Auditory System

1. The Auditory System

2. The Nature of Sound
•Sound is a longitudinal wave
•Vibrations cause compressions & rarefactions
–Medium is usually air, but sound travels in any
elastic medium
•All waves have basic characteristics:
–Amplitude
–Frequency
–Wavelength
•These properties have particular names with
reference to sound
–Amplitude = loudness or volume
–Frequency = pitch

3. Properties of Waves

4. Sound Frequency
•Units of frequency are cycles/second
(complete waves/second)
–Measured in hertz (Hz)
•Humans with normal hearing can hear in
the frequency range 20 Hz to 20,000 Hz

5. Path of Sound
•Sound waves enter the outer ear
•They pass along the ear canal to the eardrum
•Sound waves bounce off the eardrum, making it vibrate.
•The eardrum is connected to 3 tiny bones, the ossicles
•The vibrations pass along these bones.
•The 3
rd
of these bones, the stapes, presses against the oval
window in the cochlea.
•The vibrations pass into the fluid inside the cochlea.
•Here, they shake thousands of tiny hairs that stick into the
fluid from hair cells.
•As the hairs vibrate, the hair cells generate nerve signals
•The nerve signals travel along the auditory nerve to the
hearing center of the brain.

6. A Diagrammatic Representation

7. Structure of the Ear
•The ear is divided into three parts:
•The outer ear
•The middle ear
–amplifies the sound and transfers it to the
inner ear
•The inner ear
–converts the sound waves into action
potentials and transmits them to the
brain.

8. Divisions of the Ear

9. Structures of the Ear
•1.Ossicles
•2. Semicircular canal
•3. Cochlea
•4. Auditory nerve
•5. Eustachian tube
•6. Middle ear
•7. Tympanic membrane
•8. Auditory canal

10. The Outer Ear
•The pinna –
–the visible part of the ear
–directs sound waves into the middle ear
–involved in localizing sounds in vertical plane
•Auditory canal –
–The tube connecting the center of the pinna with the
tympanic membrane
–channels sound into the middle ear
–can be analyzed (to first approximation) as a open-
closed pipe.
•Both the pinna and auditory canal impose filtering
characteristics based on unique shapes.

11. The Middle Ear
•Tympanic Membrane – Ear drum: cone-
shaped membrane that converts sound to
mechanical vibration
•Ossicles - three serial bones that conduct
sound vibrations from the tympanic
membrane to the cochlea
•Eustachian Tube – connects the middle ear
and throat
–equalizes pressure on either side of tympanic
membrane

12. The Ossicles
•Three small bones conduct sound vibrations from
the tympanic membrane to the oval window of the
cochlea
–The malleus – attached to the tympanic membrane
–The incus – connects the malleus to the stapes,
–The stapes – the foot of the stapes is attached to the
oval window
–Also known as the hammer, anvil, & stirrup
•Ossicles act as a mechanical transformer,
converting pressure on the eardrum to pressure on
the oval window
–Amplifies signal 20 – 30x.

13. Picturing the Ossicles

14. Attenuation
•The acoustic reflex
•Loud noise triggers two sets of muscles:
–Stapedius muscle
–Tensor tympani
•One tightens the eardrum
•The other pulls the stirrup away from
the oval window.
•Change sound conduction from the
tympanic membrane to the cochlea

15. The Inner Ear
•Semicircular canals
–control balance
•The cochlea
–A double-walled, fluid-filled tube, curled into a
snail shell shape with 2 ½ turns
–transforms pressure variations to neural impulses.
•Organ of Corti
–Inside the cochlea
–Contains actual receptors, hair cells
•Bone conduction to inner ear is also significant.

16. The Middle & Inner Ear

17. The Cochlea
•Two major chambers
–scala tympani & scala vestibuli
•Stapes footplate sits on the oval window
–opens onto the scala vestibuli at the base of
the cochlea
•At the apex, the scala vestibuli communicates
with the scala tympani via a hole, the
helicotrema
•At the base, the scala vestibuli ends at the
round window which is covered by a membrane

18. Picturing the Cochlea

19. The Cochlea in Cross-section
•This section shows the
coiling of the cochlear duct
(1) which contains
endolymph, and the scala
vestibuli (2) and scala
tympani (3) which contain
perilymph. The red arrow is
from the oval window, the
blue arrow points to the
round window. Within the
modiolus, the spiral
ganglion (4) and auditory
nerve fibres (5) are seen.

