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Environmental Stimulus Energy
Sound waves resulting from the compression and the decompression of air (usually) molecules as they radiate out in all directions from a vibrating object.
The movement back and forth of air molecules.
Characteristics of Sound Waves
- 1. Frequency
- 2. Amplitude
- 3. Timbre
The rate of vibration of the sound wave.
The adult human ear can transduce sound waves that vibrate at about what?
15 cycles per second up to about 20,000 cycles per second (15 to 20,000 hertz (Hz))
The frequency of the sound wave corresponds to what?
The sound sensation we call pitch. The higher frequency, the higher the pitch.
This refers to the amount of pressure there is in a sound wave. This is indexed by the number of molecules compressed into a given area, which may be conceptualized as the force exerted by the vibrating object on the molecules.
What does the amplitude of the sound wave correspond to?
The sound sensation called loudness. The more molecules compressed into a given space (the greater the force exerted by the vibrating object on the molecules), the louder the sound.
What is the loudness of a sound measured in?
Units known as decibels (dB).
This refers to the purity of the sound wave (tone).
The sound that results when a vibrating object (e.g. a tuning fork) produces a sound wave that vibrates at a single frequency. Such pure tones are very rare.
The sound that results when complex vibrating objects that have many parts produce sound waves that contain a particular mix of frequencies. That is, each part of the vibrating object produces its own frequency of sound wave. For the most part, the sounds we hear are complex sounds.
What characteristic of sound waves enable us to identify the particular sounds we hear?
Timbre helps us identify the particular sounds we hear (e.g. a familiar voice or the song of a particular kind of bird).
Anatomy of the Ear: Outer Ear
- 1. Pinna
- 2. Auditory Canal
- 3. Tympanic Membrane
The flesh and cartilage attached to each side of the head. The pinna can alter the reflections of sound waves and enable the organism to locate the direction the sound is coming from, especially sounds coming from the front or back. Not very useful for human beings.
A tunnel about an inch in length leading from the pinna to the tympanic membrane (eardrum). Air molecules travel through the auditory canal and strike the tympanic membrane.
Anatomy of the Ear: Middle Ear
The air-filled space beginning at the eardrum which contains three very small bones that are collectively known as the ossicles. These small bones, the malleus (hammer), incus (anvil) and stapes (stirrup), carry sound waves (vibrations) from the eardrum to the oval window. Two muscles are also part of the middle ear.
Connected to the hammer, this muscle dampens the vibrating of the ossicles when it contracts by increasing tension on the eardrum.
Connected to the stirrup, this muscle dampens the movement of the stirrup so that the amplitude of pressure against the oval window is reduced when it contracts.
Anatomy of the Ear: Inner Ear
The part that is important for hearing, it is a snail-shaped structure. The conchlea contains three fluid-filled chambers, the scala vestibuli, scala media, and the scala tympani. These structures are also known as the vestibular canal, the conchlear canal, and they tympanic canal.
Organ of Corti
This structure rests on the floor of the scala media supported by the basilar membrane. It contains the receptor cells, known as hair cells, which transduce the vibrating sound waves into a neural impulse. The hair cells are located on the basilar membrane beneath the surface of another overhanging membrane known as the tectorial membrane.
Number of Hair Cells in the Human Ear
Been estimated to be about 28,500; 25,000 in an outer layer and 3,500 in an inner layer.
What makes up the eighth cranial nerve?
Hair cells have excitatory synapses with sensory neurons that make up this cranial nerve.
Auditory Pathways to the Brain
- 1. Auditory Nerve (Cranial Nerve VIII)
- 2. Cochlear Nucleus
- 3. Superior Olive
- 4. Inferior Colliculus or Medial Geniculate Nucleus of the Thalamus
- 5. Primary Auditory Cortex
How is the Auditory Nerve (Cranial Nerve VIII) formed?
By a bundle of axons from bipolar neurons whose dendrites synapse with the hair cells in the cochlea.
Where do the cell bodies forming the auditory nerve collect?
The spiral ganglion located just outside the cochlea.
Axons of the Auditory Nerve
Enter the brain stem at the medulla region. There are two auditory nerves, one from each ear.
When entering the brain stem, the axons of the auditory nerve form synapses with neurons in the cochlear nucleus.
From the cochlear nucleus, information from each ear travels to a nucleus known as the superior olive located on both sides of the brain stem. In other words, information from each ear travels to both sides of the brain.
The crossing pathways of information from each ear.
From the superior olive, the information travels up both sides of the brain stem via this pathway.
Auditory information traveling up the lateral lemniscus projects to the: Inferior Colliculus
This midbrain structure is important for enabling us to orient body reflexes to sound.
Auditory information traveling up the lateral lemniscus projects to the: Medial Geniculate Nucleus of the Thalamus
The thalamus is a major relay station for information traveling from the sensory systems to higher centers in the cortex of the brain.
