Neuroimaging correlates of hearing loss, tinnitus, and hyperacusis

Abstract

possibly related conditions on the brain. To that end, it is crucial to gain insight into the characteristic differences and adaptation of the central auditory system that accompany hearing loss, tinnitus, and hyperacusis. This first chapter gives a short overview of the auditory system and the ramifications if the system is compromised due to hearing loss and associated symptoms such as tinnitus and hyperacusis. The pathway from sensation to auditory perception is described, starting at the peripheral auditory system and progressing to the auditory cortex. Furthermore, I present the current state of knowledge on subcortical and cortical substrates of hearing loss, tinnitus, and hyperacusis. Then, I will give a short introduction to magnetic resonance imaging (MRI), the method used to investigate the auditory system's structural and functional characteristics in this work. Finally, I will provide an overview of the subsequent chapters within this thesis. SOUND PERCEPTIONSound perception arises from a complex system that translates the variations in pressure, which form propagating sound waves, into an audible sound. The human ear has the incredible capacity to detect pressure variations as small as 2 x 10 -5 Newtons per square meter. This detection threshold has led to the convention of expressing the level of a sound that travels through the air in sound pressure level relative to this reference intensity (dB SPL) (Fletcher and Munson, 1933). Decibels (dB) are commonly used to describe the intensity of a sound on a logarithmic scale. The practical use of this becomes evident when you consider that the human auditory system can code for a 10^1 3 -fold increase in sound intensity from a just noticeable sound to a sound that is painfully loud: an impressive dynamic range.The sensitivity of hearing is dependent on the frequency of sound. The frequency spectrum that the human auditory system can encode ranges from 20 Hz to 20 kHz. The dB Hearing Level scale (dB HL) reflects this frequency-dependent difference in sensitivity and is expressed relative to standardized or average thresholds. The dB HL scale is used to describe hearing thresholds that are depicted with an audiogram. The perceptual correlate of sound intensity is the loudness of a sound, which is additionally affected by the bandwidth, frequency, and temporal characteristics of a sound (Fletcher and Munson, 1933;Behler and Uppenkamp, 2016). The subjective loudness of pure tones can be expressed with a phon scale, as is illustrated in figure 1.1. This scale equates the loudness of pure tones of a particular frequency to a reference tone of 1000 Hz at set intensities and was originally devised in 1933 by Fletcher and Munson. Therefore, they are often called Fletcher-Munson curves. These equal loudness contours have since been revised and incorporated into an international standard (ISO). A BSignificance Statement Age-related hearing loss is the most prevalent sensory impairment in the elderly population. Older individuals are subject to the cumulative effect...