Transcranial direct current stimulation over left inferior frontal cortex improves speech fluency in adults who stutter

Abstract

Stuttering is a neurodevelopmental disorder affecting 5% of children, and persisting in 1% of adults. Promoting lasting fluency improvement in adults who stutter is a particular challenge. Novel interventions to improve outcomes are required, therefore. Previous work in patients with acquired motor and language disorders reported enhanced benefits of behavioural therapies when paired with transcranial direct current stimulation (tDCS). Here, we report the results of the first trial investigating whether tDCS can improve speech fluency in adults who stutter. Thirty adult men who stutter completed a randomized, double-blind, controlled trial of anodal tDCS over left inferior frontal cortex. Fifteen men received 20 minutes of 1-mA tDCS on five consecutive days while speech fluency was temporarily induced using choral and metronome-timed speech. The other 15 men received the same speech fluency intervention with sham stimulation. We predicted that applying anodal tDCS to the left inferior frontal cortex during speech production with temporary fluency inducers would result in longer-lasting fluency improvements. Speech fluency during reading and conversation was assessed at baseline, before and after the stimulation on each day of the five-day intervention, and at 1 and 6 weeks after the end of the intervention. TDCS combined with speech fluency training significantly reduced the percentage of disfluent speech measured 1 week after the intervention compared with fluency intervention alone. At 6 weeks after the intervention, this improvement was maintained during reading but not during conversation. Outcome scores at both post-intervention time points on a clinical assessment tool (the Stuttering Severity Instrument – version 4) also showed significant improvement in the group receiving tDCS compared with the sham group, in whom fluency was unchanged from baseline. We conclude that tDCS combined with behavioural fluency intervention has the capacity to improve fluency in adults who stutter. tDCS thereby offers a potentially useful adjunct to future speech therapy interventions for this population, for whom therapy outcomes are currently limited.