Can you hear me now?

1. Can you hear me now? Hearing

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

9. 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

10. 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

11. 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.

12. 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

13. 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

14. 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.

15. 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

16. 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