Abstracts

Rationale: Since the end of the 1990s, the clinical interest in transcranial direct current stimulation (tDCS) has increased significantly (Lefaucheur et al., Clin Neurophysiol., 128(1): 56-92, 2017). Epilepsy, with its abnormal electrical activity, should be an ideal candidate to rebalance the cortical excitability thanks to tDCS. It could be used for reducing the epileptic discharges and for restoring the cognitive EEG biomarkers affected by epilepsy._x000D_ _x000D_ In this study, we focused on face recognition and the visual ventral stream because this brain function and localization are often impacted in epilepsy. Until now, all studies used scalp EEG to assess the electrophysiological neuromodulation of the face recognition EEG biomarkers, i.e. the N170 evoked potential. Civile et al (Neuropsychologia, 143: 107470, 2020), among others, performed tDCS and reported significant differences of the N170 amplitude on the right hemisphere after tDCS. The two objectives of this study were to:_x000D_ - Use fast visual periodic visual stimulations (FPVS) to avoid the disadvantage of evoked potential analysis in the temporal domain_x000D_ - Use intracerebral EEG (stereoelectroencephalography, SEEG) to directly quantify the neuromodulation in different deep brain regions of the visual ventral stream (VVS)_x000D_
Methods: Six drug resistant epileptic patients were included. Multi-contact SEEG electrodes sampled mainly the VVS. Fast periodic visual stimulation was performed at 6Hz with images of single non-face objects (everyday objects, animals, fruits, vegetables) and faces. Faces appeared periodically every 5 images, which represent an apparition frequency of 5/6=1.2Hz. TDCS was performed at -2mA or +2mA during 3 successive sessions (sham-tDCS-sham) of 10 min-FPVS (Figure 1). SEEG signals were analyzed in the frequency domain using fast Fourier transformation (FFT) and SEEG evoked responses were statistically compared before, during and after tDCS.

Figure 1. Experimental design of tDCS during FPVS paradigms.

Results::Subjects had a total of 1,093 intracerebral contacts (mean of 156 ± 21 contacts per subjects)._x000D_ The most important and significant neuromodulation was observed at 6Hz (global visual response). At -2mA, SEEG evoked responses after tDCS were significantly lower within the controlateral temporal lobe (amplitude difference about -35% between before and after tDCS). At +2mA, SEEG evoked responses statistically decreased in all ROIs of the VSS (amplitude difference about -38 to -95% between before and after tDCS) (Figure 2).

Figure 2. Face selective responses’ amplitudes (± SE) at 6Hz frequency in each ROI for the cathodal sub-group during the different phases.

Conclusions: TDCS induces intracerebral electrophysiological changes during cognitive processes. Further investigations are required to precisely characterize these effects and the tDCS methodology to use. Cognitive impairments related to epilepsy could be blocked or reduced thanks to this non-invasive transcranial electrical stimulation technique._x000D_
Funding: French ANRT grant (CIFRE)