P 125. Transorbital alternating current stimulation strengthens oscillatory activity and functional connectivity in patients with visual system damage: A resting-state EEG study

Abstract

Introduction Non-invasive brain stimulation is an emerging treatment option for patients suffering from visual system damage and persistent visual field loss. Transorbital alternating current stimulation (tACS) applied daily for 10days was shown to improve patients‘ detection abilities (Sabel et al., 2011) and subjective quality of vision (Gall et al., 2011). Further, 10days tACS treatment progressively increases alpha band power in the resting state EEG of the treated subjects (Schmidt et al., in press). However, immediate neurophysiological after-effects of tACS have not been studied so far. Objective To investigate immediate neurophysiological effects of a single tACS session in patients with visual system damage. Methods Optic neuropathy patients were randomly assigned to either placebo ( n =7) or verum group ( n =7). The stimulation was delivered with electrodes attached below and above each eye. Current parameters were individually adjusted: amplitude was set above the phosphene threshold, and frequencies ranging from 8 to 22Hz were used. Resting state eyes-closed EEG (20 electrodes acc. to 10–20 system) was recorded immediately before and after the tACS. Power density and coherence were calculated. Coherence graphs, representing the same number (40) of strongest connections (Fig. 2), were created for each subject and characterized by graph measures indicating of the small-world structure. Changes in EEG parameters were correlated with the extent of visual system damage as assessed by perimetry. Results One session of tACS: (i) increased power of delta, theta and beta bands at the occipital region; (ii) increased theta and beta bands coherence between occipital electrodes (Fig. 1); (iii) altered the structure of beta band coherence graphs, as indicated by decreased characteristic path length (Fig. 2). Coherence increase was more profound in subjects with less extensive damage ( r =0.94; p =0.016). Conclusions One session of tACS modulates brain activity and connectivity in patients with visual system damage. The neuromodulation includes increase of power (delta, theta, beta bands) and coherence (theta, beta), and results in a modification of coherence network topology (beta) towards more optimal small-world pattern. These neurophysiological effects are stronger in subjects with mild optic nerve damage. Further, tACS induced neuromodulation seems to be non-specific, as power of frequencies not included in the stimulation protocol (delta, theta) also increased. Further studies need to elucidate how long these changes persist and how they relate to perceptual functioning and vision restoration. Funding The study was funded by the Otto-von-Guericke University, Magdeburg (see Figs. 1 and 2).