Using spontaneous optoacoustic emissions to probe frequency selectivity

Abstract

Hearing -a sensation that seems to easily appear to us in our daily life. But the sense of hearing is much more than just as if perceived incidentally. Hearing is the key element in human communication and plays an important role in cultural aspects as in language and music. We use acoustical information every day, while always depending on the two coiled structures in our skull, each of them about 3.5 cm long -the cochleae. The cochlea is fundamental for a highly frequency-selective acoustic perception. Frequency selectivity is the ability to discriminate between sounds with different frequencies. The focus of this thesis lays on the application of spontaneous otoacoustic emissions to measure frequency selectivity of the cochlea and thus mainly at the level of the peripheral auditory processing.Understanding.the.precise.mechanisms.of.the.inner.ear.physiology.that.define.the. high resolution of frequency selectivity, will expand our understanding of cochlear mechanics.and.species-specific.differences.as.well.as.the.evolutionary.development.of. the auditory system. This.first.chapter.introduces.the.general.properties.of.acoustic.cues..Further,.the. anatomy.and.physiology.of.the.human.ear.will.be.outlined.and.briefly.compared.with. the characteristics of the avian ear morphology and sound transmission. Secondly, what spontaneous otoacoustic emissions are and how they are used to test frequency selectivity of an individual will be explained. Finally, the following chapters of this thesis will be introduced. Properties and perception of soundSound in air can be understood as an oscillating movement of air molecules that causes differences in density and pressure. The unit of sound pressure is pascal (Pa). The pressure variations act as traveling pressure waves that spread away from the sound source causing them. In sophisticated communication cues (as speech) such pressure variations are complex, but can be separated into their smallest components -pure tones.Any pure tone consists of a sinusoidal waveform. Changes in increased and decreased density and pressure create the waveform of a pure tone with molecule condensation peaks and rarefaction troughs. Thus, a sound wave consists of repeating cycles of peaks and.troughs..Each.waveform.has.a.specific.frequency.that.describes.how.many.of.these. waves pass a given point in one second. The frequency is consequently a measure of cycles per second, expressed in hertz (Hz). I want to emphasize here that pitch is the subjective percept of frequency. This percept correlates with the frequency as high frequencies are perceived as high pitch sounds and low frequencies as low pitch sounds. 14Chapter 1 Figure 1.2. A schematic illustration of the cross-section of the cochlea..(A).The.cochlear.spiral.is.viewed.from.the.side..The.three.fluid-filled.compartments.that. spiral.around.the.central.axis.can.be.seen..(B).A.magnification.of.the.median.compartment,.the.scala.media,.reveals.the.organ.of.Corti. When the channels are open, potassium ions (K + ) enter the HC which causes depola...