Recently, we reported effects of repeatedly applied anodal transcranial direct current stimulation (tDCS) on tactile acuity learning of the dominant (right) index finger (IF) and concomitant neural changes as investigated by functional magnetic resonance imaging (fMRI) (Hilgenstock, Weiss, Huonker, & Witte, 2016). Relying on the same study, this article focuses on the structural underpinnings of these findings as investigated by voxel‐based morphometry (VBM) and diffusion tensor imaging (DTI).
tDCS is the application of a weak current exerting local effects on membrane potential (Bindman, 1965; Purpura & McMurtry, 1965) and neurotransmitter release (e.g., Clark, Coffman, Trumbo, & Gasparovic, 2011; Hone‐Blanchet, Edden, & Fecteau, 2016) as well as global effects on network functioning (e.g., Bachtiar, Near, Johansen‐Berg, & Stagg, 2015; Kim, Stephenson, Morris, & Jackson, 2014; Polania, Nitsche, & Paulus, 2011; Polania, Paulus, & Nitsche, 2012). Therefore, tDCS has widely been applied to study cognition, motor, and somatosensory functioning (e.g., Das, Holland, Frens, & Donchin, 2016; Stagg & Nitsche, 2011) but also to explore its beneficial potential in pathological states (e.g., Allman et al., 2016; Lindenberg, Renga, Zhu, Nair, & Schlaug, 2010; Lindenberg, Zhu, & Schlaug, 2012; Mori et al., 2013). With regard to the somatosensory system, anodal tDCS induces short‐term (e.g., Fujimoto, Yamaguchi, Otaka, Kondo, & Tanaka, 2014; Fujimoto et al., 2016; Ragert, Vandermeeren, Camus, & Cohen, 2008) as well as long‐term (Hilgenstock et al., 2016) effects on tactile acuity. After 5 days of anodal tDCS delivery, there was a profound and bilateral improvement of tactile acuity that persisted for at least 4 weeks. These improvements in tactile acuity were accompanied by changes in brain metabolism interpreted to indicate a more effective recruitment of neural machinery to process somatosensory information (Hilgenstock et al., 2016). Yet, despite the widespread application of tDCS and insights into changes in brain metabolism and connectivity, structural changes in the gray matter (GM) and white matter (WM) compartment of the brain in response to its repeated application have hardly been investigated.
Recently, Allman et al. (2016) were the first to show the capability of tDCS to induce structural changes in the GM compartment of the brain in response to repeatedly applied tDCS in stroke patients undergoing a course of 9 days of tDCS delivery with concomitant daily motor training. To the best of the authors’ knowledge, no study has investigated changes in GM in the somatosensory system in response to somatosensory training or the delivery of tDCS. Studies in the blind, however, indicate changes in GM focusing on the visual system. For example, Modi, Bhattacharya, Singh, Tripathi, and Khushu (2012) observed GM decreases in the lingual gyrus (primary visual cortex), the precuneus, the cerebellum, especially lobule VIIIa and intraparietal areas as well as an GM increase in the middle frontal gyrus (BA 6). Likewise, Voss, Pike, and Zatorre (2014) observed a significantly lower GM density in parts of the visual system and adjacent regions (pre‐/cuneus) in late blinded opposed to sighted individuals.
Changes in the WM compartment of the brain have primarily been investigated by studying fractional anisotropy (FA) (Zheng & Schlaug, 2015) that indicates organizational and directional changes in the diffusivity of water molecules in the WM compartment (Basser, 1995). Lindenberg, Nachtigall, Meinzer, Sieg, and Flöel (2013) could show that effects of tDCS on motor performance in stroke patients depended on the integrity of transcallosal and corticospinal fibers as characterized by FA (Lindenberg et al., 2012, 2013). Moreover, Zheng and Schlaug (2015) were the first to provide evidence of a behaviorally relevant increase in FA of the so‐called alternate motor fibers (cortico‐rubro‐spinal and cortico‐reticulo‐spinal fibers) in response to repeated anodal tDCS in stroke patients. While there is no study investigating FA changes in the somatosensory domain in response to tDCS, Debowska et al. (2016) revealed changes after training of Braille reading in sighted individuals affecting the primary and secondary somatosensory system, the visual system, and the middle and superior frontal gyrus. In the blind, there is a decrease in WM in the visual system (BA 17, 18) and an increase in the superior frontal gyrus (Modi et al., 2012).
Thus, this article was intended to provide insight into where changes in tactile (acuity) perception emerge both in the GM and WM compartment of the brain (sham stimulation) and how these changes are modified by the repeated delivery of anodal tDCS. Moreover, we were interested in how somatosensory learning (sham stimulation) and its modification by anodal tDCS are implemented by combining findings from the analysis of VBM and DTI data with our previously reported findings from the analysis of fMRI and behavioral data (Hilgenstock et al., 2016). To this end, the analysis of fMRI data will be extended. Given the current state of research, we hypothesized that somatosensory learning and the repeated delivery of anodal tDCS will affect the visual system and its adjacent brain regions as well as prefrontal areas, especially the middle and superior frontal gyrus. There are only a few reports of sex‐specific tDCS‐induced effects (e.g., Chaieb, Antal, & Paulus, 2008; Fumagalli et al., 2010; Kuo, Paulus, & Nitsche, 2006). Yet, to also investigate the possibility of sex‐specific effects of tDCS in the somatosensory domain, we conducted additional exploratory analyses.[…]