Asymmetric intra- and interhemispheric interactions during covert and overt sentence reading.

Covert and overt sentence reading evoke lateralized activations in overall bihemispheric networks. We assumed that the study of functional connectivity may reveal underlying principles of functional lateralization. Left-lateralized activations could relate to stronger reading-related modulation of intrahemispheric functional connectivity in the left than the right hemisphere. Alternatively, left-lateralization could result from suppression of contralateral processing and thus reflect asymmetric interhemispheric interactions. To address this issue, this functional MRI study investigated the regional lateralization of covert and overt German sentence reading in 39 healthy participants. Further, it revealed the modulation of the lateralized brain regions' functional connectivity and their contralateral homotopes by covert and overt reading (psychophysiological interactions). Left-lateralization during covert reading was associated with stronger intrahemispheric coupling particularly in the left dorsal stream rather than with suppression of contralateral processing. Lateralization during overt sentence reading instead went along with additional recruitment of right perisylvian cortices involved in articulation by asymmetric positive heterotopic interhemispheric interactions. Given the paucity of interhemispheric anti-correlations with homotopic regions, functional lateralization is likely a consequence of a task-dependent interplay between asymmetric positive intra- and interhemispheric coupling.