Chapter 11: Sound, The Auditory System, and Pitch Perception

Overview of Questions

 If a tree falls in the forest and no one is there to hear it, is there a sound?

 What is it that makes sounds high pitched or low pitched?

 How do sound vibrations inside the ear lead to the perception of different pitches?

 How are sounds represented in the auditory cortex?

Pressure Waves and Perceptual Experience



Two definitions of “sound”



Physical definition - sound is

pressure changes

in the air or other medium.



Perceptual definition - sound is the

experience

we have when we hear.

Sound Waves

 Loud speakers produce sound by:  The diaphragm of the speaker moves out, pushing air molecules together called condensation.

 The diaphragm also moves in, pulling the air molecules apart called rarefication.

 The cycle of this process creates alternating high- and low-pressure regions that travel through the air.

Figure 11.1 (a) The effect of a vibrating speaker diaphragm on the surrounding air. Dark areas represent regions of high air pressure, and light areas represent areas of low air pressure.

Figure 11.1 (b) When a pebble is dropped into still water, the resulting ripples appear to move outward. However, the water is actually moving up and down, as indicated by movement of the boat. A similar situation exists for the sound waves produced by the speaker in (a).

Sound Waves - continued

 Pure tone - created by a sine wave  Amplitude - difference in pressure between high and low peaks of wave  Perception of amplitude is loudness  Decibel (dB) is used as the measure of loudness  The decibel scale relates the amplitude of the stimulus with the psychological experience of loudness.

Figure 11.2 (a) Plot of sine-wave pressure changes for a pure tone; (b) Pressure changes are indicated, as in Figure 11.1, by darkening (pressure increased relative to atmospheric pressure) and lightening (pressure decreased relative to atmospheric pressure.)

Figure 11.3 Three different amplitudes of a pure tone. Larger amplitudes are associated with the perception of greater loudness.

Relative amplitudes and decibels for environmental sounds

Sound Waves - continued

 Frequency - number of cycles within a given time period  Measured in Hertz (Hz) - 1 Hz is one cycle per second  Perception of pitch is related to frequency.

 Tone height is the increase in pitch that happens when frequency is increased.

Three different frequencies of a pure tone. Higher frequencies are associated with the perception of higher pitches.

All pitches are same loudness (amplitude).

Complex Periodic Sounds

 Fundamental frequency is the repetition rate and is called the first harmonic.

 Periodic complex tones consist of a number of pure tones called harmonics.

 Additional harmonics are multiples of the fundamental frequency.

Complex Periodic Sounds - continued

 Additive synthesis - process of adding harmonics to create complex sounds  Frequency spectrum - display of harmonics of a complex sound  Attack of tones - buildup of sound at the beginning of a tone  Decay of tones - decrease in sound at end of tone

a. Complex sound b. Fundamental (200 Hz) c. 2 nd harmonic (400 Hz) d. 3 rd harmonic (600 Hz) e. 4 th harmonic (800 Hz) Left: Waveforms of (a) a complex periodic sound with a fundamental frequency of 200 Hz; (b) fundamental (first harmonic) = 200 Hz; (c) second harmonic = 400 Hz; (d) third harmonic = 600 Hz; (e) fourth harmonic = 800 Hz. Right: Frequency spectra for each of the tones on the left. (Adapted from Plack, 2005)

Complex Periodic Sounds - continued



Timbre - all other perceptual aspects of a sound besides loudness, pitch, and duration



It is closely related to the harmonics, attack and decay of a tone.

Guitar Bassoon Alto saxophone Figure 11.10 Frequency spectra for a guitar, a bassoon, and an alto saxophone playing a tone with a fundamental frequency of 196 Hz. The position of the lines on the horizontal axis indicates the frequencies of the harmonics and their height indicates their intensities.

Musical Scales and Frequency

 Letters in the musical scale repeat.

 Notes with the same letter name (separated by octaves) have fundamental frequencies that are multiples of each other.

 These notes have the same tone chroma.

 We perceive such notes as similar to one another.

Figure 11.8 A piano keyboard, indicating the frequency associated with each key. Moving up the keyboard to the right increases the frequency and tone height. Notes with the same letter, like the A’s (arrows) have the same tone chroma.

Range of Hearing

 Human hearing range - 20 to 20,000 Hz  Audibility curve - shows the threshold of hearing in relation to frequency  Changes on this curve show that humans are most sensitive to 2,000 to 4,000 Hz.

 Auditory response area - falls between the audibility curve and the threshold for feeling  It shows the range of response for human audition.

pain Figure 11.9 The audibility curve and the auditory response area. Hearing occurs in the light green area between the audibility curve (the threshold for hearing) and the upper curve (the threshold for feeling). Tones with combinations of dB and frequency that place them in the pink area below the audibility curve cannot be heard. Tones above the threshold of feeling result in pain.

