Investigating the involvement of the left and right hemisphere in speech production and its correlation with handedness – a repetitive transcranial magnetic stimulation study

Purpose:
While traditional theories suggest a link between language lateralization and handedness, recent evidence indicates more complex neural networks underlying speech processing. This study aimed to investigate the involvement of the left and right hemispheres in speech production using repetitive transcranial magnetic stimulation (rTMS).

Methods:
A cohort of 58 healthy volunteers with a median age of 23 (range 19-34) were included in the study. With the use of the Edin­burgh Handedness Inventory (EHI), 43 individuals were determined to be right-handed, 13 left-handed, 2 mix-handed and subsequently subdivided according to handedness decile values. rTMS was applied with 2Hz frequency and 2 seconds of duration over the left and right hemispheres to trigger speech arrest while counting downwards.

Results:
Overall prevalence of speech arrest was 96.6%. Twenty-four subjects developed speech arrest after stimulation exclusively over the left hemisphere (42.9%), two people exclusively over the right hemisphere (3.5%) and 30 participants bilaterally (53.6%). Notably, the right-handed exhibited speech arrest predominantly over the left hemisphere, while the left-handed showed more variability. Receiver operating characteristics analysis revealed handedness deciles as predictors of speech lateralization. Indivi­duals with handedness values over the 2^nd right decile of EHI were more likely to have speech arrest over the left hemisphere, while left-handed over the 7^th left decile were more likely to show speech arrest after rTMS over the right hemisphere.

Conclusions:
This study highlights the complexity of the relationship between speech lateralization and handedness. Further research using rTMS may provide insights into the neural mechanisms underlying speech processing and enable further studies on the treatment of aphasia.