how the sound stimulation is taken to the higher centers

1. Auditory Pathways
and
Perception
Dr. Karishma R Pandey
Assistant Professor
Dept. of Basic And Clinical Physiology
BPKIHS

2. Presentation Plan…….
1) Sound transmission from source to the receptor
2) Auditory pathway
3) Cortical presentation of sound
4) Attributes of sound
5) Tympanic reflex and Masking
6) Deafness and Tests to determine the functionality of ear

3. Sound Transmission

4. The waves are transformed by the
eardrum and auditory ossicles into
movements of the foot plate of the
stapes.
These movements set up waves in the
fluid of the inner ear.
Basilar membrane is not under
tension readily depressed into the
scala tympani by the peaks of waves in
the scala vestibuli
Sound Transmission

5. When the stapes moves, both membranes move in the same direction, but
they are hinged on different axes, so a shearing motion bends the hairs.
The hairs of the inner hair cells are bent by fluid moving between the
tectorial membrane and the underlying hair cells.

6. The action of the waves on the organ of Corti generates action
potentials in the nerve fibers.
The ear converts sound waves in the external environment into action
potentials in the auditory nerves.
The wave is dissipated at the round window.

7. The primary auditory cortex :
Brodmann's area 41
Low tones: anterolaterally and
High tones : posteromedially in the
auditory cortex.
Auditory Pathway

9. The auditory association areas adjacent
to the primary auditory receiving areas
are widespread.
Brodmann's area 22: processing of
auditory signals related to speech.
Planum temporale more active on the
left side than on the right side.
Area 22 on the right side : melody, pitch,
and sound intensity.
The auditory pathways are also very plastic,
and, like the visual and somasthetic pathways,
they are modified by experience
Auditory Cortical Presentation

10. Attributesof Sound
1) Frequency
2) Intensity
3) Direction
4) Pattern

11. Pitch :
frequency (number of waves per unit of
time).
Sound waves
Repeating patterns musical sounds
Aperiodic nonrepeating vibrations  noise
Pitch discrimination : about 2000
Best: 1000- to 3000-Hz range
Poor: at high and low pitches
16-20,000 Hz audible range
Pitch in conversation
avg male voice :120 Hz
avg female voice: 250 Hz

12. Movements of the foot plate of the stapes set up a series of traveling
waves in the perilymph of the scala vestibuli.
Height increases to a
maximum and then drops
off rapidly.
Distance between this
point and the stapes is
inversely related to the
pitch of the sound
High-pitched: maximum
height near the base
low-pitched: peak near the
apex

13. Traveling wave set up by a tone produces peak depression of the basilar
membrane, and consequently maximal receptor stimulation, at one point.
Major determinant of the pitch perceived

14. 10 db= 1 bel
0 db= auditory threshold
20 db= whisper
60 db= normal conversation
80 db= shouting
120 db = uncomfortable
140 db= damages cochlear receptors
loudness :
amplitude of a sound wave
Sound intensity in bel = Log 10 intensity of the sound heard
_________________________
intensity of the standard sound

15. Direction
Horizontal plane:
detecting the difference in time between the arrival of the stimulus in the
two ears
sound is louder on the side closest to the source.
Front/ Back:
pinna is turned slightly forward.
Vertical plane:
reflections of the sound waves from the pinnal surface change as sounds
move up or down, and thus changes in the sound waves

16. Pattern :
Sequence in which different component of sound appear

17. Loud sounds initiate a reflex contraction of the middle ear muscles
(tensor tympani and stapedius).
They pull the manubrium of the malleus inward and the footplate of
the stapes outward.
This decreases sound transmission.
Function: protective,
Prevent strong sound waves from
causing excessive stimulation of the
auditory receptors.
Tympanic reflex/ Acoustic reflex

18. Masking
The hearing threshold for a given sound increases in the presence of
background sound/ noise.

19. 1) Ossicular conduction: Conduction of sound waves to the fluid of
the inner ear via the tympanic membrane and the auditory
ossicles; main pathway for normal hearing
2) Air conduction: Sound waves also initiate vibrations of the
secondary tympanic membrane that closes the round window
3) Bone conduction: transmission of vibrations of the bones of the
skull to the fluid of the inner ear; transmission of extremely loud
sounds
Conduction of Sound Waves

20. Most common sensory defect in humans.
Presbycusis:
Gradual hearing loss associated with aging,
Due to gradual cumulative loss of hair cells and neurons.
Hearing loss is a multifactorial disorder caused: both genetic and
environmental factors.
Deafness/Hearing Loss:

21. Conductive (or conduction) and sensorineural hearing loss.
Conductive deafness
Impaired sound transmission in the external or middle ear
Impacts all sound frequencies.
Causes :
1) Plugging of the external auditory canals with wax (cerumen) or foreign
bodies,
2) Otitis externa (inflammation of the outer ear, "swimmer's ear") and
3) Otitis media (inflammation of the middle ear) causing fluid accumulation,
4) Perforation of the eardrum, and
5) Osteosclerosis
Deafness

22. Sensorineural deafness
1) Most commonly the result of loss of cochlear hair cells
2) Prolonged exposure to noise damages the outer hair cells
3) Problems with the eighth cranial nerve (tumors) or
4) Problems within central auditory pathways (tumor of cerebellopontine
angle and vascular damage in the medulla)
Impairs the ability to hear certain pitches while others are unaffected.
Aminoglycoside antibiotics (streptomycin and gentamicin)
Obstruct the mechanosensitive channels in the stereocilia of hair cells and
can cause the cells to degenerate

23. Weber Rinne Schwabach
Method Base of vibrating
tuning fork placed on
vertex of skull.
Base of vibrating
tuning fork placed on
mastoid process until
subject no longer
hears it, then held in
air next to ear.
Bone conduction of
patient compared
with that of normal
subject.
Normal Hears equally on
both sides.
Hears vibration in air
after bone
conduction is over.
Conduction deafness
(one ear)
Sound louder in
diseased ear
because masking
effect of
environmental noise
is absent on
diseased side.
Vibrations in air not
heard after bone
conduction is over.
Bone conduction
better than normal
(conduction defect
excludes masking
noise).
Sensorineural
deafness (one ear)
Sound louder in
normal ear.
Vibration heard in air
after bone
conduction is over,
as long as nerve
deafness is partial.
Bone conduction
worse than normal.

24. Auditory acuity : measured with an audiometer.
Presents the subject with pure tones of various frequencies through
earphones.
At each frequency, the threshold intensity is determined and plotted on
a graph as a percentage of normal hearing.
This provides an objective measurement of the degree of deafness and
a picture of the tonal range most affected.
Audiometry