Auditory Pathways and

Perception

Dr. Karishma R PandeyAssistant Professor

Dept. of Basic And Clinical PhysiologyBPKIHS

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

Sound Transmission

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

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.

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.

The primary auditory cortex : Brodmann's area 41

Low tones: anterolaterally and

High tones : posteromedially in the auditory cortex.

Auditory Pathway

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

Attributes of Sound1) Frequency

2) Intensity

3) Direction

4) Pattern

Pitch : frequency (number of waves per unit of time).

Sound wavesRepeating patterns musical sounds

Aperiodic nonrepeating vibrations noise

Pitch discrimination : about 2000Best: 1000- to 3000-Hz range Poor: at high and low pitches16-20,000 Hz audible range

Pitch in conversation avg male voice :120 Hzavg female voice: 250 Hz

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

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

10 db= 1 bel

0 db= auditory threshold20 db= whisper60 db= normal conversation80 db= shouting120 db = uncomfortable140 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

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

Pattern :

Sequence in which different component of sound appear

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

Masking

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

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

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:

Conductive (or conduction) and sensorineural hearing loss.

Conductive deafness Impaired sound transmission in the external or middle earImpacts 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") and3) Otitis media (inflammation of the middle ear) causing fluid accumulation,4) Perforation of the eardrum, and5) Osteosclerosis

Deafness

Sensorineural deafness

1) Most commonly the result of loss of cochlear hair cells2) Prolonged exposure to noise damages the outer hair cells 3) Problems with the eighth cranial nerve (tumors) or4) 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

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.

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