This study aimed at investigating the tDCS effects of different montages on cortical activity of chronic ischemic stroke patients. We focused on spectral power changing after three kinds of tDCS montages (atDCS, ctDCS and bi-tDCS) and the difference of after-effects among them. We are also interested in the after-effects over time and the relation between changing of alpha and clinical factors of stroke patients. There were four important findings from our study: (1) the tDCS effect was limited to the alpha rhythm of opening-state. (2) atDCS increased the alpha power especially alpha1 (8–10 Hz) in local and other areas. bi-tDCS affected the alpha power as well, but in a smaller area and mainly focused on alpha2 (10–13 Hz). ctDCS had no effect on alpha rhythm. (3) No significant difference of alpha band was found over the observed time range after stimulation. (4) The change of alpha especially alpha2 in contralateral hemisphere induced by atDCS was related to time since stroke, and alpha2 in ipsilateral hemisphere induced by bi-tDCS to motor impairment level.
Alpha band of EEG has been proved by several studies to be a brain rhythm involved in several cerebral functions, ranging from sensory-motor processing to memory formation [22, 23]. Ischemic stroke showed an attenuation of normative, faster activity, particularly in the alpha band (8–12 Hz) [24, 25]. Alpha band of stroke patients was found to be locally reduced in brain regions critical to observed motor or cognitive behavioral deficits. The decrease of alpha band synchrony was found related to cognitive and motor deficits in post-stroke patients [26]. Some study showed that motor recovery could be predicted by increased alpha-band functional connectivity in motor-related areas [27]. So we supposed that the increasing of alpha band may be beneficial to stroke recovery.
Previous studies investigating the changing of cortical activity after tDCS through rs-EEG power spectrum analysis were mainly focused on healthy people and showed response difference among stimulation montages. Some research showed increase of alpha band after atDCS in healthy people, but not after ctDCS, which is similar to our result in stroke subjects. Notturno et al. [20] found a higher low alpha band power post- than pre-atDCS over motor related regions, but not for ctDCS. Spitoni et al [28] explored the tDCS effect over the right posterior parietal area in healthy subjects and found that the effect was limited to the alpha band, and atDCS significantly affected the alpha band whereas the ctDCS did not elicit any modifications. This is consistent with our finding in stroke patients. However, the impact was shown in eye-closing but not in eye-opening state, which is different with this study. We hypothesized that this might be related to sleepiness with eye-closing state in some patients. Besides that, the difference of area of stimulation target may be another impact factor. For bilateral tDCS, studies are mainly focused on its rehabilitation efficacy on stroke patients [29–32]. A reduction of inter-hemispheric imbalance was found after a long-term effect of tDCS associated with physical therapy according to the analysis of motor evoked potential (MEP) [30]. We found no reports about EEG power spectrum following the bi-tDCS montage. In our study, both atDCS and bi-tDCS modulated alpha band, but atDCS preferred low-band alpha and bi-tDCS preferred high-band alpha. Studies have shown that different alpha components correspond to different cognitive processes. Low-band alpha rhythm was supposed to be related to anticipatory attentional processes and high alpha would indicate task-specific visuo-motor processes according to some task-related ERD/ERS study [33]. We speculated that these different changes on alpha rhythm induced by the two montages may imply that they work in different ways.
There are also some inconsistent reports with our results. Besides alpha band, some study showed power changing in other frequency band after tDCS [17, 34, 35]. In some study rs-EEG power spectrum analysis showed no difference comparing baseline with post stimulation in any of the tDCS conditions (one-hemispheric tDCS or bi-lateral tDCS) over dorsolateral prefrontal cortex in healthy subjects [36, 37]. Confounding results may be due to the difference in stimulation target, current density, time of duration and participants.
For the stimulus target area of tDCS, recent studies showed that brain stimulation leads not only to local changes of cerebral activity under the stimulated region, but also to distant changes in inter-connected brain regions throughout the brain [34, 38, 39], which is consistent with our results. Besides the local target area, we found that alpha power of some distant areas including frontal and parietal showed an increase following atDCS and bi-tDCS. Besides that, the influence of atDCS was more widespread associated with bi-tDCS.
For the duration effect of tDCS, some study reported increased alpha power during and after atDCS which persisted for 12 minutes without attenuation [18]. Spitoni et al [28] reported that the strongest change of alpha power occurred in the first 2 min after the atDCS ended, and the effect diminished systematically and was effective for approximately 8 min. We missed the first 10 min EEG information immediately after stimulation because of placing electrodes. So only the 10–30 min EEG signals after stimulation were analyzed in the present study. Although alpha power didn’t change in the 10–30 min after stimulation, they keep higher level than that of pre-stimulation, indicating that the effect maintained for at least 30 minutes with no significant attenuation.
For the clinical factors affecting modulation results, previous studies have found that tDCS stimulation efficacy may vary with time after stroke, nature and location of stroke and level of motor impairment [9, 29, 40]. We found that the change of alpha especially alpha2 in contralateral hemisphere induced by atDCS was related to time since stroke, and alpha2 in ipsilateral hemisphere induced by bi-tDCS to motor impairment level. Regression analyses confirmed that individuals’ response of alpha power change to atDCS could be predicted from their time after stroke. Stroke subjects with 3 to 6 months and longer than 20 months since stroke showed higher alpha power increase than other subjects, indicating higher response to the montage of atDCS. For bi-tDCS, alpha band power increased the most in moderately impaired subjects with respect to mild and severe impairment, implying that subjects with moderate motor impairment were more susceptive to this kind of montage. Plasticity processes were variable with different phases or degree of stroke. tDCS effects may interact with these processes. Studies have suggested that patterns of neural recovery may differ for individuals based on the severity of their stroke [41–42]. The quadratic regression model was better suited to model the variation trend than linear regression model both for atDCS and bi-tDCS, indicating a complicated relation among clinical factors and EEG parameters. The result may help explain the variable rehabilitation efficacy to individuals with different clinical stroke features using tDCS.