Audition & The Vestibular Sense

Amplitude

• The amplitude of a sound wave is its intensity

  • Sounds of greater amplitude seem louder, but a rapidly talking person seems louder than slow music of the same amplitude

Frequency and Pitch

• Frequency of sound is the number of compressions per second measured in hertz

• Pitch is the related aspect of perception

  • Sounds higher in frequency are higher in pitch

True or False: Most adult humans hear sounds from about 15 to 20 Hz up to almost 20,000 Hz

True

True or False: Children hear higher frequencies, because the ability to perceive high frequencies decreases with age and exposure to loud noises

True

Timbre

• The third aspect of sound (amplitude, pitch, and timbre)

• Tone quality or tone complexity

  • Different musical instruments playing the same note sound different, as do people singing the same note

Prosody

Conveying emotional information by tone of voice

• Example: “that was interesting” could indicate approval (it actually was interesting), sarcasm (it was really boring), or suspicion (you think someone was hinting something)

What is in the Outer Ear?

the pinna

Pinna (The Outer Ear)

• Pinna: the familiar structure of flesh and cartilage attached to each side of the head

  • The pinna helps locate the source of a sound by altering the reflections of sound waves

  • We have to learn to use this information because all pinnas are shaped differently

Where do sound waves pass through?

Sound waves pass through the auditory canal, enter the middle ear

• Middle ear, structure that had to be evolved when ancient fish evolved into land animals.

• Because animal tissues respond to water vibrations almost the same way that water itself does, fish have relatively simple hearing receptors that would not respond well to vibrations in the air

True or False: The structures of the outer ear and middle ear evolved to amplify sound vibrations

False

• The structures of the middle ear and inner ear evolved to amplify sound vibrations

Middle Ear

When sound waves reach the middle ear, they vibrate the tympanic membrane, or eardrum

Tympanic Membrane or Eardrum

The tympanic membrane connects to three tiny bones that transmit the vibrations to the oval window, a membrane of the inner ear.

The Smallest Bones in the Body

Are sometimes known by their

• English names: hammer, anvil, and stirrup

• Latin names: malleus, incus and stapes

What happens in the hydraulic pump?

The vibrations of the tympanic membrane amplify into more forceful vibrations of the smaller stirrup, converting sound waves into waves of greater pressure on the oval window

When the stirrup vibrates the oval window, it sets into motion the fluid in the….

Cochlea

Cochlea

The snail-shaped structure of the inner ear

Hair cells

• Also known as auditory receptors

• Lie along the basilar membrane of the cochlea

Cilia

• Not all cilia are the same height

• They move back and forth by vibration

What are cilia jointed by?

Tip links

Tip Link Function

Tip Link (Increased tension on tip link)

We slowed things and made vibration come in. Now we’re going to let the fluid flow back.

Tip Link (Decreased tension on tip link)

• You move the tips closer together

• Not many potassium (K⁺) channels open

Vibrations in the fluid of the cochlea displace the hair cells, stimulate which part of the ear?

The auditory nerve

What are the 3 canals or ducts in the cochlea?

1. Vestibular Canal

2. Tympanic Canal

3. Cochlear Duct

Vestibular Canal (Scala Vestibuli)

Contains perilymph

Tympanic Canal (Scala Tympani)

Contains perilymph

Cochlear Duct (Scala Media)

Contains endolymph

The Process of the Ear

How many mechanisms does the auditory system use and what are they?

Two mechanisms

1. Frequency Theory

2. Place Theory

Frequency Theory

The basilar membrane vibrates in synchrony with sound waves, causing auditory nerve axons to produce action potentials at the same frequency

• Example: a sound at 50 Hz would cause 50 action potentials per second in the auditory nerve

• The downfall of this theory in its simplest form is that the refractory period limits the maximum firing rate of any neuron to about 1000 Hz, far short of the highest frequencies we hear

Fill in the Blanks: A soft tone stimulates ________ neurons for each peak of the sound wave, and louder tones stimulate __________ ________ of them

A soft tone stimulates fewer neurons for each peak of the sound wave, and louder tones stimulate progressively more of them

Place Theory (Old)

This theory suggested that the basilar membrane resembles the strings of a piano

• If you sound a note with a tuning fork near a piano, you vibrate the piano string tuned to that note. However, the parts of the basilar membrane are bound together too tightly for any part to resonate like a piano string

Place Theory (Modified Version)

The basilar membrane varies from stiff at its base, where the stirrup meets the cochlea, to floppy at the other end of the cochlea, the apex

A high-pitched sound sound sets up a traveling wave that peaks sharply at some point along the basilar membrane. The point at which it peaks it identifies the frequency of the sound

• Low frequency sounds vibrate hair cells near the apex

• High frequency sounds vibrate hair cells farther toward the base

Timing Theory

The Auditory Cortex (Area A1)

As information from the auditory system passes through subcortical areas, axons cross over in the midbrain to enable each hemisphere of the forebrain to get most of the its input, but not all of it from the opposite ear. The information ultimately reaches the primary auditory cortex (area A1)

Diagram: Information from One Ear to Both Sides

What area in the temporal cortex processes the experience you have of sound source moving from one place to another?

Planum Temporale

• The area is connected to the MT which processes visual motion

• The connection helps you combine visual and auditory information to locate some things accurately

Tonotopic Map

When researchers record from cells in the primary auditory cortex while playing pure tones, they find that most cells have a preferred tone. The auditory cortex provides what researchers call a tonotopic map of sounds.

Sound Localization

You compare the responses of your 2 years to locate a sound. You compare the times of arrival at the 2 ears. A sound coming directly from the side reaches your closer ear about 600 microseconds (us) before the other (varying by the size of your head)

Interaural Time Difference (ITD)

Interaural Level Difference (ILD)

Limits of ITD and ILD

Sound reaches both ears at the same time and with the same loudness

Spectral Notches (Monaural Cue)

Sounds hitting the outer ear from different vertical positions end up with unique spectral notches

Diagram: ITD and ILD

What does your head create when you hear high-frequency sounds, with a wavelength shorter than the width of the head which makes the sound louder for the closer ear?

Sound Shadow

Phase Difference

If a sound originates to the side of the head, the sound wave strikes the 2 ears out of phase. How much out of the phase depends on the frequency of the sound, the size of the head, and the direction of the sound.

Phase differences provide information that is useful for localizing sounds with frequencies up to about 1500 Hz in humans.

How many people have amusia?

Amusia is tone deafness; 4%

For people with amuisa they have less than the average connections to the frontal cortex

What are the 2 categories of hearing loss?

1. Conductive deafness

2. Nerve Deafness

Conductive Deafness or Middle-ear Deafness

• Sometimes temporary

• If it persists can be corrected with surgery or hearing aids

Because people with conductive deafness have a normal cochlea and auditory nerve, they readily hear their own voices, conducted through the bones of the skull directly to the cochlea, bypassing the middle ear. Because they hear themselves clearly, they may accuse others of talking too softly.

Nerve Deafness, or Inner-ear Deafness

• Occurs because of damage to cochlea, hair cells, or auditory nerve

• Confined to one part of the cochlea, it impairs hearing of certain frequencies and not others

• Can be inherited, result from a disease, or from exposure to loud noises

Tinnitus

• Constant ringing in the ears (something is stimulating the cells which make your ears ring)

• Common in people who have lost their hearing but rare in people with life-long deafness

• Resembles phantom limb in some cases

Cochlear Implants

• A sensor detects sound waves and electromagnetically sends signals to the receiver below the skin

• Implant allows for tonotopic electrical stimulation of the various locations of the basilar membrane