Effects of brain polarization on reaction times and pinch force in chronic stroke
© Hummel et al; licensee BioMed Central Ltd. 2006
Received: 10 August 2006
Accepted: 03 November 2006
Published: 03 November 2006
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.
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.
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.
Rehabilitative treatments result in incomplete motor recovery after stroke. Six months after stroke, 65 percent of survivors cannot use their paretic hand into daily living activities to the extent done before [1–5]. Absence or minimal hand movements by four weeks is predictive of poor motor outcome  and only a small percentage of patients recover function to the extent of community-matched healthy subjects  resulting in marked difficulties in carrying out activities of daily living (ADL).
Recent studies demonstrated that enhancing activity in motor areas of the affected hemisphere (M1affected hemisphere) by means of anodal transcranial DC stimulation (tDCS) [8–10] or repetitive transcranial magnetic stimulation (rTMS)  results in improvements in performance of complex ADL-like tasks with the paretic hand . The tasks utilized in these studies, including the Jebsen-Taylor test (JTT), are cognitively demanding, only in part preprogrammed, and require complex online updating of sensory information as well as adequate sensorimotor integration for accurate performance . Successful performance of the JTT requires a complex pattern of activation of muscles and joints as well as the use of targets and tools  and such skilled movements engage an extensive network of brain regions [14–16]. As a consequence, ADL-like tasks included in the JTT can be fully carried out by a minority of stroke patients .
At present, it is not known if the beneficial effects of tDCS apply only to complex motor tasks in patients with reduced impairment, capable of performing ADL-like tasks or if it can also benefit performance of simpler motor tasks which can be done by patients with higher impairment levels. In this investigation, we studied the effects of anodal tDCS on performance of motor tasks that predominantly rely on M1 functioning such as maximal pinch force production and a simple reaction time task [17–20]. We hypothesized that noninvasive anodal tDCS [8, 21, 22] applied to M1affected hemisphere would improve reaction time and pinch force compared to Sham stimulation  in a group of patients with subcortical stroke. Attention, fatigue and discomfort as possible confounds were evaluated using visual analogue scales [8, 10, 23–28].
INTERVENTION(tDCS, Sham) did not influence Attention (tDCS: 6.99 ± 0.22; Sham: 6.62 ± 0.27), Fatigue (tDCS: 7.01 ± 0.20; Sham: 7.26 ± 0.22). Discomfort/pain was negligible and comparable in the tDCS (1.67 ± 0.14) and Sham sessions (1.20 ± 0.14, paired t-test, ns) on a scale of 1 (no discomfort) to 10 (maximal pain). None of the patients were able to distinguish between the tDCS and the Sham sessions.
Effects of anodal tDCS of M1affected hemisphereon reaction time
Effects of anodal tDCS of M1affected hemisphereon pinch force
Stratification of patients according to impairment levels revealed that tDCS-induced improvement was more pronounced in the more impaired group (PF = 15.7 ± 4.1%; Fig. 2B) compared to the less impaired group (PF = 7.1 ± 2.5%; Fig. 2B). Correlation analyses did not show a significant correlation between tDCS-induced improvement in PF and the patients' MRC scores (Spearman's rho [two tailed] R2 = 0.08; p = 0.27).
Effects of Sham tDCS of M1affected hemispherecompared to the no stimulation condition
Discussions and conclusion
The main finding of this double-blind sham-controlled cross-over study was that a 20 min period of noninvasive anodal tDCS applied to M1affected hemisphere resulted in transient improvements in maximal pinch force and reaction times of the paretic hand relative to sham in a group of chronic stroke patients in the absence of measurable nonspecific changes in attention, discomfort or fatigue. These effects were documented in every single patient tested with RT and in all but one with PF. All patients were blind to the intervention type as were investigators testing the endpoint measures.
