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,

F re q u e n c y C o d e

o s c illa t io n s o f th e b a s ila r m e m b ra n e

im p u ls e s in a u d ito ry n e rv e f ib e r

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.