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

2. What is it that makes sounds high pitched or low pitched?
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?

3. 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.

4. Loud speakers produce sound by:
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.

5. 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.

6. 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).

7. 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.
p is the sound pressure of the stimulus, and po is the standard sound pressure which is usually set at 20 micropascals. The standard pressure is close to the pressure of a 1,000 Hz tone at threshold in a free field. Note that adding sound pressure level (SPL) to a dB measure indicates that the standard pressure of 20 micropascals was used.

8. Figure (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.)

9. Figure 11. 3 Three different amplitudes of a pure tone
Figure 11.3 Three different amplitudes of a pure tone. Larger amplitudes are associated with the perception of greater loudness.

10. Relative amplitudes and decibels for environmental sounds

11. 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.

12. Three different frequencies of a pure tone
Three different frequencies of a pure tone. Higher frequencies are associated with the perception of higher pitches. All pitches are same loudness (amplitude).

13. 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.

14. 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
The instructor can tell the students that the sounds being discussed here are periodic sounds, which are the most widely studied type of sounds. Aperiodic sounds, which do have repeating sounds waves, are also widely heard in our environment, but have not been investigated as extensively as periodic sounds.

15. Complex sound Fundamental (200 Hz) 2nd harmonic (400 Hz)
3rd harmonic (600 Hz)
4th 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)

16. 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.

17. Guitar Bassoon Alto saxophone
Figure 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.

18. 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.

19. Figure 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.

20. Human hearing range - 20 to 20,000 Hz
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.
The instructor can note that the range from 2,000 to 4,000 Hz is the most important range for understanding human speech. In addition, the instructor can note how other animals can perceive sounds outside of the range of human hearing (elephants can hear below 20 Hz; dogs can hear frequencies above 40,000 Hz, cats above 50,000 Hz and dolphins as high as 150,000 Hz.

21. pain
Figure 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.

22. Outer ear - pinna and auditory canal
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.

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

24. Two cubic centimeter cavity separating inner from outer ear
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

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

26. Outer and inner ear are filled with air.
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.
The instructor can also note that the middle ear contains muscles, the smallest skeletal muscles in the body, which are responsible for dampening the ossicles’ vibration to protect the inner ear from damaging stimuli.

27. Figure 11. 13 Environments inside the outer, middle, and inner ears
Figure 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.

28. (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).

29. Main structure is the cochlea
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

30. Figure 11. 15 (a) A partially uncoiled cochlea
Figure (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.

31. Key structures The Organ of Corti
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
There are approximately 3,500 inner hair cells and 12,000 outer hair cells.

32. Figure 11. 16 (a) Cross-section of the cochlea
Figure (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.

33. 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.
The instructor can use the next slides to give details about how the movement of the structures works to cause transduction.

34. channels resting excites
Figure (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.

35. 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

36. 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.

37. 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 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.

38. 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.

39. 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 .
The next slide provides the opportunity to present a description of a traveling wave.

40. Figure 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.

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

42. 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.

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

44. 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

45. 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.

46. Figure (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.

47. Conductive hearing loss Blockage of sound from the receptor cells
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

48. 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

49. Presbycusis
Figure 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).

50. Figure 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.

51. 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)

52. SONIC MG CN
SON IC
MGN A1
Figure 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.)

53. 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.

54. Figure 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).

55. 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.

56. Areas in the monkey cortex that respond to auditory stimuli
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.

57. Effect of Experience on the Auditory Cortex
Musicians show enlarged auditory cortices that respond to piano tones and stronger neural responses than non-musicians.

58. The device is made up of: a microphone worn behind the ear,
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.

59. Figure 11.46 Cochlear implant device.

60. 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.