Previous reports using tDCS and rTMS showed improvements in performance of tasks that mimic activities of daily living performed with the paretic hand after noninvasive cortical stimulation of M1affected hemisphere [8–11, 29]. Performance of these tasks, like the Jebsen-Taylor test, require complex visuomotor integration and skilled coordination of force production as well as proper management of attentional and motivational resources [8, 13]. This study, designed to determine if anodal tDCS over M1affected hemisphere exerted comparable effects over simpler motor tasks, identified improvements in pinch force and reaction times in the paretic hand with this intervention but not with Sham or No stimulation. These effects suggest that anodal tDCS to M1affected hemisphere enhanced activity within the primary motor cortex [8, 22, 30], actively engaged in control of force production and simple visuomotor integration processes [17, 18, 31–33].
The lack of effects on attention, fatigue and discomfort evidenced by the analysis of visual analog scales, as well as the inability of patients to identify the stimulation and Sham sessions are consistent with results from previous investigations [8, 10, 23–28]. Furthermore, tDCS was applied over the primary motor cortex, distant from areas involved in modulating attentional and motivational processes (e.g., cingulate cortex, prefrontal cortex, limbic system) [34, 35]. Interestingly, repetition of both tasks after Sham resulted in a trend towards longer reaction times and weaker pinch force. The finding of this effect in the control experiment with both Sham (30 sec tDCS) and No stimulation (no tDCS) indicates that it was unrelated with the application of 30 sec tDCS during Sham. Most likely, it reflects mild experimental fatigue, subthreshold for detection by the VAS questionnaires [8, 10].
Recovery of motor function after stroke starts first with improvements in performance of repetitive, relatively simple motor activities like force production, evolving later to relearning of more complex motor synergies and skillful tasks. It is a common finding that while most chronic stroke patients are able to generate various levels of force, only a fraction of them are able to perform skilled motor tasks like those involved in activities of daily living tested with the Jebsen-Taylor test . Since the ability to control properly force production and visuomotor integration represents a prerequisite to meaningful skilled complex motor activity [37–39], it can be speculated that a first rehabilitative step to promote functional recovery after stroke might focus on reacquisition of these motor primaries. Subsequent training could then utilize these basic skills to train more complex actions, such as proposed in sports and musical practice programs . In this way, motor training of these primaries could be utilized to orchestrate more complex skillful motor tasks [41–43]. The finding that anodal tDCS applied to M1affected hemisphere improved force production and a simple visuomotor integration task relative to Sham suggests that these mechanisms could contribute to the behavioral gains reported by anodal tDCS of M1affected hemisphere on more complex activities of daily living [8–11]. Of note is that these improvements were present in patients with higher impairment levels unable to perform skilled ADL-like motor tasks (Fig 2).
In summary, the present study provides novel evidence that anodal tDCS of M1affected hemisphere may enhance performance of a wider range of motor tasks than previously thought, some of them relatively simple and mediated predominantly by M1 function.
Time after stroke (mo)
In an additional session (control experiment), we evaluated the effects of no stimulation (no tDCS, n = 4) and compared the results with those obtained with Sham. In all sessions patients had the scalp electrodes placed in position and the sessions had a comparable time course.
Reaction time testing
Patients were seated 60 cm in front of a 20 inch-monitor with both arms supported by a cushion. They were instructed to focus on the cross in the centre of the screen, and to bend their wrist as quickly as possible in response to the GO-signal (Fig. 5B). Each trial started with a visual warning signal ('Get ready...'), which was followed by a GO-signal at random intervals (2–6 seconds). Each block consisted of 23 wrist flexion trials. The first three trials of every block were used as practice trials and were not included in the analysis. Electromyogram (EMG) was recorded from silver-silver chloride electrodes positioned in a belly tendon montage on the skin overlying the Flexor Carpi Radialis muscle (50 Hz-2 kHz, sampling rate 5 kHz) from a Counterpoint Electromyograph (Dantec Electronics Sklovlunde, Denmark). The reaction time (RT in msec) was defined as the time interval between the GO-signal and the onset of the EMG-burst (Fig. 5B, 1A). Patients didn't receive any feedback about their performance. Reaction times were analyzed by an investigator blinded towards the INTERVENTION type (Sham, tDCS) and towards TIME (Base, Post). Trials with EMG activity in the rest period before the GO-signal were excluded from analysis.
