Occipital tACS bursts during a visual task impact ongoing neural oscillation power, coherence and LZW complexity

Little is known about the precise neural mechanisms by which tACS affects the human cortex. Current hypothesis suggest that transcranial current stimulation (tCS) can directly enhance ongoing brain oscillations and induce long - lasting effects through the activation of synaptic plasticity mechanisms [1]. Entrainment has been demonstrated in in - vitro studies, but its presence in non-invasive human studies is still under debate [2,3]. Here, we aim to investigate the immediate and short-term effects of tACS bursts on the occipital cortex of participants engaged in a change – of - speed detection task, a task that has previously reported to have a clear physiology - behavior relationship, where trials with faster responses also have increased power in γ - oscillations (50 - 80 Hz) [4]. The dominant brain oscillations related to the visual task are modulated using multichannel tACS at 10 and 70 Hz within occipital cortex. We found that tACS stimulation at 10 Hz (tACS 10) enhanced both α (8 - 13 Hz) and γ oscillations, in hand with an increase in reaction time (RT) in the change – of - speed detection visual task. On the other hand, tACS at 70Hz desynchronized visual cortices, impairing both phase - locked and endogenous γ - power while increasing RT. While both tACS protocols seem to revert the relationship reported in [4], we argue that tACS produces a shift in attentional resources within visual cortex while leaving unaltered the resources required to conduct the task. This theory is supported by the fact that the correlation between fast RT and high γ- power trials is maintained for tACS sessions too. Finally, we measured cortical excitability by analyzing Event – Related - Potentials (ERP) Lempel – Ziv - Welch Complexity (LZW). In control sessions we observe that lower γ - LZW complexity correlates to faster reaction times. Both metrics are altered by tACS stimulation, as tACS 10 decreased amplitude of the P300 peak, while increasing γ- LZW complexity. To this end, our study highlights the nonlinear cross - frequency interaction between exogenous stimulation and endogenous brain dynamics, and proposes the use of complexity metrics, as LZW, to characterize excitability patterns of cortical areas in a behaviorally relevant timescale. These insights will hopefully contribute to the design of adaptive and personalized tACS protocols where cortical excitability can be characterized through complexity metrics. Additional Title Page Footnotes: We introduce a bursting tACS protocol to study semi-concurrent tACS effects in the visual system and their impact on behavior as measured by reaction time. Burst 10 Hz tACS (tACS10) applied to the visual cortex entrained γ-oscillations and increased RTs in a change-of-speed detection visual task more than 70 Hz tACS (tACS70) or Control conditions. Burst tACS10 also decreased amplitude of the P300 peak, while increasing α-power and γ-LZW complexity. Physiological and behavioral impact of occipital tACS10 and tACS70 was frequency-specific. tACS70 reduced γ-oscillations after 20min of tACS stimulation. Cognitive task may determine cortical excitation levels as measured by complexity metrics, as lower γ-LZW complexity correlates to faster reaction times.