While mounting evidence suggests that noninvasive brain stimulation may be a useful adjunctive treatment for patients with aphasia after stroke, both TMS and tDCS have limitations that must be considered. One important caveat regarding noninvasive brain stimulation techniques is their limited spatial resolution and the difficulty of knowing precisely which region or regions of the brain are being affected. These concerns are especially applicable
to tDCS, which employs relatively large electrodes Alpelisib (typically 5 × 7 or 5 × 5 cm) for stimulation. Evidence from computer modeling studies also suggests that the distribution of current in the brain associated with tDCS can be quite diffuse, and that regions of maximal stimulation can be unpredictable, varying with factors like reference Z-VAD-FMK ic50 electrode size and position (Bikson, Datta, & Elwassif, 2009). While the spatial resolution of TMS is understood to be considerably higher than that of tDCS, evidence suggests that the degree of spatial resolution required to target specific cortical sites such as the pars triangularis is achieved more readily when rTMS is used in conjunction with image-guided navigation techniques (Julkunen et al., 2009), which are not employed by many investigators currently using TMS. Moreover, predictions
about neurophysiologic effects of brain stimulation are further complicated in stroke patients by the presence of lesions of varying size and distribution (Wagner et al., 2006). Another
important limitation of noninvasive brain stimulation techniques in aphasia is that current understanding of their neurophysiologic effects and their impact on behavior remains incomplete. For example, while low-frequency rTMS is often presumed to have inhibitory effects Casein kinase 1 and high frequency rTMS to have excitatory effects on cortical activity and related behaviors, considerable interindividual variability in these effects has been observed (Gangitano et al., 2002). Perplexingly, some studies that have employed TMS and tDCS in patients with aphasia have reported results contrary to what would have been predicted based on the findings of other investigators. For instance, recent tDCS studies have reported improvement on language performance measures in aphasic patients receiving stimulation of opposite polarities—either cathodal (Monti et al., 2008) or anodal (Baker et al., 2010)—to the left frontal lobe. Thus, while a growing body of evidence suggests that noninvasive brain stimulation techniques may be useful for facilitating aphasia recovery, specific inferences about the anatomic or functional mechanisms of TMS and tDCS in patients with aphasia must still be viewed with some caution until more data has been reported. Varying accounts of post-stroke language recovery are not mutually exclusive.