Medium is usually air, but sound travels in any elastic medium
All waves have basic characteristics:
These properties have particular names with reference to sound
Amplitude = loudness or volume
Frequency = pitch
Properties of Waves
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
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.
A Diagrammatic Representation
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.
Divisions of the Ear
Structures of the Ear
2. Semicircular canal
4. Auditory nerve
5. Eustachian tube
6. Middle ear
7. Tympanic membrane
8. Auditory canal
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.
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
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.
Picturing the Ossicles
The acoustic reflex
Loud noise triggers two sets of muscles:
One tightens the eardrum
The other pulls the stirrup away from the oval window.
Change sound conduction from the tympanic membrane to the cochlea
The Inner Ear
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 .
The Middle & Inner Ear
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
Picturing the Cochlea
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.
A Single Turn
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.
Picturing Frequency Processing
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
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
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
Picturing the Organ of Corti
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.
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
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
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
Superior olive - each side is innervated from both ventral cochlear nuclei - binaural response properties
Medial geniculate nucleus - next to LGN, auditory thalamic relay nucleus
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.
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.
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
Coding Sound Intensity
Two ways to code sound intensity
Number of active neurons and firing rates of neurons
as sound intensity increases, the deflections of the basilar membrane stimulating IHCs broaden
more and more IHCs are activated
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
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
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
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
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
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)
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
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
blockage in sound conduction: wax in ear, disarticulated ossicles, stiffening of insertion of stapes footplate into oval window.
Usually correctable with surgery
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.
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)!
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:
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.
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 )
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.
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".
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
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.
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