Abstract

It is well established that speech production and fine motor praxis are linked neurologically, with evidence indicating that shared left hemisphere networks underpin both functions. One suggestion for the crucial component of this left lateralised specialisation is that both speech and praxis rely on effective sequencing of information for their successful execution (Flowers & Hudson, 2013). Whilst the concept of speech and motor sequencing and their neural correlates is well accepted, few studies explore the behavioural characteristics of language and praxis when performed simultaneously. This study was designed to probe the mechanism behind this interaction by overloading the left hemisphere sequencing network via a dual task paradigm. Participants (n=22) performed an experimental set of motor and speech tasks, (namely a verbal fluency paradigm and an electronic peg moving task), as well as a control set of similar tasks, (namely digit recall and a box crossing tracking test), both under single and dual task conditions. The two sets of tasks differed on the extent to which they relied on motor and speech sequencing, and it was hypothesised that tasks more reliant on this type of processing (i.e. the experimental condition) would suffer a greater performance decrement under dual task conditions, thus reflecting the greater load placed on left hemisphere speech and praxis centres. Each task was performed for 2 mins for both conditions, and scores were calculated based upon number of correct responses/movements made. Ordering of single and dual presentation was counterbalanced between participants. Participants subsequently underwent functional Transcranial Doppler ultrasound (fTCD) whilst performing a word generation task, to provide a direct measure of their hemispheric language dominance. Results showed that tasks forming the experimental arm were more impaired under dual task conditions than the control tasks, suggesting they both relied on a common processing system. Most interestingly speech production scores in the experimental arm were consistently impaired under dual task conditions, whereas the control arm speech task performance remained consistent with single task conditions. Motor performance suffered less in dual task conditions over both experimental and control arms, but there was a significant effect of hand dominance on performance in dual task conditions, whereby non-preferred hand usage didn’t suffer under dual task conditions, but preferred hand did. This data indicates the common processing capacity for speech and fine motor tasks of the right hand are controlled by left hemisphere networks which can be disrupted through a dual task paradigm. This data supports theories suggesting a motor based, gestural origin for language (e.g. Corballis, 2003), and indicates that speech production is more sensitive to the effects of increased processing requirements than are motor tasks.