Impaired motor inhibition in adults who stutter - evidence from speech-free stop-signal reaction time tasks.

Idiopathic stuttering is a fluency disorder characterized by impairments during speech production. Deficits in the motor control circuits of the basal ganglia have been implicated in idiopathic stuttering but it is unclear how these impairments relate to the disorder. Previous work has indicated a possible deficiency in motor inhibition in children who stutter. To extend these findings to adults, we designed two experiments to probe executive motor control in people who stutter using manual reaction time tasks that do not rely on speech production. We used two versions of the stop-signal reaction time task, a measure for inhibitory motor control that has been shown to rely on the basal ganglia circuits. We show increased stop-signal reaction times in two independent samples of adults who stutter compared to age- and sex-matched control groups. Additional measures involved simple reaction time measurements and a task-switching task where no group difference was detected. Results indicate a deficiency in inhibitory motor control in people who stutter in a task that does not rely on overt speech production and cannot be explained by general deficits in executive control or speeded motor execution. This finding establishes the stop-signal reaction time as a possible target for future experimental and neuroimaging studies on fluency disorders and is a further step towards unraveling the contribution of motor control deficits to idiopathic stuttering.

Prenatal testosterone and stuttering.

BACKGROUND: The prevalence of stuttering is much higher in males compared to females. The biological underpinnings of this skewed sex-ratio is poorly understood, but it has often been speculated that sex hormones could play an important role. AIMS: The present study investigated a potential link between prenatal testosterone and stuttering. Here, an indirect indicator of prenatal testosterone levels, the Digit Ratio (2D:4D) of the hand, was used. As numerous studies have shown, hands with more "male" characteristics (putatively representing greater prenatal testosterone levels) are characterized by a longer ring finger compared to the index finger (represented as a lower 2D:4D ratio) in the general population. STUDY DESIGN, SUBJECTS, OUTCOME MEASURES: We searched for differences in the 2D:4D ratios between 38 persons who stutter and 36 persons who do not stutter. In a second step, we investigated potential links between the 2D:4D ratio and the multifaceted symptomatology of stuttering, as measured by the Overall Assessment of the Speaker's Experience of Stuttering (OASES), in a larger sample of 44 adults who stutter. RESULTS: In the first step, no significant differences in the 2D:4D were observed between individuals who stutter and individuals who do not stutter. In the second step, 2D:4D correlated negatively with higher scores of the OASES (representing higher negative experiences due to stuttering), and this effect was more pronounced for female persons who stutter. CONCLUSIONS: The findings indicate for the first time that prenatal testosterone may influence individual differences in psychosocial impact of this speech disorder.