Dichotic listening among adults who stutter
Degree GrantorUniversity of Canterbury
Degree NameMaster of Audiology
Dichotic listening of auditory stimuli is used to assess brain lateralisation by simultaneously presenting different stimuli to the left and right ears to determine which syllable was perceived as being the clearest. There is a limited, albeit dated number of studies that have examined dichotic listening performance in adults who stutter (AWS) and the results remain inconclusive. The aim of this research was to investigate whether AWS show a difference in the magnitude of the right ear advantage (REA) in both undirected and directed attentional tasks when compared with adults who do not stutter (AWNS). There were 14 right-handed participants, consisting of seven AWS and seven age and sex matched AWNS controls. All participants were screened for normal hearing. They completed a dichotic listening task, which included undirected and directed attentional listening tasks. Participants were to select the consonant-vowel (CV) pair they heard the clearest. The interaural intensity difference (IID) was modulated randomly during the undirected attention task. The results for the undirected task revealed: (1) a significant REA for AWS for the IID conditions of 0 to +21 dB and significant left ear advantages (LEA) for IIDs of -15 to -21 dB; (2) a significant REA for AWNS for the IID conditions of -9 to +21 dB and significant LEAs for IIDs of -18 to -21 dB; (3) laterality index scores with a significant IID effect but no significant group or group-by-ear interaction effects using parametric statistics. Further analysis of laterality using non-parametric statistics found significant differences between the fluency groups. In general, the findings in this study were revealing of differences between AWS and AWNS when performing dichotic listening tasks with speech stimuli. The primary difference observed between groups was in regards to the IID point at which a previous REA became a LEA. This “crossing-over” point occurred later for AWNS, indicating a strong left hemisphere advantage for the processing of speech. The earlier “crossing-over” for AWS would indicate that the right hemisphere was activated sooner for the processing of speech compared to AWNS. This activation of the right hemisphere is assumed to reflect more diffuse cerebral lateralisation for speech processing for the AWS and confirms past brain imaging studies. In the directed attention task, there was no significant difference between AWS and AWNS indicating that instances of stuttering may occur due to more automatic (bottom-up) speech processing. These findings have implications for theories of laterality and hemispheric asymmetry for phonological processing for AWS, which has been suggested to reflect a subgroup of AWS for whom cerebral dominance is related to their disfluency.