20. A Single Turn

21. Frequency Processing
•Occurs in the cochlea
•The base of cochlea processes high
frequencies
•The apex processes low frequencies
•From the base to the apex of the cochlea
the frequency that produces a maximal
deformation of the basilar membrane
decreases
•Auditory nerve fibers are ``tuned'' to
different center frequencies.

22. Picturing Frequency Processing

23. Cochlear Fluids
•Scala tympani & scala vestibuli are filled with
perilymph
•Inner chamber, scala media, is filled with
endolymph
–Inwardmovement of the stapes footplate causes
bulging of the round window membrane because the
fluid in the cochlea is incompressible
•The stria vascularis secretes endolymph
•Perilymph is like CSF, high Na
+
, low K
+

•Endolymph is very unusual, high K
+
, low Na
+

•There is a voltage in the scala media, the
endocochlear potential of about +80mV

24. Organ of Corti
•Part of the cochlea
–A helical band between the outer wall of the bony cochlea
and the inner bony covering of the modiolus (central axis
of the helix)
•Contains the receptors of hearing: hair cells
•Layers:
•Basilar membrane
–Hair cells rest on the basilar membrane
•Reticular lamina –
–top rigid surface that supports the stereocilia of the hair
cells
•Tectorial membrane –
–gelatinous mass with internal fibers that sits on top of
stereocilia

25. Picturing the Organ of Corti

26. Hair Cells
•The Organ of Corti contains 16,000 -
20,000 hair cells along its 37 millimeter
length.
•Each hair cell has many cilia which bend
with the vibrations of the basilar
membrane.

27. Transduction
•Sound input causes a traveling wave in the
basilar membrane and the organ of Corti
•Upward movement of organ of Corti deflects
stereocilia away from the modiolus
•Downward movement of organ of Corti
deflects them toward the modiolus
•This deflection is reflected in the receptor
potential of the inner hair cells

28. Inner Hair Cells
•Cell body below the reticular lamina sits in normal
extracellular fluid
–high Na
+
, low K
+
•Top surface, bearing stereocilia sits in endolymph
–high K
+
, low Na
+
•In silence, mechanically gated potassium channels
at tips of the stereocilia are partly open
•Resting potential is about -70mV
•E
K
, the potassium equilibrium potential is 0mV
because of the high concentration of potassium
both inside the cell and in the endolymph

29. Transduction of Inner Hair Cells
•Deflection of the stereocilia either fully opens or
fully closes the potassium channels
–The mechanism is the mechanical springs (filaments)
connecting the stereocilia
•When open, depolarization results
–inward rushing potassium tends to move the membrane
potential toward 0mV = E
K

•When closed, hyperpolarization results
–Depolarization opens voltage-gated calcium channels
–Calcium mediates release of synaptic vesicles containing
glutamate onto auditory nerve neurites
•Each IHC is innervated by about 10 auditory nerve
fibers

30. Outer Hair Cells - The Cochlear Amplifier
•Main purpose of OHCs is not to stimulate
auditory nerve fibers, but to change the
mechanical properties of the organ of Corti
–affects transduction in IHCs
•Stimulation of OHC causes inward movement
of potassium
•This contracts motor proteins in the cell
wall and shortens the cell
–pulls reticular lamina closer to basilar membrane
and causes the stereocilia of the IHCs to bend
more -
•Thus a cochlear amplifier

31. Modifying Outer Hair Cell Response
•Blocking the action of the OHC motor proteins by
drugs or sound damage reduces the sensitivity of
the cochlea
•The actions of the OHCs can be modified by
efferent nerve fibers from the brain
–the brain can modulate the sensitivity of the cochlea
•Ototoxic effects of antibiotics occur because they
damage the OHCs and reduce the sensitivity of the
cochlea.
–IHCs are not affected directly by antibiotics.

