Binaural hearing in children using Gaussian enveloped and transposed tones

Abstract

Children who use bilateral cochlear implants (BiCIs) show significantly poorer sound localization skills than their normal hearing (NH) peers. This difference has been attributed, in part, to the fact that cochlear implants (CIs) do not faithfully transmit interaural time differences (ITDs) and interaural level differences (ILDs), which are known to be important cues for sound localization. Interestingly, little is known about binaural sensitivity in NH children, in particular, with stimuli that constrain acoustic cues in a manner representative of CI processing. In order to better understand and evaluate binaural hearing in children with BiCIs, the authors first undertook a study on binaural sensitivity in NH children ages 8-10, and in adults. Experiments evaluated sound discrimination and lateralization using ITD and ILD cues, for stimuli with robust envelope cues, but poor representation of temporal fine structure. Stimuli were spondaic words, Gaussian-enveloped tone pulse trains (100 pulse-per-second), and transposed tones. Results showed that discrimination thresholds in children were adult-like (15-389 ls for ITDs and 0.5-6.0 dB for ILDs). However, lateralization based on the same binaural cues showed higher variability than seen in adults. Results are discussed in the context of factors that may be responsible for poor representation of binaural cues in bilaterally implanted children.

Binaural sensitivity in children who use bilateral cochlear implants

“…This approach shares some characteristics with a previous study that compared ILD lateralization ranges between NH and BiCI adults (Stakhovskaya and Goupell, 2017). For the current study, the DR for NH subjects was estimated to be 65 dB sound pressure level (SPL) based on the loudness growth function in NH adults for a 4-kHz tone (Allen et al, 1990), which was the carrier frequency for the pulse train presented to NH children in Ehlers et al (2016). Research has shown that children and adults show similar loudness growth functions (Serpanos and Gravel, 2000).…”

“…One of the questions that arises regarding binaural sensitivity in children with BiCIs is whether they perform similarly to or significantly worse than their peers with NH. Data from the current study were compared to NH children tested on the same task (Ehlers et al, 2016). It is worth noting that there is currently no accepted way to directly compare electric CUs to acoustic decibels (dB).…”

“…As there are two DRs that may differ between the ears, the smaller of the two was chosen in order to be conservative. ILD JNDs were then calculated as a percentage of DR for both subjects with NH (Ehlers et al, 2016) and subjects with BiCIs. For example, if a NH subject had an ILD JND of 5 dB, this would utilize 8.3% of their DR.…”

“…Therefore, although ITD JNDs could not be measured in this population using the 70.7% criterion; some degree of ITD sensitivity may be present in these subjects. Ehlers et al (2016) recently measured binaural sensitivity in children with NH using a Gaussian envelope tone (GET) vocoder, which has a high-frequency carrier and ITDs conveyed in the temporal envelopes of the bandlimited pulse train. The GET stimuli has a 4-kHz carrier frequency thus rendering temporal fine structure ITD cue unusable, but a salient envelope ITD cue.…”

“…Studies with NH children show that sound localization ability is fairly well-developed by 4-5 years of age; in fact, some children show adult-like free-field localization performance, with root-mean-square (RMS) errors as low as 8 (Grieco-Calub and Litovsky and Godar, 2010;Zheng et al, 2015). In addition, ITD JNDs in the envelope of high-frequency sounds are similar between NH children age 8-10 years and adults (Ehlers et al, 2016). Young children who use BiCIs perform significantly worse on spatial hearing tasks conducted in the free-field than their NH peers (Grieco-Calub and Litovsky, 2010;Litovsky, 2015;Zheng et al, 2015), even after several years of experience with BiCIs (Zheng et al, 2015).…”