Proefschrift_vd_Beek

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F. B. van der Beek et al.

468

Artefact rejection: Forward masking

Artefact rejection: Alternating polarity

0 1 2 3 4 5 6 7 8

Excited fibres

R16 R15 R14 R13 R12 R11 R10

R16 R15 R14 R13 R12 R11 R10 R16 R15

Scanning Selectivity

R9 R8 R7 R6 R5 R4 R2 R1

Stim

Recording

Recorded potentials (mV)

0 0.2 0.4 0.6 0.8 1 1.2 -2 -1

0 0.2 0.4 0.6 0.8 1 1.2 -1

Time from stimulus end (ms)

Patient 1

Patient 2

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

P16 P15 P14 P13 P12 P11 P10

P9 P7 P6 P5 P4 P2 P1 P3

Masker (M) Probe (P) Masked Probe (MP)

Refractory fibres

Recorded potentials (mV)

M + P - MP

0 0.2 0.4 0.6 0.8 1 1.2 Time from stimulus end (ms)

Speech perception Speech discrimination scores were obtained for all 19 adult cochlear implants users in the study group during normal clinical follow-up at predetermined intervals, starting one week after initial fitting. The data used for analysis in this study were the scores obtained at two years of follow-up. Four patients had to be excluded from the analysis, one patient since he was deceased (of a natural cause) 1.5 years after implantation, one prelingually deaf patient, and two patients for whom only scanning data and no selectivity measures were available. All subjects used the HiRes processing strategy. The standard Dutch speech test of the Dutch Society of Audiology, consisting of phonetically balanced monosyllabic (CVC) word lists, was used (Bosman & Smoorenburg, 1995). To improve test reliability, four lists (44 words) per condition were administered. Although this test is typically scored with phonemes in the Netherlands and Flanders, the data are shown as word scores, which is a more common reporting method in Anglo-Saxon countries. All testing was performed in a soundproof room, using a calibrated loudspeaker in frontal position at one metre distance. Subjects were tested in quiet at speech levels of 65 dB SPL in a CI-only condition. Speech perception Speech discrimination scores were obtained for all 19 adult cochlear implants users in the study group during normal clinical follow-up at predetermined intervals, starting one week after initial fitting. The data used for analysis in this study were the scores obtained at two years of follow-up. Four patients had to be excluded from the analy- sis, one patient since he was deceased (of a natural cause) 1.5 years after implantation, one prelingually deaf patient, and two patients for whom only scanning data and no selectivity measures were avail- able. All subjects used the HiRes processing strategy. The standard Dutch speech test of the Dutch Society of Audiology, consisting of phonetically balanced monosyllabic (CVC) word lists, was used (Bosman & Smoorenburg, 1995). To improve test reliability, four lists (44 words) per condition were administered. Although this test is typically scored with phonemes in the Netherlands and Flanders, the data are shown as word scores, which is a more common report- ing method in A gl -Saxon countries. All testing was performed in a soundproof room, using a calibrated loudspeaker in frontal position at one metre distance. Subjects were tested in quiet at spe ch level of 65 dB SPL in a CI-only condition. Analysis Signal processing was performed off-line using Matlab. eCAP amplitudes were automatically detected using Matlab software (as per Frijns et al, 2002) and plotted against the electrode positions along the array. Curves that did not show eCAP amplitudes above 0.1 mV were not included in the analysis. This criterion was not reached in 16% of the responses, mainly in the low current range. The average of the peak amplitude, for both selectivity and scanning, was 0.6 mV. The curves were normalized by taking the value at the electrode contact of interest and dividing all values along the array by this value. Next, both flanks of the selectivity and scanning curves were fitted by a 4th order polynomial. The width was defined as the number of electrode contacts (spaced 1.1 mm apart) from the stimulated contact to the point at which the normalized amplitude reduces to 0.6. For the middle con- tact both the width in the apical (EM-A) as well as the basal direc- tion (EM-B) were calculated. In cases where the minimum value did not drop to 0.6 the width was set as the limit of the array in the apical or basal direction (as per Abbas et al, 2004). In previous studies both 50% and 75% of the peak mplitu e hav been used as a measure of the width of the region of excitation (Cohen et al, 2003; Hu hes & Abbas, 2006 ). Fo this study, 60% of the peak amplitude, determined on the basis of the fitted, normalized curves, was selected as a trade-off between obtaining as many curves as possible and being able to measure differences between distinctive profiles along the array. Figure 2 shows typical selectivity curves recorded in one sub- ject. The figure shows the normalized eCAP amplitudes obtained Figure 1. A: For scanning measures a single probe electrode contact is stimulated (Stim) and the response is recorded at all electrodes individually. Recordings at electrodes further away from the stimulating electrode results in lower eCAP amplitudes (shown below electrode array). B: Selectivity measures are made by stimulating a masker electrode contact (M) immediately before the probe contact (P). The neural population stimulated by the masker will then be in a refractory state when the probe is activated, so that the response to the probe will be reduced in proportion to the amount of overlap. When the measurements of masker alone and probe alone are summed and the measurement of masker and probe combined is subtracted (M P MP) the eCAP measurement will show the overlap between the cochlear region excited by M and P. C & D: Raw waveforms of individual patients are shown with detected peaks marked (N1, circles; P1, squares). C: waveforms of an individual patients’ scanning data using the forward masking and the alternating polarity artefact rejection method. Each waveform labeled with corresponding recording electrode (R1, R2, R3…). D: waveforms of selectivity data from two individual patients (as always in selectivity measurements a forward masking technique was used). Each waveform labeled with the corresponding position of the probe (P1, P2, P3…). For personal use only. Figure 1. A: For scanning measures a single probe electrode contact is stimulated (Stim) and the response is recorded at all electrodes individually. Recordings at electrodes further away from the stimulating electrode results in lower eCAP amplitudes (shown below lectrode ar ay). B: Selectivity measures are made by sti ulating a masker electrode co tact (M) immediately b fore the probe contact (P). The neural population stimulated by the masker will then be in a refractory state when the probe is activated, so t at the response to the probe will be reduced in proportion to the amount of overlap. When the measurements of masker alone and probe alone are summed and the measurement of masker and probe combined is subtracted (M + P - MP) the eCAP measurement will show the overlap between the cochl ar region excited by M and P. C & D: Raw waveforms of individual patients are shown with detected peaks marked (N1, circles; P1, squares). C: waveforms of an individual patients’ scanning data using the forward masking and the alternating polarity artefact rejection method. Each waveform labeled with corresponding recording electrode (R1, R2, R3…). D: waveforms of selectivity data from two individual patients (as always in selectivity measurements a forward masking technique was used). Each waveform labeled with the corresponding position of the probe (P1, P2, P3…). Int J Audiol Downloaded from informahealthcare.com by Leids University on 11/27/13

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