Hearing and Deafness

Outer, middle and inner ear

Types of hearing loss Conductive Hearing Loss defect in the external or middle ear sound not getting into inner ear severe hearing loss up to 60 dB (talking) can interfere with language development middle ear effusions mild loss: up to 30 dB (whisper)

Cochlea cross-section

Organ of Corti

Organ of Corti in action

Reponse of basilar membrane to sine waves Each point on the membrane acts like bandpass filter tuned to a different frequency: high freq at base, low at apex. Each point vibrates at frequency of pure tone (-> phase locking)

Reponse of basilar membrane to sine waves Each point on the membrane acts like bandpass filter tuned to a different frequency: high freq at base, low at apex. Each point vibrates at frequency of pure tone (-> phase locking)

Outer Hair cell

Types of hearing loss - 2 Sensori-neural (SNHL) Sensori-neural (SNHL): Outer-hair cell damage (and inner) from cumulative exposure to loud sound Breakdown of cochlear frequency analysis Only partial help from hearing aids

Symptoms of SNHL (from OHC loss) Raised thresholds: – helped by amplification Wider bandwidths: –no help possible Recruitment (restricted dynamic range): –partly helped by automatic gain controls in modern digital aids Often accompanied by tinnitus

Conductive vs Sensori-neural deafness Mostly a combination of OHC and IHC damage

Normal vs Impaired Hearing

Demo of hearing loss Normal SNHL sloping loss mild at low frequencies severe at high frequencies from "Perceptual consequences of cochlear damage" by Brian C J Moore. OUP 1995.

Excitation pattern of complex tone on bm

Cochlear excitation patterns NormalBandwidths three times broader

Levels of hearing loss Loss of hearing ranging from mild (<60 dB), to severe (60-80 dB), to profound (>85 dB). prevalence: 1/1000 children (severe to profound hearing loss) incidence: 1/1000 babies born deaf 1/50 infants from an NICU have significant hearing loss 1/10 children develop hearing problems at some time

Effects of SNHL on speech Speech very redundant –High-pass 1400Hz = Low-pass 1400 Hz High frequencies most affected –prosodic features best (pitch given by low freqs) –consonants worse than vowels –lip-reading helps place information in consonants Wider bandwidths -> more masking

Suitability for implant Over 2 years of age *Profound hearing loss in both ears *Does not benefit from conventional hearing aids *Has strong desire to communicate

Cochlear Implant 1. Mic. 2.Lead 3. speech processor. 4 the transmitting coil 5. Transmitting coil sends the signals transdermally to the implanted receiver/stimulator via an FM radio signal. 6. The receiver stimulator 7. The electrodes along the array stimulate the remaining auditory nerve fibers in the cochlea. 8. auditory nerve

Cochlear implant 2

Shannon Implant Demo

Types of hearing loss 3. Cortical Hearing Loss *inadequate transmission of the neural impulses from the inner ear to the temporal lobes *associated with diseases of the White Matter *results in several different types of hearing loss: 1. Cortical Deafness *affects perception of all sounds 2. Cortical Agnosia *affects perception of environmental sounds 3. Verbal Auditory Agnosia *affects perception of language and results in severe receptive dysphasia