The Ear

 Outer ear - pinna and auditory canal  Pinna helps with sound location.

 Auditory canal - tube-like 3 cm long structure  It protects the tympanic membrane at the end of the canal.

 The resonant frequency of the canal amplifies frequencies between 1,000 and 5,000 Hz.

Figure 11.11 The ear, showing its three subdivisions - outer, middle, and inner. (From Lindsay & Norman, 1977)

The Middle Ear

 Two cubic centimeter cavity separating inner from outer ear  It contains the three ossicles  Malleus - moves due to the vibration of the tympanic membrane  Incus - transmits vibrations of malleus  Stapes - transmit vibrations of incus to the inner ear via the oval window of the cochlea

Figure 11.12 The middle ear. The three bones of the middle ear transmit the vibrations of the tympanic membrane to the inner ear.

Function of Ossicles

 Outer and inner ear are filled with air.

 Inner ear is filled with fluid that is much denser than air.

 Pressure changes in air transmit poorly into the denser medium.

 Ossicles act to amplify the vibration for better transmission to the fluid.

 Middle ear muscles dampen the ossicles’ vibrations to protect the inner ear from potentially damaging stimuli.

Figure 11.13 Environments inside the outer, middle, and inner ears. The fact that liquid fills the inner ear poses a problem for the transmission of sound vibrations from the air of the middle ear.

(a) A diagrammatic representation of the tympanic membrane and the stapes, showing the difference in size between the two. (b) How lever action can amplify the effect of a small force, presented on the right, to lift the large weight on the left. The lever action of the ossicles amplifies the sound vibrations reaching the tympanic inner ear. (From Schubert, 1980).

The Inner Ear

 Main structure is the cochlea  Fluid-filled snail-like structure (35 mm long) set into vibration by the stapes  Divided into the scala vestibuli and scala tympani by the cochlear partition  Cochlear partition extends from the base (stapes end) to the apex (far end)  Organ of Corti contained by the cochlear partition

Figure 11.15 (a) A partially uncoiled cochlea. (b) A fully uncoiled cochlea. The cochlear partition, indicated here by a line, actually contains the basilar membrane and the organ of Corti.

The Organ of Corti

 Key structures  Basilar membrane vibrates in response to sound and supports the organ of Corti  Inner and outer hair cells are the receptors for hearing  Tectorial membrane extends over the hair cells

Figure 11.16 (a) Cross-section of the cochlea. (b) Close-up of the organ of Corti, showing how it rests on the basilar membrane. Arrows indicate the motions of the basilar membrane and tectorial membrane that are caused by vibration of the cochlear partition.

The Organ of Corti - continued

 Transduction takes place by: 

Cilia bend in response to movement of organ of Corti and the tectorial membrane.

 This causes bursts of electrical signals.

channels excites resting Figure 11.18 (a) Movement of hair cilia in one direction opens ion channels in the hair cell, which results in the release of neurotransmitter onto an auditory nerve fiber; (b) Movement in the opposite direction closes the ion channels so there is no ion flow and no transmitter release.

Neural Signals for Frequency

 There are two ways nerve fibers signal frequency: 

Which

fibers are responding  Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers; 

How

fibers are firing  Rate or pattern of firing of nerve impulses

Békésys’ Place Theory of Hearing

Hungarian who won Nobel Prize in 1961

 Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate.

 Békésy determined this in two ways:  Direct observation of the basilar membrane in cadavers.

 Building a model of the cochlea using the physical properties of the basilar membrane.

Biography Sketch

  Before and during World War II, Békésy worked for the Hungarian Post Office (1923 to 1946), where he did research on telecommunications signal quality. This research led him to become interested in the workings of the ear. In 1946, he left Hungary to follow this line of research in Sweden.

In 1947, he moved to the United States, working at Harvard University until 1966. After his lab was destroyed by fire in 1965, he was offered to lead a research laboratory of sense organs in Honolulu, Hawaii. He became a professor at the University of Hawaii in 1966 and died in Honolulu.

Hair cells all along the cochlea send signals to nerve fibers that combine to form the auditory nerve. According to place theory, low frequencies cause maximum activity at the apex end of the cochlea, and high frequencies cause maximum activity at the base. Activation of the hair cells and auditory nerve fibers indicated in red would signal that the stimulus is in the middle of the frequency range for hearing.

Békésys’ Place Theory of Hearing - continued

 Physical properties of the basilar membrane  Base of the membrane (by stapes) is:  Three to four times narrower than at the apex.

 100 times stiffer than at the apex.

 Both the model and direct observation showed that the vibrating motion of the membrane is a traveling wave .

Figure 11.21 A perspective view showing the traveling wave motion of the basilar membrane. This picture shows what the membrane looks like when the vibration is “frozen,” with the wave about two thirds of the way down the membrane.

