Time-frequency analysis
Based on our For each condition, time–frequency representations (TFR) of oscillatory power changes resulted from a Morlet wavelet decomposition with variable, frequency-dependent cycles (=frequency / 2) into frequency bins between 3 and 30 Hz. The length of the wavelets increased linearly from 1 cycle at 2 Hz to 15 cycles at 30 Hz to optimize the trade-off between temporal resolution at lower frequencies and stability at higher frequencies. The time window from 200 ms to 100 ms before presentation of the fixation cue was defined as baseline (-1200 ms to −1100 ms). The baseline was offset by 100 ms from fixation cue onset to minimize contamination of the baseline interval by fixation-associated activity. The change in spectral power during the preparatory period (−1000 ms to 0 ms) is reported as the logratio from the baseline period, calculated by dividing by the mean baseline power per frequency and converting to decibel (dB) by log-transformation (dB = 10 × log10(power/baseline), then averaged across trials for each condition.Since this study focused on preparatory activity and eye movements were inherent in the response period of the trials, the final epochs were limited to the time window before stimulus presentation, i.e., before the direction of the following saccade had been revealed to the participant. Thus, data were segmented into epochs time-locked to the onset of the cue stimulus containing the full 1000 ms period of fixation cue presentation (−1000 ms to 0 ms with respect to stimulus onset) for statistical analysis. (To investigate effects of DBS conditions on cortical activity on a single-trial level, time–frequency transformations were adapted for single-trial analysis by calculating TFR for each trial separately using the same Morlet wavelet decomposition and baseline correction as outlined above. For the construction of the generalized mixed-effect models that were used to predict trial outcome (correct antisaccade / error), TFR were additionally calculated for error trials, while only correct antisaccade trials were included in the single-trial mixed linear models for antisaccade latency. For the single-trial analysis, frequency ranges and time windows were determined post-hoc based on the group level results (see Results section).