Abstract
Studies to date that have used fTCD to examine language lateralisation have predominantly used word or sentence generation tasks. Here we sought to further assess the sensitivity of fTCD to language lateralisation by using a metalinguistic task which does not involve novel speech generation: rhyme judgement in response to written words. Line array judgement was included as a non-linguistic visuospatial task to examine the relative strength of left and right hemisphere lateralisation within the same individuals when output requirements of the tasks are matched. These externally paced tasks allowed us to manipulate the number of stimuli presented to participants and thus assess the influence of pace on the strength of lateralisation.In Experiment 1, 28 right-handed adults participated in rhyme and line array judgement tasks and showed reliable left and right lateralisation at the group level for each task, respectively. In Experiment 2 we increased the pace of the tasks, presenting more stimuli per trial. We measured laterality indices (LIs) from 18 participants who performed both linguistic and non-linguistic judgement tasks during the original ‘slow’ presentation rate (5 judgements per trial) and a fast presentation rate (10 judgements per trial). The increase in pace led to increased strength of lateralisation in both the rhyme and line conditions.Our results demonstrate for the first time that fTCD is sensitive to the left lateralised processes involved in metalinguistic judgements. Our data also suggest that changes in the strength of language lateralisation, as measured by fTCD, are not driven by articulatory demands alone. The current results suggest that at least one aspect of task difficulty, the pace of stimulus presentation, influences the strength of lateralisation during both linguistic and non-linguistic tasks.
Highlights
Functional transcranial Doppler sonography uses ultra-sound to measure changes in the speed of blood flow through the left and right middle cerebral arteries (MCAs) during the performance of sensory and cognitive tasks (Aaslid et al, 1982)
Right hemisphere lateralisation was established for a non-linguistic task, which was matched to the linguistic condition in task requirements
We note that the group mean laterality indices (LIs) of .84 during the rhyme judgement is lower than those LIs reported in previous studies of word generation (e.g. 2.7 (Stroobant et al, 2009); 1.69 (Bishop et al, 2009); 2.11 (Somers et al, 2011); 3.19 (Krach et al, 2006); 3.94 (Dorst et al, 2008); 2.41 (Badcock et al, 2011)
Summary
Functional transcranial Doppler sonography (fTCD) uses ultra-sound to measure changes in the speed of blood flow through the left and right middle cerebral arteries (MCAs) during the performance of sensory and cognitive tasks (Aaslid et al, 1982). The primary experimental task used has been word generation (e.g. verbal fluency as in Deppe et al (2000) and Knecht et al (1998)) or with children, sentence generation in the form of picture or video description (Lohmann et al, 2005; Bishop et all., 2009; Haag et al, 2010; Groen et al, 2012; Chilosi et al, 2014) These studies converge with findings from other neuroimaging modalities indicating a robust and pervasive leftward asymmetry in functional responses during expressive language production. In order to maximise the contribution of fTCD to the field, and to further our understanding of developmental changes in language lateralisation it would be beneficial to take a multidimensional approach to language (Bishop, 2013) by examining language lateralisation across a range of different language skills and during generation of novel material
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