4.3—the auditory system

Question 1

4.3 Learning Objectives

Question 2

4.3 Focus Questions

Question 3

Pitch

Answer 3

• pitch: the perceptual experience of sound wave frequencies.
• low-frequency sounds have long wavelengths and a low pitch.
• aimplitude: determinds its loudness; high-amplitude waves are louder than low-amplitude waves.
• loudness—a function of sound wave amplitude—is expressed through decibles (dB).

Question 4

The Human Ear (Diagram)

Answer 4

• pinna: the outer region that helps channel sound waves to the ear and allows you to determine the source or location of a sound.
• auditory canal: extends from the pinna to the eardrum.
• middle ear consists of three tiny moveable bones called ossicles known as the alleus (hammer), incus (anvil), and stapes (stirrup).

Question 5

Cochlea

Answer 5

• cochlea: a fluid-filled membrane that is coiled in a snail-like shape and contains the structures that convert sound into neural impulses.
• basilar membrane: hair-like projections line this part of the cochlea.
  
  • the pressing and pulling action of the ossicles causes parts of the basilar membrane to flex.
  • this causes the fluid in the cochlea to move, displacing these tiny hair cells.
  • when the hairs move, they stimulate the auditory nerves.
  • damange to any part of the auditory system will result in hearing impairments.
• Cochlear implants: a small microphone that detects sounds from the outside world that electronically stimulates parts of the membranes in the cochlea.

Question 6

Sound Localization

Answer 6

• sound localization: the process of identifying where sound comes from; is handled by parts of the brain stem as well as by a midbrain structure called the inferior colliculus.
• sound shadow: a phenomenon in which we localize sound by using differences in the intensity in which sound is heard by both ears.
  
  • if the source of the sound is to your left, the left ear will experience the sound more intensely than the right because the right ear will be in the sound shadow.
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Question 7

Place Theory of Hearing

Answer 7

• place theory of hearing: how we percieve pitch is based on the location (place) along the basilar membrane that sound stimulates.
• high-frequency sounds stimulate hair cells closest to the ossicles, whereas lower-frequency sounds stimulate hair cells toward the end of the cochlea.

Question 8

Frequency Theory

Answer 8

• frequency theory: the perception of pitch is related to the frequency at which the basilar membrane vibrates.
• neurons cannot fire more than 1000 times per second.
• given this limit, how can we hear sounds exceeding 1000 Hz?
• volley principle: groups of neurons fire in alternating (hence the term “volley”) fashion.
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Question 9

Primary Auditory Cortex

Answer 9

• primary auditory cortex: a major perceptual centre of the brain involved in percieving what we hear.
• the primary auditory cortex is surrounded by brain regions that provide additional sensory processing.
• secondary auditory cortex: helps us to interpret complex sounds, including those found in speech and music.
• the right hemisphere is able to detect smaller changes in pitch than the left hemisphere.
• we are not born with a fully developed auditory cortex.

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Question 10

The Perception of Music

Answer 10

• the brain areas involved in the perception of some elements of music are also involved in coordinating movements.
• individual differences in the ability to detect musical beats are linked to differences in activity in the basal ganglia.
• brain-imaging experiments show which areas of the brain are active, this does not guarantee these regions are necessary for a function to occur.
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