No Evidence of Altered Language Laterality in People Who Stutter across Different Brain Imaging Studies of Speech and Language

Abstract

A long-standing neurobiological explanation of stuttering is the incomplete cerebral dominance theory, which refers to competition between two hemispheres for "dominance" over handedness and speech, causing altered language lateralisation. Renewed interest in these ideas came from brain imaging findings in people who stutter (PWS) of increased activity in the right hemisphere during speech production or of shifts in activity from right to left when fluency increased. Here, we revisited this theory using functional MRI data from children and adults who stutter, and typically fluent speakers (119 participants in total) during four different speech and language tasks: overt sentence reading, overt picture description, covert sentence reading and covert auditory naming. Laterality indices (LIs) were calculated for the frontal and temporal lobes using the LI toolbox running in Statistical Parametric Mapping. We also repeated the analyses with more specific language regions, namely the pars opercularis (Brodmann Area 44) and pars triangularis (Brodmann Area 45). Laterality indices in PWS and typically fluent speakers (TFS) did not differ and Bayesian analyses provided moderate to anecdotal levels of support for the null hypothesis (i.e., no differences in laterality in PWS compared with TFS). The proportions of the PWS and TFS who were left lateralised or had atypical rightwards or bilateral lateralisation did not differ. We found no support for the theory that language laterality is reduced or differs in PWS compared with TFS.