P131 Phase-dependent modulations of brain activity and connectivity are dependent on cognitive state

Abstract

Question Oscillations in the theta range (4–8 Hz) are a good candidate to coordinate neuronal ensembles during cognitive control. TACS has been shown to modulate cognitive function in a phase dependent manner. In this study we tested the effect of tACS in two paradigms with different levels of cognitive demand: a low-cognitive demand choice-reaction task (CRT); and a high-cognitive demand 2-back task. Simultaneous tACS/fMRI was used to test the hypothesis that the effects of tACS on brain activity and connectivity depend on cognitive state as well as the phase of stimulation. Methods TACS was delivered over F4 and P4 (with a common return in T8), at 6 Hz, 1 mA (Fig. 1 Download : Download high-res image (363KB) Download : Download full-size image ). Experiment 1: 10 subjects performed 3 sessions of each tACS condition “Synchronized” (0°), “Desynchronized” (180°), and sham stimulation. Experiment 2: 24 subjects had concurrent tACS and fMRI (3T, Siemens). Results Experiment 1: TACS delivered in the “synchronized” condition significantly improved reaction times in the high-cognitive demanding task as compared to the sham and “desynchronized” conditions. There was no effect of tACS in the tasks with low-cognitive demands. Experiment 2: 1. Distinct effects of tACS were seen for tasks with different levels of cognitive demand and the effect of tACS on brain activation is phase-dependent (Fig. 2 Download : Download high-res image (507KB) Download : Download full-size image ). In the 2-back task, tACS delivered in the “synchronized” condition elicited an increase in activity in task-related regions. In contrast, the “desynchronized” condition resulted in increased deactivation of the default-mode network. 2. tACS modulates brain connectivity in a task- and phase-dependent manner. PPI analysis showed that 0° stimulation modulated functional connectivity between the stimulated frontoparietal regions and regions involved in cognitive control. 3. Participants that had increased brain activity during the “synchronized” condition showed faster reaction times in the 2-back task (r = −0.4917, p = 0.02). Conclusions We demonstrate that tACS modulates brain activity and functional connectivity in a phase-dependent manner, which is dependent on cognitive task performance. TACS increases task-relevant functional connectivity within regions involved in cognitive control, providing a mechanistic insight how the technique enhances cognitive function.