After studying this chapter, the student should be able to:
Diagram the transduction of sound in the outer, middle, and inner ear.
Diagram the cochlea and describe its tonotopic organization.
Outline the major auditory pathways through the brainstem, thalamus, and cortex.
Describe the interaural time and intensity difference mechanisms used by the brain for auditory localization.
Describe the major auditory processing areas in cortex.
The sense of hearing allows us to capture sound energy from the environment and informs us about the identity of the sound emitter and its location. Complex sounds are also an essential means of complex communication for biological organisms, reaching their pinnacle in human language. Sound consists of a sequence of air pressure pulses created by vibrating objects such as vocal cords. Vibrations cause compression and rarefaction of the air, which result in the characteristics of frequency, the number of cycles per second, amplitude, and sound intensity, which is usually measured in decibels, a log scale. The sensitivity of the auditory system is very close to the absolute threshold created by random movement of air molecules. Sound reception begins with mechanical modification and transduction in the outer and middle ear and neural coding in the inner ear at the cochlea. Relays in the brainstem and thalamus pass the encoded auditory input to the auditory cortex in the mid-superior temporal lobe, where higher auditory processing allows us to understand language and appreciate music.
Near the cochlea of the auditory system are the semicircular canals of the vestibular system. Hair cells in those canals respond not to external sound input but to motion along 3 axes of rotation. The neural output of the vestibular system is essential for maintaining balance and works with motion-detecting cells from the retina in central pathways that code our motion in space and allow the performance of complex, balanced motion.
As any object vibrates, it compresses air on the side it is moving toward and rarefies air on the other side. These pressure pulses are transmitted and spread by collisions of the molecules in the air as longitudinal waves. This is illustrated in Figures 13–1 and 13–2. Most objects have a natural frequency at which, when struck, they vibrate. Objects will also typically vibrate at various amplitudes at all the integral multiples (harmonics) of that frequency.
Sound has the properties of frequency and amplitude. A. A representation of the changes in sound pressure over time for a pure tone, which is a single frequency. B. An increase in amplitude. C. An increase in frequency. (Reproduced with permission from Barrett KE, Barman SM, Brooks HL et al: Ganong’s Review of Medical Physiology, 26th ed. New York, NY: McGraw Hill; 2019.)