The Auditory Stimulus Sound waves can be described in terms of their frequency and their amplitude. Frequency (measured in Hz) is the number of cycles that a sound wave can complete in 1 second; frequency is an important determinant of pitch. Humans hear pure tones with fre¬quencies between 20 and 20,000 Hz.
We detect the presence of tones best in the 1000- to 4000-Hz range. We can discriminate between two very similar tones in the 500- to 2000-Hz range, where the Weber fraction can be as small as 0.002.
Pure tones are represented by sine waves. Auditory researchers frequently use pure tones; however, complex tones are more common and represent the combination of a number of different pure tones.
Amplitude is the maximum pressure created by sound waves, often measured in decibels; amplitude is an important determinant of loudness. The Weber fraction for intensity discrimination is between 0.2 and 0.5.
The middle ear contains three bones—the malleus, the incus, and the stapes—important in reducing the effects of the impedance mismatch between air pressure and the fluids of the inner ear.
The inner ear contains the cochlea, which houses the organ of Corti, a structure that contains the auditory receptors, or hair cells. The organ of Corti lies on the basilar membrane and is covered by the tectorial membrane.
The inner ear has relatively few inner hair cells, although they monopolize most of the afferent auditory nerve fibers. In contrast, the inner ear has relatively many outer hair cells; although they share a small number of the afferent auditory nerve fibers, they receive most of the efferent fibers. Outer hair cell motility is responsible for amplifying the traveling wave on the basilar membrane, though it may be modified by efferent input.
Transduction in the inner ear is due to displacement of the stereocilia of the inner hair cells. The tip links of the stereocilia are crucial to the rapid response of hair cells to displacement.
The graded potentials of the hair cells serve as input to the afferent fibers of the auditory nerve.
The auditory nerve has nerve fibers sensitive to particular frequencies; this nerve travels to the cochlear nucleus. The auditory pathway continues to the superior olivary nucleus, then to the inferior colliculus, then to the medial geniculate nucleus, and finally to the auditory cortex.
Parts of the inferior colliculus and the auditory cortex are organized tonotopically; furthermore, some cells in the auditory cortex respond to complex characteristics of sounds. The auditory cortex is essential for sound localization, speech perception, and other complex auditory tasks.
Hearing Impairments Tinnitus is a ringing in the ears that results from a variety of causes.
A person with conductive hearing loss shows a consistent loss of hearing at all frequencies. Because the sound stimulus is not properly conducted, this person can be helped by a hearing aid. Conductive hearing loss results from outer-ear or middle-ear impairments, such as punctured eardrums, ear infections, or otosclerosis.
A person with sensorineural hearing loss shows a deficit at certain frequencies, although hearing may be normal for other frequencies; this person often shows recruitment. Thus, designing a hearing aid for people with sensorineural hearing loss is difficult because of their differential sensitivity to various pitches and loudnesses.
Auditory adaptation produces a decreased perceived loudness for a tone that is presented continuously.
Auditory fatigue occurs when a loud noise is presented and then turned off, making subsequent tones more difficult to hear. Auditory fatigue can lead to a temporary threshold shift or a permanent threshold shift.