L15 - Implants Flashcards
What is a cochlea implant?
- Bionic ears are the only aspect of human augmentation that has taken off
- Suitable for severely/profoundly deaf: congenitally-deaf children, victims of meningitis, recently deafened adults
- Consist of an external process and implant which has an electrode array which is pushed into the cochlea and electrodes stimulate different places inside the cochlea
- Early implants had one electrode, but now 24 are inserted
- Results in perceived sound
How do they put in an implant?
- Two opening in the cochlea: oval and round window
- Round window is exploited to insert the electrode = have access to middle ear and implant is pushed through the round window and into the cochlea as far as they can (under the basilar membrane) = tighten the cord to get it as close to the spiral ganglion to pick up the stimulation and separately excite the ganglion
How does a cochlea implant work?
- External sound processor analyses the sound into diff frequencies
- Sends info to round bit on back of head which then sends by induction to receiver = sends it to internal implant and electrode array = then the spiral ganglion = action potentials in the auditory nerve = allows hearing
How does the processing work?
- As we know cochlea differentiates frequencies, the processor mimics this e.g milk is separated into 8 frequency bands and each part would be sent to a diff part of the cochlea but this cannot be done as each part cannot correspond to an electrode = due to issues of current flowing from one electrode and another = resolved by having one electrode stimulated at once = pulsing in proportion to the amount of sound in each channel
Zoom in on one channel
- The pressure of waveform goes at different rates in different parts of the cochlea.
- Send off phase locked action potentials up the auditory nerve = respond to different cycles of waveform
- Extracted variation in amplitude = tone is completely unchanging
- Waveform that has diff frequencies
- Processing extracts the envelope - variation of amplitude over time
= Done for all 8 channels to create a distinct pattern of frequency changes to be communicated to cochlea - Reduces variation in pressure in waveform = stops phase-locked action potentials across the nerve
- Once envelope has been extracted = Continuous Interleaving Sampling (pulses) encoding in an implant causing a modulated pulse train
Stimulation using a vocoder = modulated noise band - Vocoder helps normal hearing person to understand what it is like to listen with an implant
How do you make a vocoder?
1) Band of noise between two frequencies will go to a particular place in the cochlea = stimulating a singular place in a normal hearing person
- Use that envelope in someone who is deaf
-Speech filtered between 1-8 freq bands, env of each band was extracted, noise filtered onto similar bands, noise bands multiplied by the envelopes of the corresponding speech bands
- That same envelope can be used to be modulated as it follows the envelope changes
- Will vary the amount of stimulation in the cochlea depending on the amplitude over time
How can we understand the performance cochlea-implant users display?
- Through understanding of the 4 frequency channels using the vocoder
What are the limitations of the processing strategies?
- Channels: at a certain point = does not make a different
- If you give people two (bilateral) = very expensive
What is the insertion-depth problem?
- Speech recognition declines with decreasing insertion depth if speech information is only delivered to the correct position in the cochlea = discarding low frequency channels = cannot be mapped
- Put frequencies wherever you can get them if some electrodes are not usable
STUDY: - Simulated each electrode with the right frequency vs whatever frequency you want to get across
- Can do this in vocoder by mismatching modulation and envelope
- When shifted, the tones sound like mickey mouse (frequencies get higher), when natural - discards the lower frequencies
- Performance drops off in the natural mapping in hearing as you throw away more low frequency info
- Did not compare to shifting mapping
What is the Speech-in-Noise Problem?
- Speech Reception Thresholds for different cochlear implant simulations and for different interferers
- Does having more channels help? Threshold for detecting/understanding speech lowers in background noise as number of channels lowers
- Can understand speech with just 4 channels with no background noise but it’s harder with background noise
- For a normal hearing person: Listening to a single opposite gender voice = easiest to hear, Modulated noise = harder to hear, Continuous noise is hardest to hear
- BUT this pattern is reversed when you have a vocoder/cochlea implant (because of the loss of phase-locking)
- Having more channels is good until a certain point where it plateaus in performance
What is the current spread problem?
- Current spread undermines you when you try to get more electrodes
- Electrode in cochlea produces an electric field that stimulates things around in, it will eventually attenuate with distance - must travel to ganglion cells but can interact with the electrode next to it - causing channel interactions
- Hard to vocode & We do not know how much current spread is in the ear and is hard to measure
What are the benefits of a short electrode? (electro-acoustic stimulation)
- Looking at people who cannot hear pure tones at given frequencies (usually high frequencies) = can get to those easily in cochlea = embed short electrodes = hybrid hearing
- Compared SRTs for short and long-electrode cochlea implants
- SRTs for simulations of these implants compared with normal performance
STUDY - People implanted with short vs long electrodes
- Performed an exp where left the low freq unprocessed and vocoding in high freq
- Perform better and lower threshold when you have combo of acoustic and electric hearing
LIMITATION: the groups weren’t random: electrodes were fitted according to their needs (had different hearing needs)
FOLLOW-UP STUDY:
- With vocoder, can use same ppts and conditioned using 16 channel vocoder and acoustic/electric stimuli
- Combinaiton of acoustic/electric helps if you have a talker as an interfere but does not help if there is noise = helpful to have short electrodes for those with residual hearing
What is the benefit of bilateral implants for speech?
- What direction sound is coming from
- Can get better understanding of speech with background noises
- If you have two cochlea implants = benefit of having one on each side
- SRTs measured for each implant individually and using both
- If you move noise to one side, sound is in accoustic shadow of head, signal to noise ratio is good
- Someone with both cochlea implants = compare binaural performance with better signal performance is similar = do not have to turn around to hear what people are saying
What was the study looking at predictions of SRM for CI users?
- How well people could hear speech in background noise
- Modelled set of data looking at CI patients listening with 1/2 ears
- Measured speech perception thresholds
- Correlation between predictions and reality
What was the predicted SRM for unilateral and bilateral cochlea implantees?
- Target is front, noise at 90 degrees
- Curves show benefits when target is in front vs symmetrical (much louder)
- Measured an 18db difference when model suggested 4db
