Chapter 11 Function of the Ear Perry C. Hanavan, Au.D.

Hearing and hearing tests Hearing range between 20 – 20,000 Hz Hearing tested between 250 – 8000 Hz Audiogram The ear is not equally sensitive to all frequencies. Different for sound field and ear phones…

Outer Ear The Mirror The Tube

Outer Ear The Mirror: The pinna is a very interesting part of the body! No one knows the function of each of the nooks and crannies of the pinna …. such as localization, speech perception, etc. Pinna serve as an “acoustic mirror”

Outer Ear The Mirror: Higher frequencies (shorter sound wavelengths) are reflected back to the ear canal by the pinna Sounds above 1500 Hz are enhanced by the “pinna effect”

Outer Ear The Mirror: The pinna effect results in a natural 5-8 dB high-frequency amplification

Outer Ear The tube: The external ear canal is 25-30 mm (1 inch) long, closed at the tympanic membrane and “open” at the entrance Marshall, et al refer to this tube as “quarter wavelength resonator” meaning that it has a 17 dB resonance (gain) near 2700 Hz in adults

Question The child’s ear canal is adult-like by which age? 1 2 3 4 5

Outer Ear The tube: People naturally hear through this resonance. Infants have shorter ear canals and their resonance can be up to 7000 Hz. Children’s ears are adult-like by age 3.

Outer Ear The tube: With mastoid cavities and large perforations, the resonance drops to about 1500-2000 Hz. Small perforations and tubes, have no effect.

Outer Ear

Outer Ear The tube: Ear canal finishes growing by age 9, and entire conchea by age 12.

Middle Ear Ear “Drum”: Actually refers to the tympanic membrane AND all of the other structures down wind… Ossicles, muscles, tendons,…

                                                         

Middle Ear Tympanic Membrane: Three layers (pars flaccida has two layers) Vibration pattern is rather complex Top portion (pars flaccida) does not transduce sounds very effectively.

Middle Ear “Matching transformer”: The only reason we have a middle ear is to be able to convert sound in AIR to sound in WATER. Fish do not need (or have) middle ears since they live in water

Middle Ear Matching Transformer: Allows us to hear 30-40 dB better than if no middle ear

Middle Ear Matching Transformer: Loss of this natural transformer means that we would have a 30-40 dB loss. Serous otitis media can give a 30-40 dB loss. Our voice is attenuated by 30-40 dB when we are under the water. Still not perfect! The middle ear is only about 66% efficient….

Middle Ear Stapedial Reflex: Attached to the third stirrup-like bone is the stapedial muscle. Found in all mammals. Protection from loud sounds (eg. our voice!)

Middle Ear Stapedial Reflex: Basis behind acoustic reflex testing in audiometry. Contracts upon high level inputs. Own voice, loud music and noise, amplified sound.

Middle Ear Stapedial Reflex: Usually contracts with inputs of 80-90 dB (ie., above the speech range of intensities). Only slightly higher for those with hearing loss. Our ear has a rudimentary high-level compression system for loud sounds.

How does this change the speech signal? Frequency Mouth Inner Ear F1 (500 Hz) 65 dB 56 dB F2 (1500 Hz) 60 dB 57 dB F3 (2500 Hz) 55 dB 64 dB

                                                                                                                                

Some common hearing ranges SPECIES Low (Hz) High (Hz) Human 20 20,000 Chimpanzee 100 Dog 50 46,000 Dolphin 1000 130,000 Bat 3000 120,000 Goldfish 2000

Problems of the outer and middle ears Conductive hearing loss Maximally 60 dB loss Medically and/or surgically treatable

Examples of conductive losses Wax (cerumen) occlusion Eardrum perforation Serous otitis media (ear infection) Otosclerosis (stiffening of the ossicles)

Inner Ear (Cochlea) Physiology: Over 100,000 moving parts stuffed into a volume smaller than the tip of your baby finger. Organ of Corti Of the 15,000 nerve endings: ¼ afferent (to the brain) from inner hair cells ¾ efferent (from the brain) to the outer hair cells

Inner Ear (Cochlea) Frequencies are arranged in the cochlea similar to a piano keyboard… High frequencies are at the “basal” end near the stapes footplate Low frequencies are at the “apical” end and are well protected by the 2 ½ turns of the cochlea …. Low frequencies have a longer longevity than higher frequencies (more peripheral).

                                                                          

                                                         

Inner Ear Outer Hair Cells: Receives “outgoing” stimuli and this functions to amplify the soft sounds (“motor” units). Usually first to die off (up to 60 dB HL, all of hair cell damage is outer hair cell…) Source of oto-acoustic emissions. (OAE)

Inner Ear Outer Hair Cell Damage: Modern hearing aids helps the “motor function” BUT… don’t really retune the hair cells. Will still need something else to help with noise reduction (eg. directional microphones).

Inner Ear Outer Hair Cell Damage: Loss of “motor function” for amplifying the quiet sounds. Loss of some “tuning” function. At more intense levels, “no real hearing loss” I can hear OK, if you just speak up!

Inner Ear Outer Hair Cells: Receives “outgoing” stimuli and this functions to amplify the soft sounds (“motor” units). Usually first to die off (up to 60 dB HL, all of hair cell damage is outer hair cell…) Source of oto-acoustic emissions. (OAE)

Inner Ear Inner Hair Cell Damage: Typical damage (in addition to outer hair cell damage) for losses above 60 dB HL.

Problems of the inner ear Sensorineural hearing loss Permanent No maximum hearing loss

Examples of sensori-neural hearing loss Presbycusis (age related loss) Noise/music exposure Certain drugs Hereditary conditions

Central Processing VIII cranial nerve takes sound up to the auditory cortex (transverse temporal gyrus) Several synapses or “weigh stations” along the way.

Central Processing Poorer representation of sound in brain Loss of tonotopic representation? Loss of one-to-one correspondence Slower neural conduction rate Possible mild demylenization? Loss of insulation of nerves

Natural Gain Characteristics of the Ear (Pinna effect) high frequency amplification Ear canal (high frequency amplification) Stapedial reflex (high level reduction) Outer hair cells (low level amplification) Central- binaural summation (3-6 amp.)

Ear Canal Resonance

Localization

Basilar Membrane-Organ of Corti

Tonotopic

Tonotopic

Auditory Nerve Tuning curve for single VIII cranial nerve fiber The sharper the tuning curve the greater the frequency specificity

Central Auditory Mechanism