CAN YOU HEAR ME NOW?
Hearing
What to Expect/Objectives
Describe what hearing is Describe the pressure waves that experiences
as sound Describe the 3 regions of the ear Outline the series of events that trigger the
electrical impulses sent to the brain Contrast place and frequency theories Explain how place and frequency theories help
us understand pitch perception Describe how we pinpoint sounds Contrast the 2 types of hearing loss and their
causes Describe how cochlear implants function Explain why deaf culture advocates object to
cochlear implants
Summary
What is hearing? We transduce air pressure waves into
neural messages that the brain interprets as sound
Hearing is highly adaptive – just like our other senses
Like sight, it relies on past experience for perception
Sound Waves
If you slam a book on the desk, the result is that stimulus energy becomes sound waves Molecules of air that bump into each other
Waves expand and compress Like waves in a pond when you toss a stone in
Ears detect the changes in air pressure Ears transform the vibrating air into nerve
impulses Brain then decodes those nerve impulses
as sound
Variations in Waves Strength = amplitude = loudness Frequency = pitch
Long waves have low frequency and therefore have low pitch
Decibels are the measuring unit for sound energy
Absolute threshold for hearing is zero decibels
Every 10 decibels is a 10- fold increase in sound
Normal conversation is 60 db, whisper is 20 db
Prolonged periods over 85 db can create hearing loss
The Ear
The ear converts sound waves into neural activity through a mechanical chain reaction
1. Outer ear channels the sound waves through the auditory canal to the eardrum Eardrum is a tight membrane that vibrates with the
waves 2. Middle ear then transmits the eardrum’s
vibrations through a piston made of 3 parts to the cochlea Hammer, anvil and stirrup = parts of piston Cochlea is a snail-shaped tube in the inner ear
3. Cochlea’s membrane vibrates moving fluid that is in the tube
4. Ripples in the basilar membrane which is lined hair cells
The Ear Continued
5. Hairs bend 6. Bending of hairs trigger impulses in
nearby nerve fibers 7. Nerve fibers converge to create the
auditory nerve 8. Message send through thalamus to
temporal lobe’s auditory cortex
Hairs!
Very sensitive Damage to them can result in hearing loss Cochlea has 16,000 of them Move them by as much as the width of an
atom and it will trigger a neural response Can be damaged easily Loudness is measured by the number of hair
cells that respond If a hair cell loses sensitivity to soft sounds, it
can still detect loud sounds Why someone with hearing loss can hear loud
sounds just the same as someone without hearing loss
Pitch!
Chirp or roar? There are 2 theories on how we hear pitch Place Theory
Helmholtz Different sound waves trigger different activity
in different places along the basilar membrane Brain recognizes pitch by knowing where on
the membrane the neural signal came from High frequencies=large vibrations=beginning
of membrane Low frequencies=vibrations=end of membrane
Pitch 2
Frequency Theory Brain can read pitch from the frequency of
neural impulses Example – if a sound have a frequency of
100 waves per second then those 100 waves per second travel up the auditory nerve, thus there are more neural impulses triggered and our brains know it is higher pitched.
Houston…We Have a Problem
Both theories have “holes” in them Place theory: Can’t explain low-pitched sounds
because the neural signals that are generated cannot be localized so neatly on the basilar membrane
Frequency Theory: Neurons cannot fire faster than 1000 times per second, so how can we hear sounds that we know are above 1000 waves per second (upper third of the piano keyboard).
So…. Place theory best explains how we hear high pitch Frequency theory best explains how we hear low
pitch Combination handles everything in between
Where Did That Sound Come From? The placement of our ears allows for
stereophonic (3D) hearing That is why our ears are where they are
and why we have 2 of them We are great at hearing things on either
side of us, but not directly above us, below us, ahead or behind.
There is parallel processing in hearing too
Hearing Loss
Ear is intricate and delicate, therefore it is “injury prone”
Conduction hearing loss – problem with mechanical system that gets waves to cochlea
Sensorineural hearing loss – damage to hair cell receptors or associated nerves
Once tissue is dead it remains dead Generally caused by heredity, aging and
exposure to loud noises Hearing aids can amplify sound Have found a way to generate new hair cells
in other animals – hope for humans.
Cochlear Implants
Bionic ear! Electronic device that translates sounds into
electrical signals Wired to the cochlear nerves Conveys the info to the brain This is a hotly debated treatment Object to using the devices on children that
became deaf before they learned to speak They argue that it is not a disability and
therefore people should not be labeled as disabled
Sensory Compensation
When you lose one sense, the others tend to compensate and become stronger/more efficient and effective
If an area of your brain is supposed to be used for hearing is not used for hearing then your brain will use it for something else
So…by giving hearing back it will lessen sensory compensation
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