Pinch force testing
Patients were seated in an armchair with both arms relaxed. Maximal pinch force 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 of the paretic hand and were instructed to squeeze the gauge as hard as they could for 1–3 seconds (Fig. 4C). Patients didn't receive any feedback about their performance. Nine muscle strength measurements were averaged in each block. Maximal pinch force (PF) was elicited by pinching the gauge between the thumb and the index finger of the paretic hand. Trials in which pinch force was produced during the rest period where excluded from analysis.
Noninvasive anodal tDCS
Anodal tDCS was delivered for 20 minutes in the tDCS session and for up to 30 seconds in the Sham session using a Phoresor® II Auto (Model No. PM850, IOMED®, Salt Lake City, Utah 84120 USA) through two gel-sponge electrodes (TransQE from IOMED®, surface area was 25 cm2 for each electrode) embedded in a saline-soaked solution. For both interventions (tDCS and Sham) current was increased in a ramp-like fashion at the onset of the stimulation [8, 21] eliciting a transient tingling, burning sensation on the scalp that disappeared over seconds, consistent with previous reports [8, 21, 23]. Current (1 mA) remained on for 20 minutes in the tDCS session and for up to 30 seconds in the Sham session. At the end of the session, tDCS was turned off slowly over a few seconds, a procedure that does not elicit additional perceptions [8, 21].
The patients and the investigator performing motor testing and data analysis were blinded to the type of intervention (tDCS or Sham) as described previously [8, 10, 23]. The tDCS device remained out of the patients' view at all times. Another unblinded investigator's sole participation in the study consisted in administering the interventions. Participants were explicitly asked after the experiments whether they are able to determine which of the experimental session was "real" stimulation and which one was "not real, sham" stimulation.
In seven patients, the anode was positioned on the projection of the hand knob area  of the primary motor cortex of the affected hemisphere on the patient's scalp by using coregistered MRI for neuronavigation (Brainsight®). In 4 patients in whom MRI was not available at the time of testing, the anode was centered on the optimal scalp position for activation of the paretic First Dorsal Interosseus (FDI) muscle using an 8-shaped TMS Magstim coil, a procedure that matches the projection of the anatomical hand knob . The cathode was placed on the skin overlying the contralateral supraorbital region.
Reaction time and pinch force data were normally distributed as evaluated by Kolmogorov-Smirnov test. For statistical analysis the three baseline (RT1 to RT3, PF1 to PF3) and the three post intervention measurements (RT4 to RT6, PF4 to PF6) were pooled (Fig 5A). Repeated measures ANOVA (ANOVARM) was used to evaluate the effects of INTERVENTION(tDCS, Sham) and TIME(Base, Post) on RT and on PF. Paired t-tests were used to evaluate reports of discomfort/pain. For the control experiment paired t-tests were used to compare the No stimulation with the Sham condition. Non-parametric Friedman test was used to evaluate the effects of INTERVENTION(tDCS, Sham) on attention and fatigue, which were not normally distributed. Conditioned on significant p-values (p < 0.05), post-hoc testing was performed and corrected for multiple comparisons when necessary. All data are expressed as mean ± SE.
Activities of daily living
Modified Ashworth Scale for Grading Spasticity
Baseline before tDCS as intervention
Baseline before Sham as intervention
- BaseNo Stimulation:
Baseline before No Stimulation as intervention
Jebsen-Taylor Hand Function Test
Primary motor cortex
- M1affected hemisphere:
Motor cortex of the affected hemisphere
Medical Research Council scale
Pinch force testing
After tDCS as intervention
After Sham as intervention
- PostNo Stimulation:
After No Stimulation as intervention
Repetitive transcranial magnetic stimulation
Reaction time testing
Transcranial DC stimulation
Visual analog scale
This research was supported by the intramural research program of NINDS, NIH, a grant from the Alexander von Humboldt Foundation (Feodor-Lynen) to F. Hummel and by the Rehabilitation Medicine Scientist Training Program (RMSTP), grant number: 5K12HD001097, to P. Celnik.
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