Some experiments and fitting issues associated with patients having a cochlear implant in one ear and normal hearing in the other

Transcription

1 Some experiments and fitting issues associated with patients having a cochlear implant in one ear and normal hearing in the other Bob Carlyon, MRC CBU, Cambridge, U.K....and a cast of thousands

2 Co-authors MRC CBU: Olivier Macherey & John Deeks Addenbrookes Hospital Dave Baguley Patrick Axon John Briggs Joanne Muff Advanced Bionics Paddy Boyle Univ. Leiden Johan Frijns Jeroen Briaire Randy Kalkman Univ. Bordeaux Rene Dauman Xavier Barreau

3 Cochlear implants 101 Transmitter Receiver/Stimulator Electrode array Speech processor

4 Effect of electrode position on pitch Pitch increases as one stimulates more basal electrodes. This shift in the place of excitation roughly mimics effects of increasing frequency in normal hearing. This information is usually enough when programming a CI. It doesn t tell us: Which acoustic frequency would produce the same place of excitation corresponding to a given electrode. In other words: how does the pitch of electric stimulation compare to that produced in acoustic hearing?

5 Acoustic-electric pitch comparisons I ll describe some experiments with 5 patients having normal hearing in one ear and a CI in the other. Main question: for a given electrode, what is the frequency of an acoustic sound that produces the same pitch? Who cares? Scientists Basic information about which auditory nerve fibres are excited by a sound, & how this varies with e.g. level, hearing loss... Clinicians Implications for surgery & fitting in cases of residual lowfrequency hearing?

6 Clinical implications As implantation criteria relax, more & more patients have residual low-frequency hearing in implanted or non-implanted ear With residual same-side hearing, one approach is to insert short electrode array, so as to preserve residual hearing. But how far?? APEX BASE For the audiologist, suppose a patient has good hearing up to, say, 500 Hz in the unimplanted ear. Given that the head shadow effect is quite small at low frequencies, should you still programme CI to present low-frequency information to most apical electrodes?

7 Acoustic-electric pitch matches Simple prediction: experiments with animals have presented acoustic pure tones, and measured which AN fibres respond most to tones at each frequency. Greenwood (1969) summarised findings and calculated a frequency to place formula that best fit the data for each species. This formula takes into account the length of the basilar membrane (BM), and can therefore be used to make predictions for humans, based on length of human BM Initial findings: CI patients with residual hearing in unimplanted ear compared pure tones to high-rate (e.g pps) electric pulse trains. Tones adjusted to a frequency 1-2 octaves lower than predicted by Greenwood (Boex et al, 2006; Dorman et al, 2007)

8 Complications and Solutions Hearing impairment Hearing loss in un-implanted ear. Test patients with NH in one ear Plasticity Pitch match between ears may change with implant use (Reiss & Turner). Test early on Temporal codes Pitch is influenced not only by place of excitation but also by temporal pattern of firing; may differ between high-rate electric pulse trains and pure tones Use novel stimuli that produce similar temporal response in each ear Measurement issues High-rate pulse trains and pure tones sound different. Results can be contaminated by non-sensory biases (e.g. context) Perform sanity checks

9 Present study: Patients Patients have one deaf ear with tinnitus and one normal hearing ear. Implanted with Advanced Bionics HiRes 90k device for tinnitus alleviation. We test before patients have heard acoustic and electric sounds together (e.g. at switch-on) and also at later times. 2 implanted in Bordeaux (B1-B2) 3 implanted in Cambridge (C1-C3)

10 Present study: Protocol Study designed to evaluate effects of CI on tinnitus Crossover design: 3 months listening in live speech mode 3 months listening to sounds via mp3 player (via AUX input) Order ot two modes randomised across patients Questionnaire at start & end of each phase Pitch matches performed before speech mode; these matches informed the maps programmed into CI At end of tinnitus study, patients re-programmed with standard clinical map Tinnitus study still ongoing & I won t discuss those results

11 Stimuli a) 500-ms 1031-pps pulse train to 1 electrode vs tone b) 500-ms 12 or 25-pps pulse train to 1 electrode vs 12 or 25-pps acoustic pulse train, passed through 100-Hz-wide bandpass filter (slopes= 48 db/octave) Aud nerve fibres fire at same time to each pulse (e.g., 25 times/ sec) Which fibres fire depends on filter setting (acoustic) or electrode (electric) Amplitude Freq (Hz)

