Effects of brain polarization on reaction times and pinch force in chronic stroke

Abstract

Background: Previous studies showed that anodal transcranial DC stimulation (tDCS) applied to the primary motor cortex of the affected hemisphere (M1affected hemisphere) after subcortical stroke transiently improves performance of complex tasks that mimic activities of daily living (ADL). It is not known if relatively simpler motor tasks are similarly affected. Here we tested the effects of tDCS on pinch force (PF) and simple reaction time (RT) tasks in patients with chronic stroke in a double-blind cross-over Sham-controlled experimental design.

Results: Anodal tDCS shortened reaction times and improved pinch force in the paretic hand relative to Sham stimulation, an effect present in patients with higher impairment.

Conclusion: tDCS of M1affected hemisphere can modulate performance of motor tasks simpler than those previously studied, a finding that could potentially benefit patients with relatively higher impairment levels.

Figures

Figure 1

(A) Effects of tDCS on RT (representative trials). RT, measured as the time between the GO-signal and the onset of EMG response, is shown at baseline (RTBase) and post intervention (RTPost) in representative trials in the tDCS (left) and Sham (right) sessions. Note the shorter RTPost than RTBase after tDCS but not Sham (highlighted in gray). X axis shows time (msec) and Y axis shows EMG activity (mV) (B) Effects of tDCS on RT (individual subjects) RT after tDCS and Sham relative to baseline (BASE) in all subjects (values > 100 indicate longer RT, whereas those < 100 indicate faster RT relative to baseline). Note that RT improved in all subjects with tDCS, while most subjects experienced longer RT with Sham, probably reflecting mild fatigue over the length of the experimental session (see Discussion). Group analysis showed that RT improvements were significantly different from Sham (paired t-test, p < 0.05), an effect present in all subjects (see individual subject connecting lines). (C) Effects of tDCS on RT (group data) tDCS shortened significantly RT (RTBase, paired t-test, p < 0.05). Y axis shows reaction times in msec.

Figure 2

Effects of tDCS and impairment of patients. Patients were stratified in two groups according to their ability to perform skilled ADL-like motor tasks. Less impaired patients (n = 7) were able to perform the Jebsen-Taylor-Task (JTT) and more impaired patients were not able to perform the JTT. After stratification tDCS-induced improvement of reaction time (A) and pinch force (B) was calculated for each group. Note, that the improvement was larger in the more impaired group.

Figure 3

(A) Effects of tDCS on PF (individual subjects). Pinch force after tDCS and Sham relative to baseline (BASE) in all subjects (values > 100 indicate stronger PF, whereas those < 100 indicate weaker PF relative to baseline). Note the significant improvement in PF after tDCS compared to Sham, present in all but one patient (paired t-test, p < 0.05, see also individual subject connecting lines). (B) Effects of tDCS on PF (group data) Group data showed a non-significant trend of increased pinch forces with tDCS (POST) compared to pinch forces during baseline (BASE; paired t-test, p = 0.18).

Figure 4

(A) Effects of Sham stimulation on RT compared to No stimulation. Reaction times were comparable during Sham stimulation and No stimulation with slight slowing of reaction times during POST compared to BASE. (B) Effects of Sham stimulation on PF compared to No stimulation Forces were comparable during Sham stimulation and No stimulation with slight decrease of pinch force during POST compared to BASE.

Figure 5

(A) Experimental design of a single session. In this study we used a double-blind, crossover study design with 2 sessions (tDCS and Sham). Half of the patients started with tDCS and the other half with Sham. Each session started with baseline determinations (BASE) of reaction times (RT1–3) and pinch force (PF1–3), followed by a 30 min break in which tDCS electrodes were placed. Then tDCS or Sham was applied in a counterbalanced double-blind design followed by post intervention measures (RT4–6 and PF4–6). All patients described their level of attention toward the task (range: 1–10; 1 = no attention, 10 = highest level of attention) and their perception of fatigue (range: 1–10; 1 = highest level of fatigue, 10 = no fatigue) four times in each session (VAS1-VAS4), and their sense of discomfort/pain after each session ended (range: 1–10; 1 = no discomfort/pain, 10 = maximal discomfort/pain) using visual analog scales (VAS) that have good internal consistency, reliability, and objectivity [24, 25, 27, 28]. Instructions to the patients were identical for all Sessions. In 4 patients an additional session was performed as a control experiment to evaluate the effects of No Stimulation compared to Sham stimulation. (B) Reaction Time Testing during a Visuo-Motor Task Patients were seated in a comfortable armchair and were instructed to focus attention on a cross in the centre of a video screen, and to bend their wrist as quickly as possible in response to a GO-signal presented on the screen. Trials were started with a visual warning signal ('Get ready'), followed by a GO-signal at random intervals (2–6 seconds). Blocks consisted of 23 wrist flexion trials with the first three trials of a block used as practice trials. These trials were not included in the analysis. EMG was recorded from silver-silver chloride electrodes positioned in a belly tendon montage on the skin overlying the Flexor Carpi Radialis muscle. Reaction times (RT) were defined as the time interval between the GO-signal and the onset of the EMG-burst in the Flexor Carpi Radialis muscle. Patients didn't receive any feedback about there performance. (C) Pinch Force Testing Task Patients were seated in a comfortable armchair with both arms relaxed. Maximal pinch strength of the paretic hand was measured according to a protocol with good validity and test-retest reliability [49, 50]. Patients held the arm of a dynamometer between the lateral aspect of the middle phalanx of the index finger and the thumb pad. Trials were started with a warning signal ('Get ready'), followed by a GO-signal at random intervals (2–6 seconds). Patients were instructed to squeeze the gauge as hard as they could for 1–3 seconds after GO-signal. Patients didn't receive any feedback about there performance. Blocks consisted of nine consecutive trials.