Audition and Mechanical Senses

Sound and the Ear
Under optimum conditions, human hearing is sensitive to sounds that vibrate the eardrum by less than one-tenth the diameter of an atom, and we can detect a difference between two sounds as little as 1/30 the interval between two piano notes (Hudspeth, 2014)

We attend to hearing in order to extract useful information

Physics and Psychology of Sound

Sound waves

• are periodic compressions of air, water, or other media. 

Ex. When a tree falls, the tree and the ground vibrate, setting up sound waves in the air that strike the ears. Sound waves vary in amplitude and frequency. 

amplitude of a sound wave = intensity

• sounds of greater amplitude seem louder, but exceptions occur. 

ex. a rapidly talking person seems louder than slow music of the same physical amplitude.

• height of each wave 

frequency of a sound = the number of compressions per second, measured in hertz (Hz, cycles per second).

Pitch -  related aspect of perception. Sounds higher in frequency are higher in pitch.

Figure 6.1 illustrates the amplitude and frequency of sounds. The height of each wave corresponds to amplitude, and the number of waves per second corresponds to frequency.

Most adult humans hear sounds starting at about 15 to 20 Hz and ranging up to almost 20,000 Hz. 

Children hear higher frequencies than adults, because the ability to perceive high frequencies decreases with age and exposure to loud noises (Schneider, Trehub, Morrongiello, & Thorpe, 1986).

ex. larger animals = elephants hear best at lower pitches, and small animals = mice hear higher pitches, including a range well above what humans hear.

timbre (TAM-ber), meaning tone quality or tone complexity. TONO

ex. Two musical instruments playing the same note at the same loudness sound different, as do two people singing the same note at the same loudness.

ex. any instrument playing a note at 256 Hz will simultaneously produce sound at 128 Hz, 512 Hz, and so forth, known as harmonics of the principal note. The amount of each harmonic differs among instruments.

People communicate emotion by alterations in pitch, loudness, and timbre.

prosody - Conveying emotional information by tone of voice

ex. the way you say “that was interesting” could indicate approval (it really was interesting), sarcasm (it really was boring), or suspicion (you think someone was hinting something)

Structures of the Ear

Rube Goldberg (1883–1970) drew cartoons of complicated, far-fetched inventions.

ex. a person’s tread on the front doorstep might pull a string that raised a cat’s tail, awakening the cat, which then chases a bird that had been resting on a balance, which swings up to strike a doorbell. (para siyang domino effect)

ang point lang dito is, the ear is complex enough to resemble a Rube Goldberg device, but unlike Goldberg’s inventions, the ear actually works.

There is an outer ear, the middle ear, and the inner ear (see Figure 6.2).

When sound waves strike the tympanic membrane in (a), they vibrate three tiny bones—the hammer, anvil, and stirrup—that convert the sound waves into stronger vibrations in the fluid-filled cochlea (b). Those vibrations displace the hair cells along the basilar membrane in the cochlea. (c) A cross section through the cochlea. (d) A close-up of the hair cells.

pinna

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

• outer ear

• sound localization. by altering the reflections of sound waves, it helps us locate the source of a sound. We have to learn to use that information because each person’s pinna is shaped differently from anyone else’s (Van Wanrooij & Van Opstal, 2005).

• After sound waves pass through the auditory canal (see Figure 6.2), they enter the middle ear, a structure that had to evolve when ancient fish evolved into land animals.

• Because animal tissues respond to water vibrations almost the same way that water itself does, fish hearing receptors can be relatively simple. But because the same receptors would not respond well to vibrations in the air, early land animals would have heard only low-frequency sounds that were loud enough to vibrate the whole head (Christensen, Christensen-Dalsgaard, & Madsen, 2015).

• To develop effective hearing on land, animals needed to evolve a way to amplify sound vibrations. The structures of the middle ear and inner ear accomplish that. When sound waves reach the middle ear, they vibrate the 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.

• these are the smallest bones in the body, are sometimes known by their English names (hammer, anvil, and stirrup) and sometimes by their Latin names (malleus, incus, and stapes).

• 20 times larger than the footplate of the stirrup, which connects to the oval window. As in a hydraulic pump, the vibrations of the tympanic membrane amplify into more forceful vibrations of the smaller stirrup. The net effect converts the sound waves into waves of greater pressure on the small oval window.

• When the stirrup vibrates the oval windo w, it sets into motion the fluid in the cochlea (KOCK-lee-uh), the snail-shaped structure of the inner ear. Figure 6.2c shows a cross section through the cochlea and its tunnels. The auditory receptors, known as hair cells, lie between the basilar membrane of the cochlea on one side and the tectorial membrane on the other (see Figure 6.2d). Vibrations in the fluid of the cochlea displace the hair cells, thereby opening ion channels in its membrane. Figure 6.3 shows an electron micrograph of human hair cells. The hair cells stimulate the cells of the auditory nerve, which is part of the eighth cranial nerve.Pitch

Perception

Place theory

• Each frequency activates the hair cells at only one place along the basilar membrane (which resembles the strings of a piano, with each area along the membrane tuned to a specific frequency) and the nervous system distinguishes among frequencies based on which neurons respond

Frequency theory

• Basilar membrane vibrates in synchrony with a sound, causing auditory nerve axons to produce action potentials at the same frequency

volley principle

• the auditory nerve as a whole produces volley of impulses for sounds up to about 4000 per second, even though no individual axon approaches that frequency (Rose, Brugge, Anderson, & Hind, 1967). However, beyond about 4000 Hz, even staggered volleys of impulses cannot keep pace with the sound waves.

Human hearing takes place below 4000 Hz

The Auditory Cortex
primary auditory cortex (area A1)

• in the superior temporal cortex

The organization of the auditory cortex parallels that of the visual cortex

ex. just as the visual system has separate pathways for identifying objects and acting upon them, the auditory system has a pathway in the anterior temporal cortex specialized for identifying sounds, and a pathway in the posterior temporal cortex and the parietal cortex specialized for locating sounds

area A1 responds to imagined sounds as well as real ones. It becomes active when people view short silent videos that suggest sound—such as someone playing a piano, or a glass vase shattering on the ground

development of the auditory system depends on experience

damage to the primary auditory cortex have trouble with speech and music, but they can identify and localize single sounds NOT DEAFNESS

cortex is necessary for processing info not just hearing

- researchers found that most cells have a preferred tone. The auditory cortex provides what researchers call a tonotopic map of sounds, as shown in Figure 6.6. Note that cells responsive to similar frequencies tend to group together. The tonotopic map differs in detail from one person to another

• Although some cells in the auditory cortex respond well to a single tone, most cells respond best to a complex sound, such as a dominant tone and several harmonics

• ex. or a tone of 400 Hz, the harmonics are 800 Hz, 1200 Hz, and so forth. We experience a tone with harmonics as richer than one without them. S