AUDITION (Chap 7.2)
Web Links for Auditory System
Receptors in the auditory system transduce vibratory energy (vibration of molecules in a medium). Sound is perceived.
Three basic characteristics of sound:
Frequency, Intensity, Velocity (FIG. 7.3)
1) Frequency: Cycles per second. 1 cycle/sec = 1 Hertz (Hz)
Audible spectrum: 20 to 20,000 Hz.
Spectrum varies for species (e.g., extends higher for dogs, bats), age.
Frequency of stimulus is associated with pitch perception.
(in vision, frequency of stimulus is associated with color perception)
2) Intensity: amount of energy in stimulus: loud or faint sounds.
We can hear a broad range of intensities:
from very faint sounds (rustling of leaves) to very loud ones: jet engine.
3) Velocity, depends on the medium in which waves travel.
In air, sound travels at 750 miles/h (1250 Km/h) (sound barrier),
Velocity is greater in denser media (wood, metal)
Function:
What? (Recognition, identification)
Where? (location)
Compare with visual system
The Auditory System
The ear (FIG. 7.5)
1) Outer: Auditory canal and tympanic membrane (eardrum)
2) Middle: 3 ossicles: hammer (malleus), anvil (incus), stirrup (stapes).
function: interface between air in outer ear and liquid in inner ear.
3) Inner ear: Oval and round windows.
Cochlea, contains the Organ of Corti (Fig. 7.5)
Organ of Corti: Tectorial membrane
Hair cells (receptors)
Basilar membrane
Stirrup pushes oval window in, forcing round window out, creating liquid wave inside cochlea.
Wave in cochlea induces vibrations in basilar membrane
Hair cells on basilar membrane collide with tectorial membrane.
Transduction:
Hairs in hair cells are bent, which opens ion channels on hair cells.
Receptor potential in hair cell is created, and transmitter is released.
Receptor potential produces action potentials in ganglion cells (spiral ganglion, in the cochlea) whose axons form the auditory nerve (8th pair).
Recognition of sound (what?)
Basilar membrane analyses the component frequencies of sound.
The basilar membrane is tonotopically organized:
Different regions of membrane vibrate with different frequencies (as a keyboard).
Base (near windows) is thick and stiff and vibrates with HIGH frequencies.
Apex (end of membrane) is thin and flexible and vibrates with LOW frequencies.
Auditory pathway (FIG. 7.6)
Spiral ganglion cells project to the cochlear nucleus, in the medulla (myelencephalon),
from here to other nuclei, but eventually to the thalamus (medial geniculate nucleus),
and from here to the auditory cortex temporal lobe, FIG. 7.7).
Neurons in primary auditory cortex respond to pure tones, but neurons in seconary auditory cortex require more complex sounds. For instance, in monkeys, neurons in secondary auditory cortex respond better to monkey calls.
This pathway is tonotopically organized (cells prefer specific frequencies).
Sound Localization (where?): requires two ears.
Auditory system can analyze differences in the intensity and in the time of sound arrival between the the two ears.
Comparison of time of arrival occurs in the Medial Superior Olives, in the medulla
Comparison of intensity of sound between the two ears is done in the Lateral Superior Olives.
The Medial and Lateral Superior Olives are in the medulla (myelencephalon) (see FIG. 7.6).
Deafness. Complete deafness is rare. Two kinds
1. Nerve deafness, or inner ear deafness: damage to cochlea or hair cells, or nerve
Bilateral lesions of the primary auditory cortex in laboratory mammals produce no permament deficit in their ability to detect sounds. However, cortical lesions can disrupt the ability to localize brief sounds, and to recognize complex sounds.
2. Conductive deafness, or middle ear deafness. Patients hear their voices.