Sensation and Perception: Ch.9

Sound wave

Fluctuation in air pressure across time, caused by vibrations of objects in the environment

Amplitude

The intensity or magnitude of displacement of a pressure wave

• Decibels (dB): units measuring physical intensity of sound on a logarithmic scale 

• Perceived as loudness: the psychological aspect of sound related to perceived intensity

Frequency

 The number of times per second that a pattern of pressure change repeats

• Hertz (Hz): unit measuring frequency of sound, where 1 Hz= 1 cycle/sec

• Perceived as pitch: the psychological aspect of sound related mainly to perceived frequency 

Audibility threshold

The lowest sound pressure level (dB) that can be reliably detected at a given frequency 

Outer ear

External portion of ear, collects and focuses sound waves

Pinna: outer, funnel-like part that collects the sound waves

Ear canal: canal that conducts sound vibrations from the pinna to the tympanic membrane 

Tympanic membrane: eardrum at the end of the ear canal, vibrates in response to sound

• Like the border in between outer and middle ear

Middle ear

Transfers and amplifies sound vibrations from the tympanic membrane to the oval window

Ossicles: 3 tiny bones

1. Malleus (hammer): receive vibration from the tympanic membrane

2. Incus (anvil): connects malleus to stapes

3. Straps (stirrup): transmits sound to oval window 

They all vibrate one another 

Oval window- flexible opening to cochlea which the stapes transmits vibration to the fluid inside 

• Like the border in between the middle and inner ear

Inner ear

Converts sound vibrations into neural signals

Cochlea: tiny, coiled structure where sound is transduced in the organ of Corti to the auditory nerve 

• Eventually ends up in the auditory nerve that ends up in our brain to be processed 

Organ of corti

A structure on the basilar membrane that is composed of hair cells and dendrites of auditory nerve fibers

Basilar membrane

A plate of fibers that forms the base of the cochlea

Auditory nerve

A collective of neurons that convey information from hair cells in the cochlea to the brainstem 

Hair cell

A cell that transduces mechanical movement in the inner ear into neural activity sent to the brain

Stereocilia

Hairlike extension on tips of hair cells that initiate the release of neurotransmitters when bent

Place code

Information about a sound wave’s frequency is coded by the place among the cochlea that has the greatest mechanical movement

• High frequency → base, near oval window

• Low frequency → apex, at the end 

Temporal code

Information about a sound wave’s frequency is coded by the timing of neural firing as it relates to the period of the sound

• High frequency → fast firing

• Low frequency → slow firing 

Neural pathway order

1. Coclea

2. Brainstem

   1. Cochlear nuclei: first brainstem nucleus at which auditory nerve synapses 

   2. Superior olive: brainstem region where inputs from both ears converge

   3. Inferior colliculus: midbrain nucleus in the auditory pathway, mostly from contralateral ear 

3. Thalamus

   1. Medial Geniculate Nucleus: Part of the thalamus that relays auditory signal to the temporal cortex

4. Cortex 

   1. Primary auditory cortex (A1): in the temporal lobe, responsible for processing basic acoustic information

   2. Secondary auditory cortex (A2): adjacent to (and gets input from) A1, responds to more complex characteristics of sounds 

Tonotonic organization: neurons are organized anatomically in order of frequency  

Conductive hearing loss

Hearing loss caused by problems with the outer or middle ear

• Problems with amplification of sound

• Outer and middle ear issue

Solution: Hearing aids- amplify sound

Sensorineural hearing loss

Hearing loss caused by defects in the cochlea or auditory nerve

• Problem with transduction of sound 

• Inner ear issue

Solution: 

• Cochlear implant

• Deliver sounds directly to the auditory nerve 

• Works as an electrical help to artificially simulate

Presbycusis

Normal, age-related hearing loss

• Typically affects high frequency and low amplitude sounds 

• Hearing loudness gets worse (high frequency and low sounds)

• Worse in men than women 

Hearing loss external factors

• Obstruction of the ear canal- blocks amplification

• Ear infection- fills middle ear with fluids

• Excessive noise exposure- damage to hair cells