Brain oscillations emerge from nonlinear interactions across anatomically connected neural populations, alternating between transiently coordinated and desynchronised states. Transcranial alternating current stimulation (tACS) can modulate these dynamics, but most work has focused on frequency and amplitude, leaving waveform shape comparatively unexplored. Here we used a whole-brain model of delay-coupled Stuart--Landau oscillators constrained by empirical human structural connectivity to determine how sinusoidal, square, triangular, sawtooth and pulsed stimulation reshape spontaneous alpha-band activity. All waveforms were applied at the same frequency and amplitude to the posterior parieto-occipital regions. Network responses were quantified using the Kuramoto order parameter, spectral entropy and metastable oscillatory modes of transient alpha bursts. Sinusoidal and pulsed stimulation produced the strongest effects, increasing global synchrony while reducing metastability and spectral entropy, consistent with a transition from a fluctuation-rich regime to a coherent and spectrally concentrated state. These waveforms also transformed intermittent alpha bursts into prolonged or near-continuous oscillatory episodes. In contrast, square, triangular and sawtooth stimulation reduced synchrony while largely preserving metastability, producing weaker and more fragmented modulation. These findings identify waveform shape as a key determinant of rhythmic stimulation effects and a principled parameter for neuromodulation design.
Sharma, V., Tiesinga, P. H. E., Cabral, J.
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