Impact of transcranial direct current stimulation on somatosensory transfer learning: When the secondary somatosensory cortex comes into play

Transfer learning is an immanent feature of perceptual learning. Yet, despite the increasingly widespread application of transcranial direct current stimulation (tDCS) to study learning, transfer effects in response to tDCS have not been studied. Therefore, the present study investigated the transfer of tactile acuity evoked by repeatedly applied anodal tDCS over the left primary somatosensory cortex (S1) over the course of five days from the dominant (right) to the non-dominant (left) index finger (IF). There was a complete transfer of improvement of the right IF to its contralateral homologue by follow-up four weeks later. Changes in tactile acuity of the left IF in the tDCSanodal condition were accompanied by a significant longitudinal change in functional connectivity between the left S1 and the right secondary somatosensory cortex (S2) assessed at day five of tDCS delivery and four week later. Moreover, we observed a close link between tactile acuity and (changes of) functional connectivity of the right S2 in the tDCSanodal condition identifying the S2 as neural correlate to mediate the transfer of tDCS effects in the somatosensory domain. These findings provide unprecedented evidence of transfer effects evoked by tDCS, implicate the S2 in somatosensory transfer learning and provide evidence in support of models of perceptual learning allocating learning to the reweighting of connections between different levels of processing.