Masking Level Differences and Binaural Intelligibility Level Differences in Children with Down Syndrome
Binaural hearing results in a number of listening advantages relative to monaural hearing, including enhanced hearing sensitivity and better speech understanding in adverse listening conditions. These advantages are facilitated by the ability to detect and utilize interaural cues within the central auditory system. It is well understood that infants and children with Down syndrome have unique auditory structures resulting from congenital and acquired influences (e.g., Balkany, Mischke, Downs, & Jafek, 1979; Becker, Armstrong, & Chan, 1986; Bilgin, Kasemsuwan, Schachern, Paparella, & Le, 1996; Blaser, et al., 2006). Structural anomalies within the central auditory system and alterations in synaptic communication could adversely affect binaural hearing. However, binaural hearing capabilities have not been examined in these children. This study was designed to determine if the binaural abilities of children with Down syndrome are compromised relative to those of typically-developing children on a masking level difference task and a binaural intelligibility level difference task. Participants included typically-developing children aged 3 to 13 years (N=46) and children with Down syndrome aged 6 to 16 years (N=11). In addition, typically-developing adults (N=6) and adults with Down syndrome (N=3) were included to obtain an estimate of adult performance for the experimental tasks. Despite normal hearing sensitivity children with Down syndrome in this study had higher masked thresholds for our experimental stimuli and exhibited less release from masking than typically-developing children. These observations suggest that children with Down syndrome have similar speech recognition in noise as typically-developing children aged 3 to 5 years. The binaural advantages afforded to typically-developing children, such as enhanced hearing sensitivity and better speech understanding in adverse listening conditions, were not present for children with Down syndrome in this study. The reduced binaural benefit experienced by children with Down syndrome suggests that they will require more favorable signal-to-noise ratios than typically-developing children to achieve optimal performance in adverse listening conditions. This has important implications for the planning of educational and therapeutic interventions for individuals with Down syndrome.