Search

The rate pitch discrimination ability of cochlear implant (CI) users is poor compared to normal-hearing (NH) listeners. At low pulse rates, the just noticeable difference (JND) is on average 20% of the base rate, while NH listeners can discriminate small frequency differences of 0.2% at 1 kHz. Recent investigations suggest that double pulses with short interpulse intervals (IPIs) may have a beneficial effect on rate pitch discrimination in CI users. In a first experiment psychophysical tests were carried out to establish whether rate pitch in CI users could be improved by applying double pulses with equal amplitude and short IPIs. Pulse trains with base rates of 200 and 400 pps, composed of either single pulses or double pulses with IPIs of 15, 50, and 150 μs were presented. In a second experiment pairwise comparisons were carried out between pitch of a pulse train composed of alternating double and single pulses with pitch of pulse trains composed of single pulses. The alternating pulse train had a base rate of 400 pps, the pulse trains with solely single pulses had base rates of 200, 300, and 400 pps. The loudness and pitch perception of the different stimulus types were evaluated and compared. A significant loudness difference was found between single and double pulses for both pulse rates. The JND for pitch discrimination between double-pulse IPIs had a high inter-subject variability, and no significant group effect was found. No subject reported a pitch change between double pulse and single pulse stimulation. In contrast, most of the subjects recognized a change in pitch between single-pulse trains and pulse trains with alternating double and single pulses. The latter was lower in pitch than the single-pulse train stimulation. To conclude, using (equal amplitude) double pulses instead of single pulses in a pulse train does not effect pitch perception. Instead, loudness differs between double pulses and single pulses with the same amplitude. [ABSTRACT FROM AUTHOR]

Abstract: The impact of jitter on rate pitch discrimination (JRPD) is still a matter of debate. Previous studies have used adaptive procedures to assess pitch discrimination abilities of jittered rate pulses (Dobie and Dillier, 1985; Chen et al., 2005) or have used jitter detection thresholds (Fearn, 2001). Previous studies were conducted in a relatively small number of subjects using either a single-electrode cochlear implant (Dobie and Dillier, 1985, n = 2) or the Nucleus multi-channel devices (Fearn, 2001, n = 3; Chen et al., 2005, n = 5). The successful application of an adaptive procedure requires a monotone psychometric function to achieve asymptotic results. The underlying psychometric function of rate jitter has not been investigated so far. In order to close this knowledge gap, the present study determines psychometric functions by measuring of JRPD with a fixed stimulus paradigm. A rather large range of temporal, Gaussian distributed jitter standard deviation 0, 1, 2, 3, 4 ms was applied to electrical pulse patterns. Since the shape of the underlying probability density function (PDF) may also effect JRPD, a uniform PDF was alternatively applied. 7 CI users (8 ears, high-level performers with open-speech perception, MED-EL Pulsar/Sonata devices, Innsbruck, Austria) served as subjects for the experiment. JRPD was assessed with a two-stage forced choice procedure. Gross results showed decreasing JRPD with increasing amounts of jitter independent of the applied jitter distribution. In conclusion, pulse rate jitter affects JRPD and therefore should be considered in current coding strategies. [Copyright &y& Elsevier]

Objectives Most cochlear implants implement stimulation strategies which apply sequential electrical pulses to encode acoustic signals such as speech, noise, and sounds via electrical stimulation of the auditory nerve. Parallel stimulation of adjacent electrodes has been employed in recent cochlear implant (CI) systems in an endeavor to improve coding of pitch information (e.g. FS4-p fine structure with parallel signal processing MED-EL, Innsbruck, Austria; VCIS, AB Corp., Sylmar, CA, USA). We investigated whether parallel stimulation of three adjacent electrodes enhances rate pitch perception compared with single electrode stimulation. Methods Most comfortable loudness (MCLs) levels were assessed in single and multi electrode condition in 12 subjects (15 ears, PULSARci100/SONATAti100 implant, MED-EL). Rate pitch discrimination was determined by means of an adaptive procedure (two-interval two-alternative forced choice, 2I2AFC) at individual MCL in the single- and multi-electrode condition at base frequencies of 100, 200, 283, 400, and 566 pulses per second (pps) (single electrode condition: electrode 5, multi electrode condition: electrode 4, 5, 6; PULSARci100/SONATAti100 implant: 12 electrode contacts; 1, most apical; 12, most basal). Results To achieve MCL in the multi-electrode condition significantly higher stimulation current compared with single stimulation was required. No significant difference between single- and multi-electrode condition just noticeable differences in rate discrimination (JNDR) group was found. In contrast, a pairwise comparison of individual results in a subgroup recruited out of successfully completed runs at high base rates showed statistically an improved rate discrimination in 17 of 24 runs in the multi-electrode condition. Therefore, a potential effect of parallel stimulation on rate discrimination is conceivable. Discussion The results in a subgroup of this study indicate that, compared with single-electrode stimulation, synchronous multi-electrode stimulation of three adjacent electrodes shows improvement rate discrimination in 17 of 24 test runs (binomial and χ2 test, P = 0.05) but did not result in statistically better JNDRs (best averaged improvement 19.8% at base rate 400 pps). [ABSTRACT FROM AUTHOR]