Sound-Direction Identification, Interaural Time Delay Discrimination, and Speech Intelligibility Advantages in Noise for a Bilateral Cochlear Implant User

OBJECTIVESTo characterize some of the benefits available from using two cochlear implants compared with just one, sound-direction identification (ID) abilities, sensitivity to interaural time delays (ITDs) and speech intelligibility in noise were measured for a bilateral multi-channel cochlear implant user. METHODSSound-direction ID in the horizontal plane was tested with a bilateral cochlear implant user. The subject was tested both unilaterally and bilaterally using two independent behind-the-ear ESPRIT (Cochlear Ltd.) processors, as well as bilaterally using custom research processors. Pink noise bursts were presented using an 11-loudspeaker array spanning the subject’s frontal 180° arc in an anechoic room. After each burst, the subject was asked to identify which loudspeaker had produced the sound. No explicit training, and no feedback were given. Presentation levels were nominally at 70 dB SPL, except for a repeat experiment using the clinical devices where the presentation levels were reduced to 60 dB SPL to avoid activation of the devices’ automatic gain control (AGC) circuits. Overall presentation levels were randomly varied by ±3 dB. For the research processor, a “low-update-rate” and a “high-update-rate” strategy were tested.Direct measurements of ITD just noticeable differences (JNDs) were made using a 3 AFC paradigm targeting 70% correct performance on the psychometric function. Stimuli included simple, low-rate electrical pulse trains as well as high-rate pulse trains modulated at 100 Hz.Speech data comparing monaural and binaural performance in noise were also collected with both low, and high update-rate strategies on the research processors. Open-set sentences were presented from directly in front of the subject and competing multi-talker babble noise was presented from the same loudspeaker, or from a loudspeaker placed 90° to the left or right of the subject. RESULTSFor the sound-direction ID task, monaural performance using the clinical devices showed large mean absolute errors of 81° and 73°, with standard deviations (averaged across all 11 loudspeakers) of 10° and 17°, for left and right ears, respectively. Fore bilateral device use at a presentation level of 70 dB SPL, the mean error improved to about 16° with an average standard deviation of 18°. When the presentation level was decreased to 60 dB SPL to avoid activation of the automatic gain control (AGC) circuits in the clinical processors, the mean response error improved further to