Orchestration of oscillatory activity to improve visual motion discrimination

This thesis describes advances in the use of novel configurations of non-invasive brain stimulation over the visual system allowing to modulate of modifying electro-physiological activity, interregional interactions and by it, visual behavior such as motion discrimination capacity in healthy subjects. We have implemented three experimental protocols that include the application of a motion discrimination and integration task in combination with bifocal transcranial Alternating Current Stimulation (i.e. tACS) over the primary visual cortex (i.e. V1) and medio-temporal areas (i.e. V5), while varying some of the “orchestra” parameters in each study. The common objective pursued in the three studies presented in the upcoming chapters was to evaluate physiological changes induced by tACS combined with the visual task, leading to enhanced visual performance expressed by the accurate distinction of the generalized movement orientation of a kinetogram. After introducing the scientific rationale of this thesis in Chapter 1, Chapter 2 describes whether applying two phase-shifted (Alpha) ɑ-tACS conditions (Anti-Phase and In-Phase tACS) within the V1-V5 network were able to positively modulate behavior compared to Sham tACS. Our results suggest that the active Anti-Phase condition significantly increased visual motion discrimination compared to In-phase tACS which rather tended to decrease performances. These two case scenarios were associated with opposite changes in Alpha-Gamma oscillatory modulation (i.e. V1 phase – V5 amplitude coupling) determined by multichannel EEG. Based on these findings, in Chapter 3, we describe testing the effects of modulating Alpha-Gamma interregional interaction. Hence, two conditions V1ɑV5Ɣ tACS and vice versa, V1ƔV5ɑ tACS, were behaviorally and electrophysiologically evaluated. The results suggested that there was a common electrophysiological feature between the two Verum tACS, which contrasted with Sham tACS, expressed through WPLIƔ (i.e. Weighted Phase Locking Value) connectivity. Furthermore, WPLIɑ and ZPAC V1amplitude – V5phase (i.e. Z-scored Phase Amplitude Coupling) were the inter-areal mechanisms in which both Verum conditions differed to explain the variance of their corresponding group behavior. However, the electrophysiological changes did not lead to significant difference in behavioral measures.In Chapter 4, we combined the knowledge gained in the first two studies and thus, we time-locked, short bursts of phase-shifted ɑ-tACS to the visual stimulus onset. This permitted to find out that, despite the phase difference between the tACS conditions (i.e. In-Phase vs. Anti-Phase), there was a generalized augmentation of the performance after verum stimulation compared to the results with Sham. This amelioration was generally associated with changes in causal PSI (i.e. Phase Slope Index) flows in Ɣ, whereas specifically the Θ-Ɣ modulation permitted to explain the differences in behavior between Verum and Sham. Moreover, dynamic PSI-causal β bottom-up and top-down flows revealed the mechanisms behind each type of Verum stimulation. These studies provided first interesting evidence that physiology-inspired bifocal tACS applied to the visual network might be used to modulate visual behavior and respective underlying mechanisms. The induced electrophysiological and behavioural effects achieved are complex and need to be studied in more details in upcoming studies.