12 Methods Pitch matching Electric & Acoustic sounds presented in pairs; electric stimulation fixed. Patient adjusts acoustic frequency for next presentation (until done). Wide (e.g. 1-2 octaves) range of starting frequencies. After each match, switch to a different stimulus. Constant stimuli Electric & Acoustic sounds presented in pairs; electric stimulation fixed. Acoustic comparison selected at random from 7-8 stimuli. Acoustic stimuli differ in (centre) frequency over 1-2 octave range. Obtain Point of Subjective Equality (PSE) from 50% point on psychometric function. Switch to different stimulus after 10 trials/point. All stimuli presented in monopolar mode, each pulse 100 μs / phase

13 How to fool yourself Patient B2 Pitch matching, electrode 6. 1-oct wide starting range. 25-pps acoustic vs electric 1031 pps vs pure tone BUT Each match correlates with starting frequency Subsequent match with electrode 9 gave same results!

14 Is all hope lost? Patients are comparing sounds which can have very different qualities Often, the acoustic sound has a clearer pitch BUT In some cases, the 25-pps or (more often) 12-pps acoustic and electric stimuli sound almost identical There are some conditions, which when fulfilled, allow us to trust the pitch judgements obtained One such case comes from patient C1, when we present sounds with similar temporal qualities to the two ears.

15 A better result.. Patient C1 Pitch matching, electrode oct wide starting range. 12-pps acoustic vs electric 1031-pps vs pure tone No correlation between final match and starting frequency N.B. Results with pure tone less reliable

16 Non-sensory biases We ve found non-sensory biases in a large number of cases, using a variety of methods Biases also found with normal-hearing subjects when comparing pure tones in one ear to bands of noise in the other Need to perform sanity checks to make sure patients are doing what you ask them to do! Previous research probably contaminated by non-sensory biases When you do the checks, you can get good, reliable results.

17 Reliable results MATCHED CF (Hz)

18 Relationship to CT scans (Frijns, Kalkman, Briaire) CT scans (obtained just after implantation) analysed and electrode position converted to centre frequency (CF) based on predictions of Greenwood (1990) function Neural model (Frijns et al) Predictions of the two models are generally similar For each of 4 subjects, the trustworthy pitch estimates were superimposed on these functions.

19 Relationship to CT scans

20 Relationship to CT scans In most cases, matches are close to the predictions of Greenwood s model No hint of previously reported trend, where matches were 1-2 octaves below predictions In one case subject C1 matches are substantially higher than prediction based on CT scan: So what s going on here, then?

21 Electrode slippage Could the high matches for C1 be due to electrode array slipping out somewhat, prior to our measurements? Subject was re-scanned after pitch matches completed: array has slipped!

22 Patient C2 re-scan Revised predictions now closely follow pitch matches: Matches on electrodes 1 & 10 re-checked on day of scan Pitch matches can reveal electrode slippage! As CT scans usually obtained only once, you usually don t know where electrodes are

23 Implications for fitting During the speech mode phase of study, patients received an experimental map based on pitch matches. This was v. different to standard clinical map

24 Implications for fitting Our rationale was that for frequencies << 1000 Hz, the head shadow would be quite small, so that the pt would have a lightly-attenuated copy of the sound reaching the normal-hearing ear We also wished to make the map sound as natural as possible, so as to maximise tolerance to the CI (so pt would wear it, & thereby improve tinnitus) All 3 patients implanted in Cambridge responded well to the map, although we don t know whether a standard map would have done just as well.

25 Implications for fitting At end of tinnitus study C1 was provided with standard map. Following her re-scan she was given choice between a new map that fitted her pitch matches (and therefore also the Greenwood predictions), and a compromise (20% lower). She chose the map closest to pitch matches, even though it had 4 most basal electrodes de-activated She subsequently contacted us to say that with new map she was using CI substantially more

26 Implications for fitting

27 Summary Electro-acoustic matches affected by range biases This may explain low matches obtained previously, in patients only having lowfrequency hearing Sanity checks needed before trusting matches Reliable matches close to predictions of Greenwood s formula Pitch matches can reveal electrode slippage Patients with normal contralateral hearing easily tolerate removal of low-frequencies from CI map

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29 How to fool yourself again Patient B1 Constant stimuli, electrode pps vs pure tone 25 pps vs 25 pps Acoustic frequency (CF): Hz (1.7 oct) BUT Function shifts markedly when we use a different range of acoustic sounds ( Hz). Similar biases occur even with NH listeners when comparing tones & bands of noise => not just us

30 Another better result Patient C1 Constant stimuli, electrode pps vs 12 pps Acoustic frequency (CF): Hz (2 oct) Hz (2 oct) Similar results obtained with the 2 ranges Need to perform sanity checks