Hearing

• Development of the Ear

  • "Good hearing" was only possible with developed middle ear ossicles
  • In insects, the hearing was developed independently approximately 20 times
  • Development of the ear is already complete by the 22nd week of pregnancy in humans
  • Developmental disorders lead to malformations of the outer, middle and inner ear
  • More than 60% of congenital hearing loss is genetic
  • The ear combines the balance and the hearing system

  
• Structure of the Auditory System

  • Peripheral Part
  • Central Part
• Inner ear:

  • Cochlear Component (concerned with hearing),
  • Vestibular Component (stationary balance)
  • Semi-Circular Component (balance in motion)
• Anatomy of the Cochlear

  • Spiral, bony chamber (like a snail), 2½ turns
  • Bony pillar (Modiolus) with cochlear nerve
  • Three compartments: Scala vestibulu and s. tympani contain perilymph, Scala media contains endolymph
  • Spiral organ: Hairs of outer and inner hair cells are connected with the tectorial membrane and the aud. nerve, Basilar membrane is flexible (narrow and thick near oval window, wide and thin at apex) and codes the frequency
  • Potential difference: +155 mV between endolymph (+85 mV) negativ loaded hair cells (–70 mV)

  
• Pathway of Sound Waves

  1. Sound wave vibrate the tympanic membrane

  2. Auditory ossicles vibrate. Pressure is amplified ~20 times

  3. Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli

  4. Sounds with frequencies below the hearing range travel through the helicotrema and do not excite hair cells

  5. Sound in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells

  
• Basilar Membrane Function


  • Vibrations come into the cochlear via the oval window (stapes)
  • Sound waves make the basilar membrane vibrate
  • A maximum movement of the basilar membrane at a particular frequency is named resonance frequency
  • Low frequency sound → excitement apical, High frequency sound → excitement basal
  • Inner hair cells are exited at its corresponding place and with the rate of a certain frequency -> pitch of a tone

  
• Role of Outer Hair Cells

  • Most fibers connected to the outer hair cells are efferent fibers (brain to cochlear)
  • Act on and move the basilar membrane
  • Increases the responsiveness of the inner hair cells (amplification the motion of the basilar membrane)
  • Protect the inner hair cells from damage (negative feedback loop)
• Measurement of Hearing Threshold

  relative hearing threshold (pure tone audiogram, unit [dB HL]: "hearing level"), absolute hearing threshold (measured physically, unit [dB SPL]: "sound pressure level")
• Types of Hearing Loss

  • Bone Conduction Hearing Loss ->problem transferring sound waves anywhere along the auditory pathway through the outer ear, tympanic membrane, or middle ear (ossicles) to the cochlear
  • Sensorineural Hearing Loss->root cause lies in the inner ear, sensory organ, or the nerve.
  • Combined Hearing Loss
  • Auditory Synaptopathy/Neuropathy ->disease of the auditory nervous system relating to the dysfunction of synapses (no transmission)
  • Central Hearing Loss ->no apparent damage to the structures of the ear
• Air and Bone Conduction (BC) Treatment

  Air Conduction (AC): can only be treated by active amplification of sound

  
  

  Bone Conduction (BC): surgical intervention possible
• Sensorineural Hearing Loss


  Inner ear Hearing Loss

  
• Conductive Hearing Loss


  Sound Transportation
• Mixed Hearing Loss


  Inner Ear and Sound Transportation
• Medical Solutions for Hearing Loss

  • Hearing Aids
  • Active Bone Conduction and Middle Ear Implants
  • Cochlear Implants
  • (Gene Therapy)
• Hearing Aids


  • Amplification of the acoustic sound
  • Requires dynamic range compression
  • Additional features: Directional microphones, Noise reduction, Wireless Connectivity (Smartphones, TV)
  • ITE,BTE,RIC
• Active Bone Conduction Implants

  • Indication criteria: High bone conduction hearing loss (e.g. Stenosis), Mild- to moderate inner ear hearing loss, Single-Sided Deafness
  • Transducer screwed to the bone (best coupling!)
• Active Bone Conduction Implants

  • Advantages: Uncomplicated and fast surgery, Implant can not dislocate (screwed to the bone)
  • Disadvantage: Excitement of the Cochlear is not side dependent (Transmission of the sound via bone conduction to the contralateral ear)
• Active Middle Ear Implants

  • Amplify the vibrations in the middle ear
  • Connected to ossicles or the round window
• Active Middle Ear Implants

  • Advantages: Excitement of the Cochlear is side dependent
  • Disadvantages: More complex surgery,Quality of coupling the FMT to middle ear can be weak
• Cochlear Implant (CI)

  Active neuroprosthesis which directly stimulates the auditory nerve in the inner ear with electrical impulses

