The effect of transcranial direct current stimulation on reading adaptation in adults who stutter

The reading adaptation effect in stuttering refers to decreased stuttering frequency during repeated readings. This study investigated whether anodal transcranial direct current stimulation (tDCS) over the left primary motor cortex (M1) enhances reading adaptation in adults who stutter.

Two consecutive studies were conducted. Study 1 was a randomized, double-blind, sham-controlled trial with twenty adults who stutter (14 males, 6 females) assigned to either an anodal tDCS (2 mA, 20 min) or a sham stimulation group. Participants read a passage five times during stimulation. Study 2 was an exploratory secondary single-case analysis of data from six participants (5 males, 1 female) who demonstrated measurable stuttering during reading (>1 % syllables stuttered) in Study 1. Reading adaptation was quantified as percentage reduction in stuttering frequency from first to fifth reading trial.

Study 1 showed no statistically significant difference in reading adaptation between anodal and sham groups (Mann-Whitney U = 30, p = 0.137), though a moderate effect size (r = 0.34) suggested a potentially meaningful difference. Study 2 revealed variable individual responses, with three participants showing greater reading adaptation under anodal stimulation and three showing greater adaptation under sham stimulation.

Anodal tDCS over the left M1 did not consistently enhance reading adaptation in adults who stutter, suggesting that the relationship between M1 excitability and reading adaptation is more complex than hypothesized. Future research should investigate alternative neural targets, stimulation parameters, and individual differences in stuttering severity to better understand the effects of neuromodulation on speech motor adaptation in speech disorders.