The influence of handedness on hemispheric interaction during word production: insights from effective connectivity analysis.

Overt picture naming causes activation in both left and right hemispheres as demonstrated by functional magnetic resonance imaging (fMRI) studies. Unexpectedly, a previous experiment revealed that there was no difference in brain lateralization related to handedness. To understand the mechanisms behind this phenomenon and to investigate the influence that each hemisphere exerts over the other in more detail, we analyzed the effective connectivity in the naming network. More specifically, we examined the impact of handedness on hemispheric interactions. Four brain regions activated for correct naming in the previous fMRI experiment served as volumes of interest: Bilateral fusiform gyrus (FG) related to visual-semantic processing and bilateral Brodmann's area (BA) 44 related to phonological processing. We defined three dynamic causal models (DCMs) representing different symmetrical connectivity patterns, compared them using Bayesian model selection, and considered differential connection strengths. For the majority of 31 participants, the same model featuring early integration of visual-semantic processing and independent phonological processing was optimal. This also held true if right-handers, left-handers, and ambidexters were considered as independent groups. For left-handers, the connection of right FG and right BA 44 was significantly stronger compared with the other two groups, and it was significantly stronger than its left-hemisphere equivalent. In right-handers, the according left-hemisphere connection was significant, but connectivities were rather symmetrical. These findings reveal significant and unique right-hemisphere contributions to word production.