***Relying on a unique expertise developed in my laboratory, this research program will provide novel evidence characterizing central gain mechanisms, the most recent hypothesis to explain loudness disturbances. In addition, determining how central gain can be modulated by stress will provide new hypotheses concerning auditory and non-auditory factor interaction in normal auditory function and its modification by aging. **