  
• Cochlear Implant (CI) Indication criteria

  • Hard of hearing patients (or acquired deaf) with no sufficient speech understanding with a hearing aid
  • Deafened patients (must have heard in the past),
  • Acquired Single-Sided Deafness
  • Deaf born children
  • The auditory nerve must functionally exist! → MRT diagnostic prior to implantation
• Cochlear Implant (CI) Signal Processing

  • Frequency dependent depolarisation of synapsis via an electrical field
  • Field distribution accounts to simulation mode (monopolar = wide, bipolar = narrow)
  • Power consumption (monopolar << bipolar)
• Cochlear Implant (CI) Sound Processing Strategies

  • F0F1F2 (fundamental frequency and formants)
  • MPEAK (Multipeak)
  • SPEAK (Spectral Peak Strategy)
  • ACE (Advanced Combination Encoders)
  • CIS (Continuous Interleaved Sampling)
  • FS4 (Fine Structure Processing)
• SPEAK (Spectral Peak Strategy)

  
  

  Samples the incoming acoustic signal, converts that signal to the frequency domain, and identifies 6–10 peaks in the acoustic spectrum
• CIS (Continuous Interleaved Sampling)

  • All channels are stimulated in every cycle
  • Monopolar stimulation (faster stim. Rate)
  • Representation of the full frequency range in every stimulation cycle
• FS4 (Fine Structure Processing)


  • Same as CIS but: Timing of the stimuli of the 4 most apical (low frequency) electrodes is set to the zero crossing of the bandpass filtered signals, Rate coding of these 4 FSP channels, Imitates physiologically "normal hearing", Improved pitch and speech perception -> music
• FS4 (Fine Structure Processing)


  Same as CIS but: Timing of the stimuli of the 4 most apical (low frequency) electrodes is set to the zero crossing of the bandpass filtered signals, Rate coding of these 4 FSP channels (FSP: Fine Structure Processing), Imitates physiologically "normal hearing", Improved pitch and speech perception -> music
• CIS (Continuous Interleaved Sampling)


  Old strategy, Stimuli are not timed to acoustic wave
• How to fit a CI-user (example)

  1. Presentation of single channel electrode stimuli

  2. Adjust the perceived loudness to a comfortable level for each electrode (C-Value)

  3. Adjust the hearing threshold for each electrode (T-Value)

  4. Sweep stimulus over all electrodes to compare equal loudness of all electrodes

  5. Go to "live" mode (activate microphone) and adjust loudness for spoken speech

  6. On the first days of activation, the CI sounds often "unnatural" or "robotic". A limited speech/number perception is normal

  7. With assistant of specialized speech therapist (training of hearing with CI) the speech perception improves over the next weeks and months

  8. The sound gets more and more "normal" after some months
• Pitch Perception with a CI

  • Speech transmission and speech perception with CI is fine for most CI users
  • Music perception is bad for approximately haft of CI users
  • Normal hearing ~3500 inner hear cells, CI users: 12-22 electrode channels
• How to improve pitch perception?

  Place dependent stimulation rates, Mismatch between electric and acoustic stimulation
• If the electrical stimulation rate of a CI electrode corresponds to the physiological pitch of the stimulation site, a better (more precise, more tonal, more accurate, ...) pitch perception is conveyed.
• Electric-acoustic pitch comparison in SSD patients

  1. Insertion angle estimation

  2. Frequency mapping (6 apical electrodes) and calculated electrical stimulation rates

  3. Individually measured insertion angles [degree]

  4. Individually calculated stimulation rate [pulses per second]

  5. Experimental setup
• Results: Frequency mapping (6 apical electrodes) and calculated electrical stimulation rate


  • Fixed rate vs. place dependent rate
  • Median of interquartile range (individual subject data per rate)
  • Insertion angles are highly variable, especially in the apical region
  • Electrodes with insertion angles > 375⁰ (i.e. < 466 Hz) and place-adapted stimulation rates: Pitch matches closer to Greenwood function, Variation of pitch matches decreased
• Totally Implantable Cochlear Implant


  All implant components are placed under the skin, Challenging: Power supply (rechargeable battery) and microphone (body sound)
• Optical Cochlear Implant


  Optogenetic approaches for hearing restoration, Genetic modification of biological tissue enabling control of cells by light, Much higher frequency selectivity (more channels)
• CI- and Electric-Acoustic Stimulation (EAS) Simulation


  Restgehör-Anteil, cochler implant, electric-acoustic stimulation, without simulation, residual low frequency hearing, electric-acoustic stimulation and noise
• Mechanically Gated Ion Channels

  • Stereocilia protrude into the K+ rich endolymph
  • Tips of the longest stereocilia hairs are connected to the tectorial membrane
  • Stereocilia are bound together by tip links ->mechanically gated ion channels
  • In quiet the basilar membrane is in rest, few channels are open, cell is slightly depolarized
  • pulling on the tip links opens the ion channels
  • K+ and Ca2+ from endolymph flow through and create a receptor potential
  • action potential in cochlear nerve