Lecture - auditory Flashcards
sound
A vibrating object creates alternating waves of condensation and rarefaction in a medium
The pattern of condensation and rarefaction is
propagated away from the vibrating source like ripples in water
sound intensity progressively decreases with
distance from the source
velocity of sound in air
750 mph (1250 km/h, sound barrier)
velocity is greater in
denser media (wood, metal, water).
pure tone
single sinusoidal waveform. pressure is plotted as a sine wave. frequency is the number of cycles per second, or Hertz. amplitude is the magnitude of the pressure wave measured in decibels (dB).
perceiving pure tones
pure tones don’t exist in the natural world. the frequency of the sound will determine the pitch you perceive. The amplitude of the sound determines its loudness.
Audible frequency spectrum:
20 - 20,000 Hz
complexity
determines the timbre.
natural sounds are
complex patterns of vibrations.
fourier analysis
breaks natural sounds down into sine waves.
complex relationship between natural sounds and
perceived frequency.
the decibel scale
dB = 20(logP1/P0)
Because the dynamic range of the ear is so great, sound amplitude is expressed on a ratio scale (log 10), not an interval (linear) scale.
0 dB
corresponds to the average human’s absolute threshold for hearing.
6 dB increase
corresponds (approximately) to a doubling of sound pressure
1 Pa (unit of pressure, “Pascal”) =
1N/m2 (1 Newton/meter2)
Sound waves enter the
auditory canal of the ear and then cause the tympanic membrane (the eardrum) to vibrate. This sets in motion the bones of the middle ear, the ossicles, which trigger vibrations of the oval window: hammer (malleus), anvil (incus), and stirrup (stapes).
the ear pathway
Sound wave > eardrum > ossicles (hammer, anvil, stirrup) > oval window
Vibration of the oval window sets in motion the
fluid of the cochlea, which is possible due to the movement of the round window.
The cochlea’s organ of Corti
is the auditory receptor organ
outer and middle ear are
filled with air
inner ear is
filled with fluid which is not compressible as air is. The ossicles amplify the vibration and the round window allows movement of the fluid in the cochlea.
the oval and round windows have membranes so
they move, but liquid does not enter nor escape (from and to the middle ear).
The basilar membrane vibrates in response to
to sound hitting the eardrum and the frequency of the sound is encoded by the place on the basilar membrane that is
maximally vibrated by the sound.
The basilar membrane is narrower and stiffer at the
basal end.
The basilar membrane is wider and less tightly stretched at the
apical end
Different frequencies produce
maximal stimulation of hair cells at different points along the basilar membrane
the basilar membrane and the auditory pathway organization is
Tonotopic (frequency)
Vibrations travel from the
base to the apex of the basilar membrane. High frequencies peak near the base, but
lower frequencies continue on and peak at the apex.
organ of corti is composed of two membranes
basilar membrane and tectorial membrane
basilar membrane
auditory receptors (hair cells) are mounted here
Tectorial membrane
rests on top of the hair cells
Transduction is produced by
the movement of hairs. Stimulation of hair cells triggers action potentials in the auditory nerve.
Basilar membrane vibration causes deflection of hair bundles
The motion of the traveling wave creates shearing forces on the hair cell (stereocilia)
hair cells
in humans there are about 3,750 inner hair cells per ear
tip links
Extracellular filaments that physically connect hair cells together. spring gate ion channels (mechanical).
Shearing movement of basilar and tectorial membranes
deflect hair cells
Tension on tip link filaments leads to
opening of the ion channels
The base of each hair cell is
contacted by a process from one or more spiral ganglion cells.
the somas of the ganglion cells are in
the center of the cochlea, and the axons form the auditory nerve (8th cranial pair)
monoaural pathway
recognition of sound (what). anterior auditory pathway to prefrontal cortex.
binaural pathway
localization of sound (where). requires computation of differences between both ears in time of arrival and sound intensity. posterior auditory pathway to posterior parietal cortex.
Cochlear nucleus and olives are in the
myelencephalon.
The head absorbs sound energy
so that sound intensity is greater in one side of the head than on the other
The brain is also sensitive to
millisecond differences in arrival time between the ears
The lateral and medial superior olives react to
differences in what is heard by the two ears
Medial olives
compute arrival time differences
lateral olives
compute amplitude differences
of areas in primary auditory cortex
two or three (tonotopic)
of areas in secondary auditory cortex
about seven areas (tonotopic). do not respond well to pure tones and have not been well-researched.
Functional columns
cells of a column respond to the same frequency
Lesions of auditory cortex in rats results in
few permanent hearing deficits
Lesions in monkeys and humans hinder
sound localization and pitch discrimination
total deafness is rare due to
multiple pathways
conductive deafness
damage to the ossicles
nerve deafness
damage to the cochlea. Partial cochlear damage results in loss of hearing at particular frequencies.
echolocation in bats
doppler shifts in frequency
