Download The Auditory System

The Auditory System
Dr. Kline
FSU
What is the physical stimulus for
audition?

Sound- vibrations of the molecules in a
medium like air.

The hearing spectrum for humans is
approximately between 20 to 20,000 Hz.
3 basic physical properties of
sound

1. Amplitude: intensity of sound

2. Frequency: cycles per second

3. Complexity: shape of wave
The psychological aspects of
sound

1. Loudness: perception of amplitude

2. Pitch: perception of frequency

3. Timbre: perception of quality of sound
Parts of the Ear

Outer Ear

1. Pinna: the visible flap of skin on our
heads, collects & funnels sound waves into
our auditory canal.

2. Auditory canal: the length & shape
cause it to resonate in response to
frequencies entering the ear.
Middle Ear

3. Tympanic membrane (eardrum): vibrations
strike the tympanic membrane causing it to vibrate
at roughly the same frequency as the sound waves
that strike it.

4. The ossicles: the malleus (hammer), the incus
(anvil), & the stapes (stirrup) vibrate. These bones
transmit the vibration from the middle ear to the
inner ear.
The stirrup triggers vibrations of the oval window
which moves the fluid of the cochlea.
Inner Ear

5. Cochlea: is the snail-shaped structure that
contains the auditory receptors.

The cochlea has 3 fluid filled chambers:
 scala vestibuli, scala tympani, & scala media.

Within the cochlea is the Organ of Corti.
 This structure is composed of the basilar
membrane on which the hair cells are located & the
tectorial membrane which rests above it.

Shearing action of Organ of corti, causes action
potentials.
The Auditory Pathway

Both Ipsilateral & Contralateral
Ear--cochlear nucleus—superior olivary nucleus
(medulla)—inferior colliculus (midbrain)—medial
geniculate nucleus (Thalamus)—Primary auditory
cortex.

-Info from ear to the contralateral hemisphere is
dominant.

-Auditory cortex is tonotopically organized,
Auditory Pathways
• 60% of the cells in the
cochlear nucleus of
each ear project to the
contralateral olivary
nucleus.
• Cells in the olivary
nucleus thus receive
binaural input.
Pitch Perception

1.Frequency Theory: basilar membrane
vibrates in synchrony with the sound source
& causes action potentials to occur at about
the same frequency.
 ( a 100 Hz tone, would have 100 action
potentials per second in the auditory nerve)



Pro: good for low frequencies
Con: bad for high frequencies,
Frequency Code
oscillations of the
basilar membrane
impulses in auditory
nerve fiber
Pitch Perception



2. Place Theory: basilar membrane is
tuned to specific frequencies at different
locations on the membrane & vibrates
whenever that frequency is present.
Pro: good for high frequencies
Con: bad for low frequencies
Pitch Perception

Volley theory- Neurons may fire in different
phases (time-locked) that when taken together
may code the pitch.
If all of these neuron fibers are taken together they
may produce a volley of impulses by various
fibers that as a whole code the pitch.

The volley theory seems to work for tones up
5000 Hz.
Volley Principle
Summary of theories
1. Low Frequencies are coded by frequency of
nerve impulses (up to 50 Hz).

2. High frequencies are coded in terms of the
place along the basilar membrane which shows the
greatest activity. (over 5000 Hz)

3. For intermediate frequencies (from 60 to 5000
Hz) pitch is coded through a combination of
Volley & place.
Sound Localization

1. Monaural cues– use one ear
 A. Pinna - funnels in sound waves; is used to
help locate sounds in space.

B. Head movements – you can move your head
to locate a sound.

C. Doppler effect – sounds coming toward you
will be perceived as “louder” & “higher” pitched
than sounds moving away from you.
2. Binaural cues

A. Inter-aural Intensity cues - the difference in
loudness between the two ears will help us locate
where the sound came from.

When the sound source is off to the side of the
head, the head casts a “shadow” through which the
sound must pass through.

This decreases the intensity of the sound on the far
side (ear farther away from the sound).

works best for high frequencies.
Sound Localization
B. Inter-aural timing cues

Good for explaining low frequencies.

When the sound source is off to the side of the
head, there will be a longer lag in when the sound
reaches the ear farthest away.

The ear closest to the sound source, will hear the
sound first.
Sound localization
Summary: Localization
1. We localize low frequencies (to 1500 Hz)
by differences in the phase or timing of
onset of the sound.

2. We localize high frequencies (above
3000 Hz) by intensity differences.

3. We seem to be less accurate for
intermediate frequencies.