PSYA01H3 Chapter Notes - Chapter 5: Basilar Membrane, Oval Window, Middle Ear

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20 Apr 2012
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Psychology- Second Half of Chapter 5
SENSATION
Audition
-vision involves the perception of objects in three dimensions, at variety of
distance, with multitude of colours and textures
-complex stimuli may occur at a single point in time or over an extended
period
-may involve an unchanging scene or rapidly changing one
-other sense analyze much simpler stimuli (odour/taste) or depend on time
and stimulus change for the development of a complex perception
-example: to perceive a solid object in three dimensions by means of touch,
must manipulate it-turn it over in our hands or move our hands over its
surface
- Stimulus must change over time for a full-fledged perception of form to
emerge
-same for audition: we hear nothing meaningful in an instant
SOUND
-sound consists of rhythmical pressure changes in the air
-as object vibrates, causes air around it to move
-when object in phase of vibration in which it moves toward you, it
compresses molecules of air
-as it moves away it pulls molecules of air further apart
-as pressure arrives at ear, it bends eardrum in
-following wave of negative pressure(molecules pulled farther apart) causes
eardrum to bulge out
-sound waves measured in frequency units of cycles per second called
Herts(Hz)
-human ear perceives vibrations between 30-20 000 Hz
-produce corresponding changes in sensations of loudness, pitch
-example: consider a loudspeaker, a device that contains a paper cone moved
back and forth by a coil of wire located in a magnetic field. Alteration in the
electrical current transmitted from an amplifier to this coil cause coil(and
paper cone) to move back and forth. If cone begins to vibrate more rapid,
pitch of sound increases(that is if the cone moves in and out over a greater
distance)
-third perception dimension, timbre, corresponds to the complexity of the
sound vibration
THE EAR AND ITS FUNCTIONS
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-refer to the ear, usually mean what anatomists call pinna-the flesh-covered
cartilage attached to side of head
-pinna performs only small role in audition
-helps funnel sound through ear through ear canal toward the middle and
inner ear, where the business of hearing gets done
-eardrum is a thin, flexible membrane that vibrates back and forth in response
to sound waves and passes these vibration on to the receptor cells in the
inner ear
-eardrum attached to the first set of three middle ear bones called the
ossicles
-three ossicles are hammer(malleus), anvil(incus), stirrup(stapes), because of
their shapes M.I.S.
-these bones act together, in lever fashion to transmit the vibrations of the
eardrum to the fluid-filled structure of the inner ear that contains the
receptive organ
-bony structure that contains the receptive organ is called the cochlea
-the cochlea is filled with liquid
-bony chamber attached to the cochlea(the vestibule) contains two openings:
oval window and round window
-the stirrup presses against a membrane behind an opening in bone
surrounding the cochlea called oval window, transmitting sound waves into
the liquid inside the cochlea where it can reach the receptive organ for
hearing
-cochlea is divided into three chambers by two membranes
-Basilar membrane: a sheet of tissue that contains auditory receptor cells
-As footplate of stirrup presses back and forth against membrane behind oval
window, pressure changes in the fluid above the basilar membrane cause
basilar membrane to vibrate
-Basilar membrane varies in width and flexibility, different frequencies of
sound cause different parts of the basilar membrane to vibrate
-High-frequency sounds cause end near oval window to vibrate
-Middle frequency sounds cause middle to vibrate
-Low frequency sounds cause tip to vibrate
-Basilar membrane can vibrate freely only if fluid in lower chamber of cochlea
has somewhere to go-unlike gases, liquids cannot be compressed
-Free space provided by round window
-Basilar membrane flexes down, displacement of fluid causes membrane
behind round window to bulge out
-Basilar membrane flexes up, membrane behind round window bulges in
-Sounds detected by neurons-auditory hair cells located on basilar membrane
-Auditory hair cells: transduce mechanical energy caused by flexing of
basilar membrane into neural activity
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-Cells possess hair-like protrusions called cilia
-Ends of cilia embedded in rigid shelf-tectorial membrane, that hangs over
basilar membrane like balcony
-When sound vibration cause basilar membrane to flex, cilia are stretched-this
pull is translated into neural activity
-Mechanical force exerted on cilia, electrical charge across membrane is
altered-cause is not fully known, although pressure on cilia is known to
increase calcium flow into the hair cells
-Calcium currents very fast, alter temporal duration of depolarization
-Change in electrical charge cause transmitter substance to be released at a
synapses between auditory hair cell and dendrite of neuron of auditory
nerve, similar to way that bipolar cells connect to ganglion cell in retina
-One hair cell connected to many auditory neurons, unlike case with visual
cells, one hair cell has a large effect on subsequent nerve activity
-Auditory system deal with sounds that persist for long intervals-intervals long
enough to deplete transmitter substance in normal synapses, unlike in visual
stimuli that can disappear
-Synapses between hair cells and auditory nerve therefore differ in function
-Depolarization is stronger—result is more reliable signal passed along
auditory nerve
DETECTING AND LOCALIZING SOUNDS IN THE ENVIRONMENT
-sounds differ in loudness, pitch, timbre
-come from particular locations
-ear’s ability to distinguish sounds by their timbre depends on its ability to
distinguish loudness and pitch
Loudness and Pitch:
-originally thought neurons of auditory system represented pitch by firing in
synchrony with vibrations of the basilar membrane
-learned that axons cannot fire rapidly enough to represent high frequencies
that we can hear
-young ear hear frequencies of more than 20 000 Hz, but axons cannot fire
more than 1000 times per second
-therefore: high frequency sounds, must be encoded in some other way
-sounds of different frequencies stimulate different groups of auditory hair
cells located along the basilar membrane (different frequency sounds cause
different parts of basilar membrane to vibrate)
-high-frequency/medium-frequency sounds, brain is informed of pitch of a
sound by activity of different sets of axons from auditory nerve
-medium-frequency sounds waves reach ear, middle basilar membrane
vibrates, auditory hair cells located in region are activated
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