Primary Auditory Cortex
From the medial geniculate nucleus in the thalamus, auditory information is projected to primary auditory cortex located in the temporal lobe of the brain.
Leads from primary auditory cortex to an area in the prefrontal cortex that enables us to identify what the sound represents.
Leads from primary auditory cortex to an area in the prefrontal cortex that enables us to locate where the sound originated in the space.
- 1. Conduction Deafness
- 2. Nerve Deafness
- 3. Central Deafness
This occurs when sound waves are not properly transmitted to the cochlea.
What causes conduction deafness?
Damage to the tympanic membrane or the tiny bones of the middle of the ear (hammer, anvil, and stirrup). This is often caused by disease, infection or tumorous bone growth. Also, loud noise could rupture the eardrum.
How can conduction deafness be corrected?
Through surgery and/or use of a hearing aid that amplifies the sound stimulus.
Can people with conduction deafness hear themselves talk?
Yes, since vibrations from the vocal cords are conducted directly to the cochlea via bones of the skull.
Results from damage to the cochlea, the hair cells or the auditory nerve.
What causes nerve deafness?
It can be inherited, result from prenatal problems, or be caused by disorders in childhood. It may also be caused by repeated exposure to loud noise or reactions to certain drugs (especially drugs taken in childhood).
How can nerve deafness be corrected?
It cannot be corrected at this time.
Damage to the auditory cortex does not lead to an inability to hear and respond to simple sounds, unless the damage includes subcortical areas as well. However, advanced auditory processing such as the ability to recognize combinations or sequences of sounds (e.g. speech or music) is impaired.
How can central deafness be corrected?
It cannot be fixed.
Coding in the Auditory System
- 1. Coding for Pitch perception
- 2. Coding for Loudness perception
- 3. Coding for Location of a Sound
Coding for Pitch Perception
- 1. The Frequency Theory
- 2. The Volley Principle
- 3. The Place Theory
- 4. Coding in the Primary Auditory Cortex
The Frequency Theory
We perceive certain pitches when the basilar membrane vibrates in synchrony with the sound wave, causing the axons of the auditory nerve to produce action potential at the same frequency.
When does the frequency theory appear true?
For very low frequency sounds up to about 100 cycles per second.
Up to how many cycles/second can we her sounds?
Up to about 20,000 cycles/second
The Volley Princible
The auditory nerve as a whole can produce volleys of impulses up to about 4000 cycles per second, even though no individual axon can produce such frequencies of action potentials itself.
When does the volley principle seems to code for pitch?
When the frequency of the sound wave is somewhere between about 100 cycles per second and 4000 cycles per second.
The Place Theory
We perceive certain pitches because each area along the basilar membrane is tuned to a specific frequency and vibrates whenever that frequency is present. In other words, each frequency activates hair cells at only one place along the basilar membrane and the brain distinguishes frequencies, and thus pitch, by what neurons are activated.
When does the place theory appear to be true?
For sound waves above 4000 cycles per second.
Higher frequency sounds
Cause a maximum displacement of hair cells near the base of the cochlea (where the stirrup meets the cochlea).
Lower Frequency Sounds
Produce maximum displacement of hair cells near the apex of the cochlea.
Coding in the Primary Auditory Cortex
Each cell in this cortex responds best to tones at a given pitch.
What do cells responding to a given tone in the auditory cortex do?
Cluster together providing a map of the sounds. This map is referred to as a tonotopic map.
Coding for Loudness Perception
Increases in the amplitude of the sound wave cause the basilar membrane to be displaced over increasingly large areas and with increasing vigor. Thus, increases in the amplitude of the sound wave eventually causes an increase in the number of neural impulses per second arriving at the auditory cortex. This is the code for loudness- the more impulses per second arriving at the cortex, the louder the perceived sound.
What stimulates more hair cells in the cochlea?
More expansive displacement of the basilar membrane
What excites neurons with higher thresholds for firing that would not otherwise fire at action potential?
More vigorous (higher amplitude) displacement of the basilar membrane bends the hair cells enough.
Coding for Location of a Sound
- 1. Low frequency sounds
- 2. High frequency sounds
- 3. Locations of neurons sensitive to differences in time of arrival of the sound
- 4. Location of neurons sensitive to intensity differences in sound
Low Frequency Sounds
The primary cues for locating low frequency sounds in space are time of arrival at the ear (most useful for sounds with a sudden onset) and phase differences (useful for sounds up to about 1500 cycles per second in frequency).
High Frequency Sounds
The primary cue for locating high frequency sounds in space is the intensity (loudness) of the sound (sounds above 2000-3000 cycles per second).
Locations of Neurons Sensitive to Differences in Time of Arrival of the Sound
- 1. Medial Superior Olive
- 2. Inferior Colliculus
- 3. Auditory Cortex
Location of Neurons Sensitive to Intensity (Loudness) Differences in Sound
Lateral Superior Olive