Dynamic current focusing for loudness encoding in cochlear implants: a take-home trial

Figures & data

Figure 1. The concept of loudness coding with dynamic current focussing (DCF) and monopolar (MP) stimulation. The upper bar for each loudness step shows the auditory nerve with the excitation pattern in grey. The lower bars show the implanted electrode array with the electrode contacts in grey. In the DCF strategy, the amplitudes of the main and neighbouring electrode contacts are increased equally up to the threshold level (T-level). To increase the loudness from the T-level, σ is decreased as a function of the stimulus level, resulting in a broader excitation pattern and higher loudness level. In MP mode, the amplitude of the main electrode contact is increased as a function of the stimulus level, resulting in broad current spreads at all loudness levels. M-level, most comfortable level.

Table 1. Characteristics of the study subjects.

Subject	Gender	Age (yrs)	CI side	Duration of implant use (yrs)	CVC (Ph%)	Electrode array	Clinical strategy	Clinical settings		DCF settings
								Rate (pps)	Pulse width (µs)	Rate (pps)	Pulse width (µs)	Mean M-level σ
S01	Male	64	R	3	89	HiRes90K HiFocus MS	Optima	1217	54.8	599	38.6	−0.22
S02	Male	68	L	12	88	HiRes90K HiFocus 1J	Optima	2560	26.0	1020	35.0	0.32
S03	Male	61	R	2	89	HiRes90K HiFocus MS	Optima	3093	21.6	1020	35.0	0.10
S04	Female	64	R	1	85	HiRes90K HiFocus MS	Optima	2007	33.2	1020	35.0	−0.08
S05	Female	59	R	5	87	HiRes90K HiFocus 1J	HiResF120	3093	21.6	1020	35.0	0.28
S06	Female	70	L	8	90	HiRes90K HiFocus 1J	Optima	2395	27.8	765	44.9	0.27
S07	Female	58	L	1	80	HiRes90K HiFocus MS	Optima	2970	22.4	829	43.1	0.43
S08	Male	70	L	7	90	HiRes90K HiFocus MS	Optima	3039	21.6	1020	35.0	0.36
S09	Female	47	R	4	84	HiRes90K HiFocus 1J	HiResF120	3039	21.6	1020	35.0	0.32
S10	Female	42	R	3	80	HiRes90K HiFocus MS	Optima	2750	24.2	765	44.9
S11	Female	66	L	2	85	HiRes90K HiFocus MS	Optima	2970	22.4	1020	35.0	0.15
S12	Female	61	R	3	90	HiRes90K HiFocus MS	Optima	3039	21.6	1020	35.0	0.32
S13	Male	68	L	1	92	HiRes90K HiFocus MS	Optima	3039	21.6	1020	35.0	0.07
S14	Male	67	L	1	74	HiRes90K HiFocus MS	Optima	2750	24.2	865	38.6	0.39
S15	Male	64	R	1	88	HiRes90K HiFocus MS	Optima	3039	21.6	1020	35.0	0.01
S16	Female	73	L	2	88	HiRes90K HiFocus MS	Optima	3039	21.6	1020	35.0	0.22
S17	Male	34	L	3	87	HiRes90K HiFocus MS	Optima	1768	37.7	925	38.6	0.16
S18	Female	54	R	10	78	HiRes90K HiFocus 1J	Optima	1865	35.9	1020	35.0	−0.01
S19	Female	50	R	14	86	Clarion CII HiFocus 1J	Optima	1428	46.7	750	47.6	0.35
S20	Female	68	R	10	88	HiRes90K HiFocus 1J	HiRes	725	43.1	846	42.2	−0.03

L: left ear; R: right ear; CVC: consonant-vowel-consonant; Ph%: percentage phonemes correct on a standard monosyllabic (CVC) word test at 65 dB; DCF: dynamic current focussing; M-level: most comfortable level; HiFocus MS: HiFocus Mid-Scala; Mean M-level σ: mean σ at most comfortable level over all 14 electrode contacts. Excluded subjects are shaded in grey.

Figure 2. Individual loudness growth functions on an 8-point loudness scale (y-axes). The stimulus levels were calculated as follows: $\text{Loudness Level }\left(\text{in dB}\right)=20·{\log }_{10}\left(\mathrm{V}/{\mathrm{V}}_{\mathrm{ref}}\right),$ where ${\mathrm{V}}_{\mathrm{ref}}$ was calibrated for each electrode contact using BEPS + software. For understanding speech, the loudness levels “2” (very soft sound) to “5” (the most comfortable loudness) were considered the most relevant. This loudness range is shaded in grey. The areas under the curves (AUCs) are indicated for the two strategies. DCF: dynamic current focussing.

Table 2. Differences in the areas under the loudness curves (ΔAUCs) between the two strategies for each subject at the indicated electrodes.

Subject	Electrode 3	Electrode 9	Electrode 14	Absolute average
S01	+29%	+28%	−5%	21%
S02	−26%	−50%	+12%	30%
S03	+78%	+59%	+306%	147%
S04	+20%	+89%	−13%	41%
S05	+68%	−13%	+39%	40%
S06	−18%	−11%	−33%	21%
S08	−33%	−20%	+61%	38%
S09	+170%	+282%	+123%	192%
S11	+39%	+150%	+126%	105%
S12	+6%	+7%	+95%	36%
S13	+68%	+161%	−14%	81%
S15	+11%	+40%	+15%	22%
S16	+1%	+24%	+208%	78%
S17	+63%	+45%	+46%	51%
S18	+97%	+80%	−33%	70%
S19	+79%	+8%	+11%	33%
S20	−14%	−45%	−28%	29%

The (absolute) average AUC of the three electrode contacts is also shown and expressed as the percentage of the AUC for the MP strategy: $\Delta \mathit{AUC}\left[\mathrm{\%}\right]=\mathrm{ }{\mathit{AUC}}_{\mathit{DCF}}–\mathrm{ }{\mathit{AUC}}_{\mathit{clinical}}/{\mathit{AUC}}_{\mathit{clinical}}\mathrm{ }·100.$

Figure 3. Individual and mean spectral ripple thresholds at 45 dB (a) and 65 dB (b) for the 17 study subjects using their monopolar (MP) clinical strategy (baseline and control measurements) and dynamic current focussing (DCF). Error bars represent ± 1 standard error of the mean.

Figure 4. Individual and mean modulation detection thresholds (MDTs) for the 17 study subjects at 65 dB SPL, using their monopolar (MP) clinical strategy (baseline and control measurements) and dynamic current focussing (DCF). Error bars represent ± 1 standard error of the mean.

Figure 5. Individual and mean speech intelligibility in noise (Dutch Matrix test) with fixed speech at 45 dB SPL (a) and 65 dB SPL (b) for the 17 study subjects using their monopolar (MP) clinical strategy (baseline and control measurement) and dynamic current focussing (DCF). The y-axis is the speech-to-noise ratio for which 50% of the words were repeated correctly. Error bars represent ± 1 standard error of the mean.

Figure 6. Subjective rating of processing strategies (SSQ) concerning quality of sound (a) and speech understanding under multiple listening conditions (b).