The investigators recently completed a clinical trial evaluating whether upper limb function in those with SCI could be improved by pairing rehabilitation with conventional tDCS. Overall, the investigators found that conventional tDCS showed promise in improving upper limb function, with an observed effect size ranging from 0.23 to 0.58. Using data from this trial, the investigators performed a retrospective analysis to determine whether the level of functional benefit the subject experienced was related to the strength of tDCS current delivered to the cortical areas dedicated to weak muscles. The investigators approximated the delivery of current using models from software developed by collaborator Dr. Bikson. The current investigators' analysis found that improvements in dexterity following ten sessions of conventional, non-focal (35 cm2) tDCS were greater when the current was delivered mainly within the motor cortical areas dedicated to the weak muscles.

Expanding on the retrospective analysis, the investigators have recently conducted a pilot study to determine whether focality of tDCS can directly change functional benefits in individuals with SCI. Specifically, the investigators conducted a single-session cross-over study evaluating the effect of conventional, sham, and a focused tDCS on grip strength and corticospinal pathway excitability (n=2). For the current study, the investigators utilized high-definition tDCS (HD-tDCS) to deliver a more focal current with tDCS. HD-tDCS delivers current through five small electrodes (0.8 cm2) placed in an "X-shaped" configuration via a simple cap. This configuration leads to an 80% greater concentration or focusing of the electric field within the targeted region compared to conventional, non-focal tDCS.

In the current cross-over study, outcomes were assessed before and immediately following a 30-minute application of tDCS paired with task training. Sessions were spaced one week apart to allow for sufficient wash-out. The Investigators found that HD-tDCS had a substantial impact on grip strength compared to conventional tDCS. Most notably, HD-tDCS improved grip strength (+15.9 N) while conventional tDCS reduced grip strength (-5.5 N). With the added grip strength following HD-tDCS, subjects reported the new ability to grip a bottle of water and hold small objects, indicating the greater potential for clinical impact compared to conventional tDCS. Overall, raw changes in grip strength translated to a 40% advantage of HD-tDCS compared to conventional tDCS, wherein a change greater than 15% was defined as clinically significant. The Investigators also observed that the single application of HD-tDCS could result in greater increases in corticospinal excitability of the weak triceps muscle compared to conventional tDCS. This is not surprising since focusing the electric field has been shown to elicit greater, longer-lasting increases in corticospinal excitability in healthy individuals; an effect that likely occurs since more current flow is less variable and arrives at cortical targets after HD-tDCS (98% improvement over conventional tDCS). Based on the investigators' previous findings, HD-tDCS is expected to deliver upwards of a 40% advantage and provide clinically significant gains compared to conventional tDCS for the proposed clinical trial.

The investigators' collective finding that focality of tDCS can directly change functional benefit and corticospinal excitability is corroborated by others. Of note, work in stroke aphasia has shown that targeting tDCS current to the cortical regions activated during speech improves the functional benefit of tDCS. Similar findings have been reported in fibromyalgia and working memory. Thus, in combination with others, the investigators' result provides the impetus to study whether the application of focal tDCS would lead to even greater improvements in upper limb function than that achieved with conventional, non-focal tDCS.

The current study will directly build off the investigators' initial clinical trial and pilot cross-over study. The investigators will perform a clinical feasibility trial and test for the first time whether the application of HD-tDCS in SCI elicits more robust increases in the excitability of residual corticospinal pathways and function of the paretic upper limbs than the application of conventional tDCS.

Twenty-four subjects with a chronic C2-C6 SCI will be enrolled in a randomized, assessor-blinded, clinical feasibility trial (See Section 1.3). Prior to randomization, subjects will complete a two-week baseline control phase. The data from the baseline control phase will allow each patient to serve as their own control since variability due to disease heterogeneity and tDCS can be accounted for in the data analysis. After completion of the baseline control phase, subjects will be randomized into two groups: HD-tDCS + massed practice training or conventional tDCS + massed practice training. Randomization will be performed using a generalized randomized block design. Subjects within a block will be matched on strength of the weak triceps (i.e., maximum volitional contraction measured by electromyography; EMG). Computer-based software for randomization will be used. Immediately after the intervention, subjects will undergo functional and neurophysiologic testing at post-test and follow-up (four weeks after intervention). Since conventional and HD-tDCS applications visually look different, only assessors collecting and analyzing the neurophysiologic and upper limb functional outcomes will be blinded to the tDCS paradigm delivered.