Neural and Behavioral Sensitivity to Interaural Time Differences Using Amplitude Modulated Tones with Mismatched Carrier Frequencies

Abstract

Bilateral cochlear implantation is intended to provide the advantages of binaural hearing, including sound localization and better speech recognition in noise. In most modern implants, temporal information is carried by the envelope of pulsatile stimulation, and thresholds to interaural time differences (ITDs) are generally high compared to those obtained in normal hearing observers. One factor thought to influence ITD sensitivity is the overlap of neural populations stimulated on each side. The present study investigated the effects of acoustically stimulating bilaterally mismatched neural populations in two related paradigms: rabbit neural recordings and human psychophysical testing. The neural coding of interaural envelope timing information was measured in recordings from neurons in the inferior colliculus of the unanesthetized rabbit. Binaural beat stimuli with a 1-Hz difference in modulation frequency were presented at the best modulation frequency and intensity as the carrier frequencies at each ear were varied. Some neurons encoded envelope ITDs with carrier frequency mismatches as great as several octaves. The synchronization strength was typically nonmonotonically related to intensity. Psychophysical data showed that human listeners could also make use of binaural envelope cues for carrier mismatches of up to 2-3 octaves. Thus, the physiological and psychophysical data were broadly consistent, and suggest that bilateral cochlear implants should provide information sufficient to detect envelope ITDs even in the face of bilateral mismatch in the neural populations responding to stimulation. However, the strongly nonmonotonic synchronization to envelope ITDs suggests that the limited dynamic range with electrical stimulation may be an important consideration for ITD encoding.

Bilateral Cochlear Implants in Children: Binaural Unmasking

“…It has been reported that in postlingually deafened adults place (mis)matches may have an effect on the size of the electrical BMLD [Long et al, 2007], which means that possibly larger BMLDs would be obtained in these children with other electrode pairs. On the other hand, as shown in the literature, binaural sensitivity does not disappear with a shift in place of stimulation [Blanks et al, 2007;Long et al, 2003;van Hoesel, 2004]. Research with normal-hearing subjects has shown that envelope delays for carrier frequencies separated by up to 2 or 3 octaves can be perceived, a result that was confirmed by neural recordings in rabbits [Blanks et al, 2007].…”

“…On the other hand, as shown in the literature, binaural sensitivity does not disappear with a shift in place of stimulation [Blanks et al, 2007;Long et al, 2003;van Hoesel, 2004]. Research with normal-hearing subjects has shown that envelope delays for carrier frequencies separated by up to 2 or 3 octaves can be perceived, a result that was confirmed by neural recordings in rabbits [Blanks et al, 2007]. A spacing of 10 mm on the basilar membrane corresponds to a frequency range of about 2.5 octaves [Greenwood, 1990] and to a range of 13 electrodes in the Nucleus CI.…”

Sensitivity to Interaural Time Differences with Combined Cochlear Implant and Acoustic Stimulation

“…Blanks et al (2007) however show that, for a simpler psychophysical task or an animal model, there is sensitivity to ITD when the interaural frequency difference increases up to several octaves.…”

Effect of mismatched place-of-stimulation on binaural fusion and lateralization in bilateral cochlear-implant users

“…In NH subjects, ITD thresholds as a function of mismatch have been measured using amplitude-modulated (AM) signals (Henning, 1974;Nuetzel and Hafter, 1981;Blanks et al, 2007;Blanks et al, 2008). The consistent trend in the results from these studies has been that with increasing mismatch, ITD JNDs increased.…”