MotM Submission

The left cerebral hemisphere is specialised and dominant for language comprehension and production and patients with left hemisphere damage often display profound language disruption. In contrast, following right hemisphere damage, disruption to language is less perceptible. Current research acknowledges a critical role for the right hemisphere in processing inferred or implied information by maintaining relevant facts and/or suppressing irrelevant ones but the exact role of the right hemisphere and its coordination with the left is still under investigation (Johns, Tooley and Traxler, 2008; Tompkins, Klepousniotou & Scott, 2013).

The present study aimed to investigate hemispheric differences in the processing of metaphorical language through the application of transcranial direct current stimulation (tDCS) over Broca's area in the left hemisphere and its homologue in the right hemisphere. Twenty (N=20) young healthy, right-handed, native English speakers participated in an auditory priming semantic judgement task. Three types of priming sentences were used: novel metaphors, conventional metaphors and literal sentences. Target words were either related to the literal meaning of the prime sentences, the metaphorical meaning, or were unrelated. During stimulation (anodal tDCS, cathodal tDCS and sham tDCS), participants were asked to judge whether target words were related in meaning to sentence primes. Anodal, cathodal or sham stimulation was applied to electrodes F7/F8 known to tap onto Broca’s area and its homologue in the right hemisphere respectively. Reaction times and accuracy rates were recorded.

The results revealed that significantly enhanced processing abilities for literal meanings and conventional metaphors only were observed after anodal (i.e., excitatory) stimulation of Broca’s area in the left hemisphere. In contrast, anodal stimulation of Broca’s homologue in the right hemisphere resulted in enhanced accuracy across all conditions. Furthermore, cathodal tDCS stimulation of Broca’s area in the left hemisphere or its homologue in the right hemisphere did not result in inhibited processing.

The present findings are in line with the Fine/Coarse Semantic Coding Hypothesis (Jung-Beeman, 2005) and corroborate previous research in underlining the distinct roles of the left and right hemispheres in processing literal and non-literal language respectively (Klepousniotou, Gracco & Pike, 2014). Furthermore, these findings have implications for the development and use of tDCS as a treatment option for aphasic post-stroke patients, as they demonstrate that the application of anodal tDCS over Broca’s area in the left hemisphere and its homologue in the right hemisphere can facilitate language processing.

• Other (NPRP, Seed, etc.)

• Arts and Sciences