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| Paper IPM / Biological Sciences / 18012 |
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Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that can modulate neuronal oscillations and influence brain function. However, its underlying mechanisms are not well understood. Entrainment according to Arnold tongue patterns by which tACS synchronizes ongoing neuronal oscillations is one of the main hypothesized mechanisms. But, it is unable to explain some experimentally observed inconsistencies. Synchronization tendency of neurons subjected to external perturbation depends on their phase response curve (PRC). The PRC determines the excitability type of neurons, i.e., an oscillator's phase shifts in response to perturbations delivered at different phases of the activity cycle, linking single-neuron dynamics to network synchronization. The PRC of a type-I oscillator is mainly positive where the phase is only advanced by a small perturbation, whereas the PRC of a type-II oscillator contains both positive and negative regions where the phase can be either advanced or delayed depending on the timing of the perturbation. We computationally analyzed a conductance based neuron described by the Morris Lecar model which can exhibit either type-I or type-II PRC. The model parameters were adjusted so that the neuron oscillates at 10 Hz in either case. The neuron was driven by 10 Hz tACS. Entrainment was defined as the 1:1 frequency locking. We found that for the type-I neuron, the 1:1 Arnold tongue region is asymmetric with respect to the endogenous oscillation, supporting that in this case tACS frequencies smaller than the endogenous frequency fail to entrain the oscillator. However, the type-II neuron can be entrained by either faster or slower tACS inputs, resulting in a more symmetric entrainment region. Our findings suggest that entrainment of neuronal oscillations by tACS may crucially depend on the dynamical properties of single neurons which determine their response to external perturbation.
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