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Revisiting the assumption of homotopic transcallosal suppression in language
Poster Session C, Friday, October 25, 4:30 - 6:00 pm, Great Hall 3 and 4
Tali Bitan1,2; 1University of Haifa, Israel, 2University of Toronto, Canada
The interactions between the two cerebral hemispheres and their implications for language performance have been the focus of interest in numerous studies. One popular model is that of transcallosal suppression between homotopic regions in the two hemispheres. This model has first emerged from the domain of spatial attention (Kinsbourne, 1982), and was adopted into the language domain to explain the increase in right hemisphere (RH) involvement after left hemisphere damage in aphasia, as a result of release from transcallosal suppression (Heiss et al. 2003). This assumption gained popularity following evidence that patients’ language performance improved after neuromodulatory suppression of their RH language homologs (Naeser et al. 2005). However, while these findings have alternative interpretations, there is no direct neuroimaging evidence for transcallosal suppression in the language network, like there is in the motor domain (Gerfkes et al. 2008). Here we show functional connectivity results from four fMRI studies in healthy children and adults, and post-stroke patients with aphasia, using a variety of tasks and languages, showing no evidence for transcallosal suppression, but rather excitatory connections, among homotopic language areas. In study 1 (Bitan et al. JNS, 2010) children (9-15) were scanned during performance of a rhyming task on pairs of spoken English words. Effective connectivity analysis showed reciprocal excitatory connectivity between left and right superior temporal gyri. The strength of this connectivity was modulated by word type, and was negatively associated with language abilities in females, but showed no inhibition. In study 2 (Mizrachi et al. Cortex, 2024) adults were scanned during performance of semantic relatedness judgment of ambiguous and unambiguous written Hebrew words. Effective connectivity analysis showed reciprocal excitatory homotopic connections between left and right inferior frontal gyri (IFG) pars opercularis and pars orbitalis for unambiguous words. Moreover, these connections were modulated by word type in ambiguous words, and differentially contributed to performance, but in no case was there inhibition between homotopic regions. In study 3 (Mizrachi et al. unpublished) adults were scanned during rest, after learning a set of novel words. Resting state functional connectivity in the language network showed excitatory connectivity between bilateral homotopic regions including IFG opercularis and triangularis, superior and middle temporal gyri. No inhibitory connectivity was found between homotopic regions. In study 4 (Chu et al. Cortex, 2018) healthy adults and post-stroke patients with aphasia were scanned during performance of a semantic sentence judgment task. Effective connectivity results showed no connectivity between right and left IFG in neither patients nor healthy controls, with excitatory right to left connectivity in primary auditory cortex correlated with language outcome. Altogether these results, drawn from a variety of tasks, as well as rest, in varied populations, do not support the popular assumption that left hemisphere language areas suppress their right homologs during rest or during task performance. There is also no evidence of suppression in the other direction in left-hemisphere stroke patients. In contrast interhemispheric homotopic connections in the language network are excitatory during rest and during some tasks, and this excitatory connectivity is contributing to language performance.
Topic Areas: Disorders: Acquired, Methods