Computational protocol: Stimulus Presentation at Specific Neuronal Oscillatory Phases Experimentally Controlled with tACS: Implementation and Applications

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Protocol publication

[…] EEG signals and trigger markers were analyzed using the FieldTrip toolbox (Oostenveld et al., ), the circular statistics toolbox (Berens, ), and custom Matlab scripts (MathWorks). Per trial (i.e., LPT trigger, recorded as EEG marker), the phase of the oscillating waveform was determined using the Hilbert transform, extracting the instantaneous phase at the onset of the trigger (function ft_preprocessing in Fieldtrip Oostenveld et al., ). Results are presented in circular phase plots. Across trials, event-related potentials over 2-s epochs, time-locked to each of the five phase-condition triggers (see insert in Figure , fourth line of stimulus protocol, communicated through LPT signals), were created to further visualize the temporal correspondence of LPT triggers to the oscillating waveform at different phase conditions.The primary measures of interest were absolute phase shift and phase consistency. We define absolute phase shift as the difference between the observed phase (the measured phase of triggers respective to the measured oscillatory waveform) vs. the desired phase (the requested phase of triggers in the stimulus protocol respective to the oscillatory time series in the stimulus protocol). We used this measure to assess the accuracy of our setup. We further assessed the reliability of our setup based on phase consistency, which we define as the range of observed phases across trials around the mean observed phase. We use two specific measures of phase consistency: the maximum offset (i.e., the largest observed difference between observed phase and mean observed phase) and the 95th percentile offset (the 95th percentile of the aforementioned differences). The latter thus shows how consistent phase offset is for the vast majority of trials, leaving out the 5% most extreme trials. In other words, 95% of all trials yielded phases closer to the observed mean phase than the 95th percentile offset. We calculated these offsets in terms of phase (degrees) and in terms of time (milliseconds). […]

Pipeline specifications

Software tools FieldTrip, CircStat
Diseases Brain Diseases