Stairways to the brain: Transcutaneous spinal direct current stimulation (tsDCS) modulates a cerebellar-cortical network enhancing verb recovery

Abstract

It has long been assumed that the language function is hierarchically organized into specific cortical areas. Here, for the first time, we present direct evidence that the spinal cord takes part in language processing. In a randomized-double blind design, sixteen aphasics underwent a language treatment combined with transcutaneous spinal direct current stimulation (tsDCS). During the treatment, each subject received tsDCS (20 min, 2 mA) over the thoracic vertebrae (IX-X vertebrae) in two different conditions: (1) anodal, and (2) sham while performing a verb naming task. Each experimental condition was run in five consecutive daily sessions over two weeks. Before and after each condition, all patients underwent a resting state functional magnetic resonance imaging (rs-fMRI). After anodal tsDCS, significant functional connectivity changes were found in a cerebellar-cortical network recruiting regions such as the left cerebellum, the right parietal and premotor cortex known to be also involved in action-related verb processing. Indeed, this increase of connectivity significantly correlated with the greatest amount of improvement found in verb naming. In line with our experimental data, we also found a greater improvement after anodal tsDCS also on untreated items of the language test but only on tasks which required the use of verbs, such as verb naming and picture description. No significant changes were found in noun naming. Thus, this evidence emphasizes, for the first time, that the neural response due to tsDCS combined with language treatment changes during the course of recovery by enhancing activity into cortical regions which influence verb processing.