Although recovery processes in visual cortex are still controversial, there is some evidence of plastic reorganisation of neural tissue after occipital stroke seen in fMRI (Brodtmann et al., 2007, 2008, 2015, Dilks et al., 2007). Bola et al. investigated the importance of fronto-occipital network dynamics in visual processing and the effect of transcranial alternating current stimulation (tACS) on those networks and on the restoration of blindness after optic nerve damage (Bola et al., 2014). All investigations regarding stroke-treatment in visual system damage were carried out in chronic phase (Alber et al. 2015) although neuroplasticity after the neural damage is pronounced in the first weeks after the damage. Since transcranial direct current stimulation (tDCS) is considered to be safe even in acute and sub-acute phase, we conducted this study not only looking at behavioural data but rather investigating neurophysiological correlates in terms of dynamics in the time course of the first 6 months after the event.
Uncover the effects of tDCS in subacute stroke patients.
Does tDCS show similar effects in stroke recovery than tACS and would it – employed in an earlier stage – show any advantages compared to tACS?
In a randomized, sham-controlled, double-blind clinical trial, nineteen sub-acute stroke patients diagnosed with unilateral posterior cerebral artery ischemia were enrolled. Patients were randomized in a verum and a sham group controlled for age, gender, mean sensitivity (MS) in threshold perimetry. The groups underwent a baseline resting-state and checkerboard-VEP EEG recording, a tDCS sham or verum treatment, then another EEG post-treatment recording after ten days and a final follow-up recording after three months.
Mean sensitivity increased significantly after treatment but dropped after 3 months in both groups (see chart). We calculated absolute change of power (post-treatment – baseline, follow-up – post-treatment; see graph) in all frequency bands to compare the change between hemispheres, groups and again link them with the change in our outcome variables. Verum and sham group showed significant differences in absolute change from baseline to post-treatment in Delta (t15=2.615, p=.020), Theta (t15=2.483, p=.025) and low Alpha (t15=2.457, p=.027), and from post-treatment to follow-up in low Alpha (t12=-2.297, p=.040) and high Alpha (t12=-2.449, p.031), but only in the intact side. In general, verum group increased in power, whereas sham group decreased in power. When we compared both hemispheres, only in sham group low Alpha (t8=-2.809, p=.023) and high Alpha (t8=-2.891, p=.020) decreased significantly more in intact than in damaged hemisphere.
Conclusion: When looking at resting state EEG power analyses, it seems that neurophysiological changes appear after some time after the stroke. Possibly, specific training induces physiological changes which could be interpreted as compensational mechanisms of the intact hemisphere. tDCS could have some protective effect in terms of stabilizing alpha power (see difference between verum und sham at t2). At follow up there is no difference anymore. Probably longer stimulation might stabilize the seen effect (mean sensitivity and alpha drop to follow up stronger in verum than in sham).