Figure 11.22 A perspective view of an uncoiled cochlea, showing how the basilar membrane gets wider at the apex end of the cochlea.

Békésys’ Place Theory of Hearing - continued

 Envelope of the traveling wave  Indicates the point of maximum displacement of the basilar membrane  Hair cells at this point are stimulated the most strongly leading to the nerve fibers firing the most strongly at this location.

 Position of the peak is a function of frequency.

Figure 11.24 The envelope of the basilar membrane’s vibration at frequencies ranging from 25 to 1,600 Hz, as measured by Békésy (1960).

Evidence for the Place Theory

 Tonotopic map 

Cochlea shows an orderly map of frequencies along its length

 Apex responds best to low frequencies  Base responds best to high frequencies

Basilar Membrane Response to Complex Tones

 Basilar membrane can be described as an acoustic prism.

 There are peaks in the membrane’s vibration that correspond to each harmonic in a complex tone.

 Each peak is associated with the frequency of a harmonic.

Figure 11.30 (a) Waveform of a complex tone consisting of three harmonics; (b) Basilar membrane. The shaded areas indicate locations of peak vibration associated with each harmonic in the complex tone .

Hearing Loss

 Two types  Conductive hearing loss  Blockage of sound from the receptor cells  Sensorineural hearing loss  Damage to hair cells  Damage to the auditory nerve or brain  Most common type is prebycusis

Hearing Loss - continued

 Presbycusis  Greatest loss is at high frequencies  Affects males more severely than females  Appears to be caused by exposure to damaging noises or drugs  Noise-induced hearing loss  Loud noise can severely damage the Organ of Corti  OSHA standards for noise levels at work are set to protect workers  Leisure noise can also cause hearing loss

Presbycusis Figure 11.34 Hearing loss in presbycusis (elder hearing) as a function of age. All the curves are plotted relative to the 20-year old curve, which is taken as the standard (Adapted from Dubno, in press).

Figure 11.35 Sound level of game 3 of the 2006 Stanley Cup finals between the Edmunton Oilers (the home team) and the Carolina Hurricanes. Sound levels were recorded by a small microphone in a spectator’s ear. The red line indicates a “safe” level for a 3-hour game.

Pathway from the Cochlea to the Cortex

 Auditory nerve fibers synapse in a series of subcortical structures  Cochlear nucleus  Superior olivary nucleus (in the brain stem)  Inferior colliculus (in the midbrain)  Medial geniculate nucleus (in the thalamus)  Auditory receiving area (A1 in the temporal lobe)

SONIC MG CN SON IC MGN A1 Figure 11.36 Diagram of the auditory pathways. This diagram is greatly simplified, as numerous connections between the structures are not shown. Note that auditory structures are bilateral -- they exist on both the left and right sides of the body -- and that messages can cross over between the two sides. (

Adapted from Wever, 1949.

)

Auditory Areas in the Cortex

 Hierarchical processing occurs in the cortex  Neural signals travel through the core, then belt, followed by the parabelt area.

 Simple sounds cause activation in the core area.

 Belt and parabelt areas are activated in response to more complex stimuli made up of many frequencies.

Figure 11.37 The three main auditory areas in the cortex are the core area, which contains the primary auditory receiving area (A1), the belt area, and the parabelt area. Signals, indicated by the arrows, travel from core, to belt, to parabelt. The dark lines show where the temporal lobe is pulled back to show areas that would not be visible from the surface. (From Kaas, Hackett, & Tramo, 1999).

What and Where Streams for Hearing



What,

or ventral stream, starts in the anterior portion of the core and belt and extends to the prefrontal cortex.

 It is responsible for identifying sounds.



Where,

or dorsal stream, starts in the posterior core and belt and extends to the parietal and prefrontal cortices.

 It is responsible for locating sounds.

 Evidence from neural recordings, brain damage, and brain scanning support these findings.

Areas in the monkey cortex that respond to auditory stimuli. The green areas respond to auditory stimuli, the purple areas to both auditory and visual stimuli. The arrows from the temporal lobe to the frontal lobe represent the

what

and

where

streams in the auditory system.

Effect of Experience on the Auditory Cortex

 Musicians show enlarged auditory cortices that respond to piano tones and stronger neural responses than non-musicians.

Cochlear Implants

 Electrodes are inserted into the cochlea to electrically stimulate auditory nerve fibers.

 The device is made up of:  a microphone worn behind the ear,  a sound processor,  a transmitter mounted on the mastoid bone,  and a receiver surgically mounted on the mastoid bone.

Figure 11.46 Cochlear implant device.

Cochlear Implants - continued

 Implants stimulate the cochlea at different places on the tonotopic map according to specific frequencies in the stimulus.

 These devices help deaf people to hear some sounds and to understand language.

 They work best for people who receive them early in life or for those who have lost their hearing, although they have caused some controversy in the deaf community.