Verbal dichotic listening in developmental stuttering: subgroups with atypical auditory processing.

OBJECTIVE: The major aim of this study was to determine whether adults with persistent developmental stuttering have atypical auditory processing.

BACKGROUND: Stuttering has been attributed to aberrant hemispheric dominance, and auditory processing deficits have been found in some adults who stutter. Dichotic listening, an indirect measure of auditory processing, has not been used to study auditory laterality in right- and left-handers who stutter. Because left-handers and people who stutter may have aberrant hemispheric dominance, it is important to examine auditory neural systems in right- and left-handed people who do and do not stutter.

METHODS: Adults with persistent developmental stuttering (n = 18) and matched controls (n = 28) were studied by simultaneous binaural (dichotic) presentation of consonant-vowel stimuli in three attention conditions: nondirected attention, attention directed right, and attention directed left. Sex-handedness groups (stutter and control) included right-handed men and women and left-handed men, but not left-handed women because this stutter subgroup could not be recruited. To study ear advantage and auditory laterality, two dependent measures were examined: percent left and right ear responses and lateralization shift magnitude. Potential relationships between degree of handedness and dichotic listening measures were also examined.

RESULTS: Matched controls and right-handed men who stutter had the expected right-ear advantage (REA) in the nondirected attention condition. In contrast, left-handed men who stutter had a left-ear advantage (LEA), and right-handed women who stutter did not have a lateral ear bias in the nondirected attention condition. Right-handed women who stutter had the greatest tendency to hear a sound that was not presented to either ear, and were relatively unable to selectively direct attention left or right. In contrast, left-handed men who stutter were able to shift attention to the left and right ear better than any other group. For the fluent control group, there were no significant relationships among degree of handedness and dichotic-listening variables. For the stutter group, degree of handedness was significantly related to percentage left and right ear response and to the lateralization shift magnitude.

CONCLUSIONS: Left-handed men who stutter and right-handed women who stutter have atypical auditory processing but differ in important ways. The left-ear bias found in left-handed men who stutter in the nondirected attention condition suggests that their right temporal lobe may be important in perceiving speech, and, therefore, they have mixed dominance. These subjects were also better at shifting attention in both directions in comparison to all other groups; thus, the right hemisphere, which is dominant for shifting attention to both right and left space, may be activated. In contrast, the right-handed women who stutter had no ear bias in the nondirected attention condition, made more perceptual errors, and had difficulty shifting attention to the left and right. Although these results suggest that right-handed women who stutter have attentional deficits, the relationship between attentional disorders and stuttering remains to be elucidated. Because right-handed men who stutter were not different from controls, aberrant hemispheric dominance cannot fully account for stuttering. Unfortunately, left-handed women were not examined in this study. Therefore, these interesting sex-handedness effects found in left-handed men and right-handed women who stutter must be interpreted with caution.