Functional connectivity in an fMRI study of semantic and phonological processes and the effect of L-Dopa.

We describe an fMRI experiment examining the functional connectivity (FC) between regions of the brain associated with semantic and phonological processing. We wished to explore whether L-Dopa administration affects the interaction between language network components in semantic and phonological categorization tasks, as revealed by FC. We hypothesized that L-Dopa would decrease FC due to restriction of the semantic network. During two test sessions (placebo and L-Dopa) each participant performed two fMRI runs, involving phonological and semantic processing. A number of brain regions commonly activated by the two tasks were chosen as regions if interest: left inferior frontal, left posterior temporal and left fusiform gyri, and left parietal cortex. FC was calculated and further analyzed for effects of either the drug or task. No main effect for drug was found. A significant main effect for task was found, with a greater average correlation for the phonological task than for the semantic task. These findings suggest that language areas are activated in a more synchronous manner for phonological than for semantic tasks. This may relate to the fact that phonological processes are mediated to a greater extent within language areas, whereas semantic tasks likely require greater interaction outside of the language areas. Alternatively, this may be due to differences in the attentional requirements of the two tasks.