Participants. All 11 participants (6 male, 5 female; 9 naïve to the purpose of the study; 20-47 years; M=29.9; SD=6.8) provided written informed consent. One participant’s data were excluded due to excessive involuntary blinking. The number of participants were comparable to previous alpha-band entrainment TMS-EEG studies49,51. NYU institutional review board approved the protocol (IRB #i14-00788), which followed the Declaration of Helsinki and safety guidelines for TMS experiments60.
Apparatus. The stimuli were presented on a ViewSonic P220f monitor. The screen resolution was 800(H) x 600(V) at 120Hz. The viewing distance was 57 cm, set by a chin-rest. The stimuli presentation code was written in MATLAB with Psychtoolbox 361,62. To linearize stimuli contrast, the monitor’s gamma function was measured with a ColorCAL MKII Colorimeter (Cambridge Research Systems).
The TMS pulses were delivered with a 70-mm figure-of-eight coil controlled by a Magstim Super Rapid2 Plus1 system. The coil, whose handle pointed rightward with respect to the midline of the skull, was supported by a mechanical arm and hand-held tangentially to the skull; its positioning was always guided by a Brainsight neuronavigation system (Rogue Research) with ~1 mm precision. Two identical TMS coils were alternated every 2 blocks of testing to prevent overheating. Infrared reflecting markers were attached to the TMS coil and the participant’s head to extract the relative 3D positions in real time; they were displayed on each participant’s MRI structural scan.
The EEG system consisted of a Brain Products actiCHamp amplifier and TMS-compatible Easycap actiCAP slim caps. The EEG cap layout, following the international 10-20 system63, included a grid of 63 TMS compatible 6mm-thick electrodes, including a ground electrode placed at Fpz and a reference electrode placed at FCz. Impedance of all electrodes after conducting gel application was kept below 25k Ohm during the whole experimental session. The EEG recording software was BrainVision Recorder. An in-house built TMS triggering and EEG event registering device with at least 0.4 ms resolution ensured timing precision at 2500 Hz EEG sampling rate64 (Fig. S1A).
Visual discrimination task. The participants performed an orientation discrimination task. Each trial began with a fixation period, followed by a 25 ms neutral pre-cue indicating that the target was equally likely to appear in the lower left or lower right visual field (Fig. 1A). After a 500 ms inter-stimulus interval (ISI), two Gabor patches and a response cue, indicating which patch was the target, appeared simultaneously. The Gabor patches (achromatic; 4 cpd; σ 0.42°) lasted 50 ms. Participants were asked to indicate whether the target patch was slightly clockwise or counterclockwise relative to vertical via a key press (right index finger for the ‘/’ key; left index.
Figure 1. Experimental protocol (A), Trial structure. Each trial began with an intertrial period (ITI), followed by a neutral pre-cue to indicate the two stimulus locations. Active or sham stimulation was delivered in the interstimulus interval (ISI) between the neural pre-cue and the Gabor stimuli. The response cue indicated which Gabor was the target. Participants responded whether the target Gabor was tilted to the left or to the right with a key press. (B), Stimulation patterns & analysis time windows. The onset of the pre-cue is at t = 0. The four red lines mark the pulse timings in the rhythmic condition. The two blue lines mark the timings of Gabors onset and offset. In the rhythmic condition, the gap between pulses was exactly 100 ms. In the arrhythmic condition, the timing of the second and the third pulses was jittered (see text and Fig S1 for the probability distribution). Time window W0 is the cycle before the first pulse. Time windows W1, W2, and W3 are the cycles between pulse pairs. Time windows W4 and W5 are the first and the second cycles after the last pulse, respectively. (C), TMS loci & directions. Each cylinder represents the TMS directional vector of one participant on the MNI brain template. The cyan discs indicate the EEG electrode positions. finger for the ‘z’ key). A high pitch (700 Hz) signaled a correct response and a low stone (400 Hz) signaled an incorrect response. Only after the participants responded, the next trial would begin. The inter-trial interval was randomly chosen among 2500, 2750, or 3000 ms.
A central fixation cross was constantly present. Participants were asked to maintain fixation at all times and to blink after each trial. An eye tracker (EyeLink1000) ensured that fixation was within a 1.5° radius invisible circle. Trials for which fixation was broken (including blinking), from cue onset to stimuli offset, were discarded and repeated at the end of the block. The contrast level of the two Gabor patches were independently titrated before the experiment. A method of constant stimuli (4-80% Gabor contrasts in 7 log steps) was used to obtain the contrast at which sensitivity (d’)65,66 reached half-of-max sensitivity of the Naka-Rushton function was defined as c50 contrast. The group average (standard deviation) of c50 contrast was 16% (4%) for both visual fields.