32. Picturing Hair Cell Movement

33. Processing Auditory Signals in the Brain
•Two major pathways:
–the dorsal pathway
–the ventral pathway
•Pathways are complex and connections not
well understood.
•Ventral Auditory Pathway:
–begins in the ventral cochlear nucleus, travels
through the Superior olive to the inferior
colliculus and MGN to the auditory cortex.

34. Major Structures of the Ventral
Auditory Pathway
•Spiral ganglion - spiral band of auditory nerve cell
bodies in wall of modiolus
•Auditory nerve - fibers enter modiolus and exit
toward the brainstem
•Ventral cochlear nucleus - brainstem nucleus -
ipsilateral innervation, monaural response
properties
•Superior olive - each side is innervated from both
ventral cochlear nuclei - binaural response
properties
•Medial geniculate nucleus - next to LGN, auditory
thalamic relay nucleus

35. Other Important Structures
•Acoustic radiation - fibers from MGN to
A1 - auditory cortex
•Auditory cortex - A1, Brodmann area
41, superior surface of temporal lobe
•Secondary auditory cortices - e.g.
Wernicke's area, etc.

37. Tonotopic Maps
•Frequency sensitivity is caused by properties of the
Basilar Membrane.
•The map of sound frequency from the basilar
membrane in cochlea is preserved
–like the retinotopic map of visual system
•When neurons synapse, they do so in an organized
pattern based on characteristic frequency.
•Systematic organization of characteristic frequency is
called tonotopy
•There are tonotopic maps on the basilar membrane,
the MGN, the auditory cortex, and within each of the
nerve relay nuclei.
• Tonotopy allows for the location of the impulse to
indicate frequency.

38. Sound Intensity
•Louder sounds cause the basilar membrane
to vibrate with greater amplitude
•More intense stimuli produce movements of
the basilar membrane over a greater
distance, which leads to the activation of
more hair cells
•More action potentials occur because of the
greater movement of the basilar membrane

39. Coding Sound Intensity
•Two ways to code sound intensity
–Number of active neurons and firing rates of neurons
•Population code
– as sound intensity increases, the deflections of the
basilar membrane stimulating IHCs broaden
–more and more IHCs are activated
•Rate code
–as sound intensity increases, the receptor potential in
IHCs grows larger
–the auditory nerve fibers fire faster
•Together they tell the brain the value of sound
intensity
–This produces the sensation of loudness

40. Coding of Sound Frequency
•Place code –
–according to the tonotopic map, different frequency
sounds cause deflections of the basilar membrane at
different places in the cochlea –
–which IHCs are activated indicates what the sound
frequency is
•Phase locking –
–for sound frequencies below 4,000 Hz, the timing of
action potentials in the auditory nerve is locked to the
cycle of compression & rarefaction in the sound wave
–timing of action potentials codes for sound frequency
•Together these two codes produce the sensation of
pitch

41. Frequency Variation
•For very low frequencies (below 200 Hz),
only phase locking codes frequency
–This is because there aren't dedicated fibers
•For medium frequencies (200-4000 Hz),
both place code and phase locking code
frequency
•For high frequencies (4000-20,000 Hz), only
place code indicates sound frequency
–phase locking stops

42. Localizing Sound in the Horizontal Plane
•Time differences between ears
–For frequencies 20-2000 Hz, the phase
locking in firing patterns from the two ears
are compared
–The difference in timing between them
specifies the location of the sound source
•Intensity differences between ears
–For frequencies above 2000 Hz, the head
produces a significant shadow on the sound
waves
–The differences in intensity between the two
ears are compared to localize the sound

43. Localizing Sound in the Vertical Plane
•Works as well with one ear as with two
ears
•Covering up the pinna eliminates this
capability
•Comparison of direct and secondary
reflected sound paths from the wrinkles
on the pinna enables us localize
vertically

44. The Auditory Cortex
•Layers similar to visual cortex
–6 of them
•A1 has a tonotopic map with low frequencies
represented anteriorly and high frequencies
represented posteriorly
•Most A1 neurons are sharply tuned for frequency
•All are binaural
–Some are excited by both left and right ears
(EE)
–Some are excited by one ear and inhibited by
the other ear (EI)

