Minimum Audible Angle, Just Noticeable Interaural Differences and Speech Intelligibility with Bilateral Cochlear Implants Using Clinical Speech Processors

Abstract

Sound localization and speech intelligibility were assessed in 5 patients implanted bilaterally with Medel C40+ or Medel C40 cochlear implant (CI) systems. The minimum audible angle (MAA) around the head in the horizontal plane was assessed in patients with bilateral CI using white noise bursts of 1000 ms duration presented from a loudspeaker mounted on a rotating boom and compared with the MAA of age-matched normal hearing controls. Spatial discrimination was found to be good in front and in the back of the head with near-normal MAA values (patients: 3–8°, controls: 1–4°). In contrast, poor performance on the sides was found (patients: 30 to over 45°, controls 7–10°). Bilateral CI significantly improved spatial discrimination in front for all patients, when compared with the use of either CI alone. Just noticeable differences (JNDs) in interaural intensity and time were assessed using white noise bursts (1000 ms duration; 50 ms linear ramp). In addition, interaural time JNDs were assessed using click trains (800 ms duration, 40 µs clicks, 50 Hz) and noise bursts in which either only the envelope or only the fine structure was shifted in time. In comparison with normal hearing controls, patients with bilateral CI showed near-normal interaural intensity JNDs but substantially poorer interaural time JNDs depending on the type of stimulus. In contrast to envelope onset/offset cues, interaural fine structure time differences were not perceived by the patients using CI systems employing the continuous interleaved sampling strategy without synchronization between their pulse stimulation times. Speech intelligibility in quiet and CCITT noise from the side (±90°) was assessed using the German HSM sentence test and was significantly better when using bilateral CI in comparison with either unilateral CI, mainly due to a head shadow effect. These favorable results are in agreement with the patients’ subjective experiences assessed with a questionnaire and support the use of bilateral CI.

Modulation Enhancement in the Electrical Signal Improves Perception of Interaural Time Differences with Bimodal Stimulation

“…This is comparable to the best thresholds found in our previous studies with single and 3-channel stimuli (Francart et al 2009. The thresholds are also comparable to thresholds found for the best bilateral CI listeners with single-channel stimuli (Laback et al 2007(Laback et al , 2004Lawson et al 1998;Litovsky et al 2010Litovsky et al , 2012Long et al 2003;Majdak et al 2006;Senn et al 2005;van Hoesel 2004van Hoesel , 2007van Hoesel and Tyler 2003). That sensitivity with multiple-channel stimuli is comparable to performance with single-channel stimuli is unlike performance often reported for NH listeners, for whom sensitivity increases with increasing bandwidth.…”

“…We are not aware of any studies that investigated bilateral CI ITD thresholds with multi-channel stimuli under direct computer control. With clinical processors, however, performance is generally poorer than in the single-channel studies (Grantham et al 2008;Laback et al 2004;Senn et al 2005). For NH listeners, Akeroyd (2003) measured JNDs in ITD for full-bandwidth vowel sounds (thus containing temporal fine structure ITD cues), and obtained values in the order of 40 μs.…”

Cochlear implants: A remarkable past and a brilliant future

“…In particular, bilateral stimulation can provide a substantial benefit in recognizing difficult speech materials such as monosyllabic words and in recognizing speech presented in competition with spatially distinct noise, in comparison to scores obtained with either unilateral implant alone (e.g., Gantz et al, 2002;Müller et al, 2002;Laszig et al, 2004;Ramsden et al, 2005;Litovsky et al, 2006;Ricketts et al, 2006). In addition, use of both implants supports an improved ability to lateralize or localize sounds (depending on which was measured in a particular study), again compared with either unilateral implant (e.g., van Hoesel and Tyler, 2003;Nopp et al, 2004;Senn et al, 2005;Grantham et al, 2007;Tyler et al, 2007). (This ability is nonexistent or almost nil with a unilateral implant, as noted before.)…”

“…Patients using unilateral CIs have little or no sound localization ability (e.g., Nopp et al, 2004;Senn et al, 2005). This reduces the effectiveness of the alerting function that could be supported by a prosthetic system for hearing and eliminates the S/N advantage of binaural hearing, especially for different locations of the speech and the noise.…”

Discrimination of Schroeder-Phase Harmonic Complexes by Normal-Hearing and Cochlear-Implant Listeners

“…Additionally, music perception and localization abilities are much worse in CI users than in normal-hearing listeners (Gfeller et al 1997(Gfeller et al , 2002(Gfeller et al , 2005Senn et al 2005;Nimmons et al 2007), highlighting the need for improvement in CI technology.…”