Disrupting the speech motor network: Exploring hemispheric specialization for verbal and manual sequencing using a dual-task approach.

OBJECTIVE: The concept of overlapping neural networks supporting both speech production and fine motor praxis is well accepted; however, few studies have explored the lateralized behavioral characteristics of both functions when performed simultaneously. METHOD: This study probes the characteristics of the dominant hemisphere by overloading cognitive processing via a novel dual-task paradigm. In 2 experiments, participants performed sets of motor and speech tasks under single-task and dual-task conditions. The sets of tasks differed as to the extent to which they relied on sequential processing, and we hypothesized that tasks more reliant on this type of processing would suffer a greater performance decrement under dual-task conditions. A reliable measure of hemispheric language dominance was obtained via functional transcranial Doppler (fTCD) ultrasound. RESULTS: Speech production scores in the experimental set (i.e., sequential processing) were consistently impaired under dual-task conditions, a distinction that was not seen in the control set. Results of Experiment 2 confirm those of Experiment 1, whereby speech scores were most strongly impaired under dual-task conditions, especially in the experimental set. Motor performance suffered less than speech performance in dual-task conditions in both the experimental and control sets across both experiments. CONCLUSION: Data suggest that the common processing capacity for speech and fine motor praxis can be disrupted through a dual-task paradigm. This novel behavioral data supports theories of a motor-based gestural origin for language and indicates that speech production is more sensitive to the effects of increased processing requirements than are motor skills. (PsycINFO Database Record (c) 2019 APA, all rights reserved).