Physiology of Hearing

Study Notes

Physically, sound waves are compressions of air, water, etc.
Amplitude of a wave refers to its intensity, but loudness differs from amplitude.
Frequency refers to the number of compressions per second (in hertz, Hz).
Human ear can detect sounds within the range of 15-20,000 Hz.
Higher the frequency, the higher the pitch.

Outer ear: consists of the pinna, which helps to locate the source of a sound.
Middle ear: contains the tympanic membrane (eardrum), which vibrates the sound to three bones (hammer, anvil, and stirrup).
Inner ear: contains the oval window and the cochlea, with three channels with liquid: scala vestibule, scala media, and scala tympani.

Theories of sound perception: place theory, frequency theory, and current perspective (which combines elements of both).
Place theory: our perception of sound depends on where each component frequency produces vibrations along the basilar membrane.
Frequency theory: basilar membrane synchronizes with a sound, producing action potentials of the same frequency.
Volley principle: attempts to account for the maximum theoretical limit for the neuronal firing of action potentials and the small time scales over which sound discrimination must occur.

Auditory information is processed on the primary auditory cortex A1 (part of the superior temporal cortex).
Audition has a "what" pathway (sensitive to patterns of sounds) and a "how" pathway (sensitive to sound location).
Development of the auditory system requires experience.
Damage on the primary auditory cortex does not lead to deafness, but can lead to impairments in processing auditory information.
Cells in the auditory cortex are more responsive to one kind of preferred sound.

Two types of hearing impairments: conductive deafness (middle-ear deafness) and nerve deafness.
Conductive deafness: caused by e.g. tumors or diseases.
Nerve deafness: caused by damage in the cochlea, hair cells, or auditory nerve.
Nerve deafness can be inherited or caused by prenatal conditions (e.g. lack of oxygen during birth, exposure to certain drugs or diseases).

With two ears, we can localize the source of a sound using three cues:
- Difference in intensity between the ears.
- Difference in time of arrival.
- Phase difference (used with low frequencies), with loudness differences used with high frequencies.