Neural mechanisms underlying stuttering: evidence from bimanual handwriting performance.

Left- and right-handed male and female stutterers were compared with fluent speakers on a bimanual handwriting task. On each trial four words were read to the subject. After repeating the words, subjects had to write the initial letters as quickly as possible using the two hands simultaneously and without visual guidance. As a group, stutterers were slower, made more mirror-reversed letters, and formed letters of poorer quality than fluent speakers. The effects were the same for males and females, and the data for left- and right-handers were mirror-symmetric with respect to left and right hands. Evidence was found for two subgroups of stutterers with respect to scores on the dependent variables. It was suggested that the overall pattern of results implicates the supplementary motor area in the mediation of stuttering, possibly through relatively ungated callosal pathways.