Estimating the reduced benefit of infant-directed speech in cochlear implant-related speech processing

Abstract

Caregivers modify their speech when talking to infants, a specific type of speech known as infant-directed speech (IDS). This speaking style facilitates language learning compared to adult-directed speech (ADS) in infants with normal hearing (NH). While infants with NH and those with cochlear implants (CIs) prefer listening to IDS over ADS, it is yet unknown how CI processing may affect the acoustic distinctiveness between ADS and IDS, as well as the degree of intelligibility of these. This study analyzed speech of seven female adult talkers to model the effects of simulated CI processing on (1) acoustic distinctiveness between ADS and IDS, (2) estimates of intelligibility of caregivers' speech in ADS and IDS, and (3) individual differences in caregivers' ADS-to-IDS modification and estimated speech intelligibility. Results suggest that CI processing is substantially detrimental to the acoustic distinctiveness between ADS and IDS, as well as to the intelligibility benefit derived from ADS-to-IDS modifications. Moreover, the observed variability across individual talkers in acoustic implementation of ADS-to-IDS modification and the estimated speech intelligibility was significantly reduced due to CI processing. The findings are discussed in the context of the link between IDS and language learning in infants with CIs.

Keywords: Acoustic distance; Cochlear implant; Individual differences; Infant-directed speech; Speech intelligibility.

FIG. 1.

Schematic diagram of the approach used in the present study for measuring (1) MD between pairs of MFCCs derived from pairs of IDS-ADS stimuli, and (2) intelligibility of IDS and ADS stimuli as estimated by SRMR value for listeners with NH and SRMR-CI for those with CIs. Note that the dashed line denotes the process for creating and analyzing the noise-vocoded versions of the same pairs of stimuli, while N stands for the number of spectral channels in the noise-excited envelope vocoder. Blocks and lines with blue (dark gray) color indicate paths for processing ADS, while those with orange (light gray) color indicate paths for processing IDS. Note that SRMR and SRMR-CI were calculated only for unprocessed stimuli in order to estimate intelligibility for NH and CI listeners, respectively. In this figure, the waveforms and their corresponding MFCCs are from the utterance “See the modi?” spoken by one of the seven talkers both in IDS and ADS speaking styles. S_IDSij and S_ADSij are the i^th pair of IDS and ADS stimuli (i = {1,2,3,…,15}) for talker j (j ={1,2,3,…,7}). MFCCs_IDSij and MFCCs_ADSij are MFCC features derived from S_IDSij and S_ADSij speech stimuli, respectively. The middle two panels show MFCCs obtained from the frames of these IDS and ADS stimuli. MD_ij is the MD calculated to measure the acoustic distance between the two matrices for MFCCs_IDSij and MFCCs_ADSij. SRMR_IDSij and SRMR_ADSij are the estimated intelligibility for S_IDSij and S_ADSij speech stimuli, respectively, as heard by listeners with NH. SRMR-CI_IDSij and SRMR-CI_ADSij are the estimated speech intelligibility for the same S_IDSij and S_ADSij speech stimuli, respectively, as heard by a listener with CIs.

FIG. 2.

The mean (bar) and ±1 standard error (vertical error bar in black) of Mahalanobis distance (MD) across groups of IDS and ADS stimuli at seven levels of spectro-temporal degradation, ranging from no-degradation (natural/unprocessed) to 4-channel noise-vocoded stimuli. Green (dark gray) circles show the mean MDs for each talker derived by averaging MDs over 15 pairs of IDS-ADS stimuli for that talker.

FIG. 3.

Estimated intelligibility of speech stimuli spoken in IDS (orange; light gray) and in ADS (blue; dark gray) styles (simulated) groups of listeners with different hearing statuses: NH (estimated by SRMR) and those with CIs (estimated by SRMR-CI). The bar graphs represent average values of SRMR or SRMR-CI over the seven talkers. The vertical error bars in black show ±1 standard error.

FIG. 4.

Variability across seven female talkers in (A) acoustic distinctiveness between their IDS and ADS for unprocessed stimuli (Natural) and simulated CI speech within a 22-channel noise vocoder (22CH), and (B) the change in talkers’ speech intelligibility (SI) due to a change in their speaking style (ADS to IDS), as heard for two (simulated) listener groups with either NH (estimated by SRMR) or CIs (estimated by SRMR-CI). The data points (gray circles) are laid over a 1.96 standard error of the mean (95% confidence interval) in red (rectangle area with light gray) and 1 standard deviation shown by blue lines (vertical dark gray lines). The solid and dotted red lines (horizontal solid and dotted dark gray lines) show the mean and median, respectively.