45. Cortical Modules in the Auditory Cortex
•Each vertical column has cells sensitive to
the same frequency
•Adjacent columns in anterior-posterior
direction change frequencies in order -
tonotopy
•Adjacent columns in lateral-medial direction
change from EE to EI to EE
–like ocular dominance columns
•Analogous to cortical modules in Area 17

46. Cortical Damage
•Unlike the visual system, damage to auditory
cortex often has little effect on basic hearing
•More often ability to understand speech or
some other complex ability is lost
•Damage to cochlea, auditory nerve, or
cochlear nuclei are more typically causes of
deafness

47. Auditory Disorders
•Conduction deafness
–blockage in sound conduction: wax in ear,
disarticulated ossicles, stiffening of insertion of
stapes footplate into oval window.
–Usually correctable with surgery
•Nerve deafness
–damage to hair cells or auditory nerve fibers from
tumors, ototoxic drugs, loud sounds, etc.
–No treatment for nerve deafness, but partial loss
can be compensated for by various hearing aids.
–Prevention is important.

48. Tinnitus
•Ringing in the ears
•A common phenomenon
•Caused by hyperactivity of cochlear amplifier
•The sounds of tinnitus are actually occurring in the
cochlea and one is simply hearing them
–they originate in cochlea and mask incoming external
sounds
•May indicate sound damage, cochlear disease or
vascular abnormalities
•Tinnitus after rock concerts is very common (and
not healthy)!

49. Auditory Perception
•Like vision, auditory sensations are organized
and interpreted in the brain to create auditory
perceptions
•Like visual perception, auditory perception is
relative
•Brain also makes “assumptions”
•Basis of auditory illusions:
–Shephard Tones
–Tri-tone illusion
–McGurk Effect

50. Shepard's Tones
•Circularity of judgment of relative pitch
•These tones eliminate all relative pitch discrimination
information.
•As a result, when played in sequence, each tone
sounds higher than all tones preceding it and lower
than all tones following it (and vice versa when the
sequence is played in the opposite order).
•Since there are only twelve tones in the sequence,
played in a continuous loop, every tone sounds both
higher and lower than every other at some point in
the sequence.

51. The Tritone Effect
•Although pitch discrimination cues have been
removed from Shepard's Tones, proximity
information remains.
–2 consecutive tones are always separated by a single
semitone.
•Although you can't determine which is higher based
on the tones alone, your choice is that the second
tone is either one semitone higher or eleven
semitones lower in pitch than the first.
–It is natural for the smaller distance to be selected.
•What if the proximity cue was removed?
–If the second tone played is either half an octave higher or
half an octave lower than the first
– Result is the tri-tone effect (The midpoint of the octave is
called the tritone)

52. The Risset Scale
•This is actually a single octave of twelve notes!
•Each note, however, is actually a chord.
•Each chord is comprised of six individual notes from
six different octaves.
•The notes of each chord have the same pitch (6 C's,
6 D's) - but they are played at 6 different volumes.
•This creates ambiguous information for the listener.
•The Risset Scale blends each tone from this special
octave into the next tone, over and over again.
•This blending, combined with the complex and
ambiguous tonal information of each note, creates
the illusion of an endlessly rising or descending tone.

53. McGurk Effect
•What am I saying?
•Alternate between looking at the talking head
while listening, and listening with your eyes
shut.
•Most adults (98%) think they are hearing
"DA"
–a so called "fused response"
•the "D" is a result of an audio-visual illusion
•In reality you are hearing the sound "BA",
while you are seeing the lip movements "GA".

55. The Scale Illusion
•A scale with successive tones alternating from
ear to ear
•The scale is played simultaneously in
ascending and descending form
•When a tone from the ascending scale is in
the right ear, a tone from the descending
scale is in the left ear, and vice versa
•When heard through earphones produce a
number of illusions

56. A Variant Scale Illusion
When listening to this pattern through loudspeakers, notice
that when each channel is played separately, it appears to
shift dramatically in pitch, but when both channels are played
together, two smooth melodies are heard. The brain creates
order out of chaos.

57. Name That Tune
•Knowledge of a piece of music
influences what we hear
•All of the notes of a well known tune
are correct, but the tones are
distributed randomly across three
octaves
•In the second clip, the notes are the
same, but now they are all in one
octave