During the ISI, 4 rhythmic or arrhythmic, active or sham pulses were delivered. The ISI between the pre-cue and the first pulse was 80 ms. The experiment applied a 2 (rhythmicity types) x 2 (stimulation types) within-subject design. The rhythmic and arrhythmic stimulations were blocked. The stimulation type (active or sham) and the target side were randomized within blocks. The experiment consisted of 8 blocks, each contained 64 trials, for a total of 512 trials; 128 trial repetitions per each of the 4 experimental conditions (2 rhythmicity types × 2 stimulation types).
For the secondary goal, we found no significant effects regarding whether entrainment modulates visual discrimination65,66, either for visual sensitivity (d’; Fig. S2) or response criterion (c; Fig. S3). The error bars represent ±1 S.E.M. corrected for within-subject design67.
Transcranial magnetic stimulation. The TMS site for each participant was defined by retinotopy or phosphenes (see Individualized TMS site). Fig. 1c illustrates the vectors connecting the cortical and scalp sites for each participant. All but one participant’s stimulation vector clustered around electrodes Oz, O2 and PO4. The TMS intensity was fixed at 70% for all participants (except for one at 65% and another at 67%) of the maximal machine output, to ensure no phosphene induction during the task. The fixed upper bound of the TMS intensity was independently determined in pilot studies to ensure that the coil would not overheat in the middle of an experimental block. The sham control consisted of 4 pulses of pre-recorded TMS sounds played through a speaker attached to the coil. The coil was placed over the scalp, targeting the same area regardless of the actual stimulation type. Participants were asked to report if they saw any phosphenes at any point during the task; they reported none.
In the rhythmic condition, the pulses were 100 ms apart, aiming to induce alpha-band entrainment. In the arrhythmic condition, the timing of the first and last pulses of the burst were the same as in the rhythmic condition, whereas the timing of the second and the third pulses were randomly jittered on each trial according to a bimodal distribution synthesized from normal distributions N (±30ms, 10ms) (Fig. S1B). Before and after the experiment, we verified that the registered TMS timings on the EEG achieved the expected precision (Figs. S1C-D).
Individualized TMS site. We had magnetic resonance imaging (MRI) structural scan images and population receptive field (pRF) mapping68,69 data of the visual cortices for all but 3 participants. We used NiBabel70 to extract the right hemisphere V1 voxels corresponding to the location and size of the stimulus in the left visual field. The center of mass of the extracted voxels served as the individualized TMS target site. For the 3 participants without fMRI data, we used a TMS phosphene induction method48,71–75 to localize the optimal cortical site inducing unilateral phosphene perception in the left visual field (most likely on V1/V248,71,76–78 and probably V379,80). For these participants, the cortical site depth was arbitrarily set at 26.4mm, the average of the scalp-to-site distance of all other participants. Table S1 summarizes the TMS cortical and scalp sites in Montreal Neurological Institute (MNI) coordinates. At these individualized stimulation sites defined by pRF voxels, all but 1 participant reported seeing phosphenes; thus, the pRF and the phosphene induction method yielded comparable stimulation sites.
EEG preprocessing. The EEG recordings were digitized at 2500 Hz without any filtering. There was no re-referencing as the region of interest was the occipital cortex. The spike artifact caused by a TMS pulse typically lasted 4-10 ms. To reduce the artifact, signals within 1 ms before and 13 ms after each TMS pulse were replaced with shape-preserving piecewise cubic interpolated data (MATLAB command: interp1 with ‘pchip’ option)44,49,51,81. The exact same procedure was applied to EEG data during sham auditory pulses to ensure that any statistical differences could not be attributed to discrepancies in data processing. No trials were discarded other than those with blinks (see above, Visual discrimination task).
The continuous data were then down-sampled offline to 100 Hz and segmented into epochs containing data from 300 ms before to 900 ms after the neutral pre-cue onset. Preprocessing and analyses were carried out with MATLAB R2017a and software packages FieldTrip82 and Brainstorm83.
EEG analyses. Before the experiment, we recorded 2-min eyes-closed resting-state activity to define the individual alpha frequency (IAF) as the frequency corresponding to the maximum peak between 7 and 13 Hz on Welch’s periodogram84 (MATLAB command: pwelch). To assess if the neural activity was phase-locked to the entraining periodic stimulation, we calculated two indices for each of the 4 conditions: (1) evoked oscillatory amplitude (square-root of power), by averaging waveforms across trials first and then applying Morlet wavelets59; (2) inter-trial phase coherence (ITPC, or phase-locking value)85 by applying Morlet wavelets to each trial and calculating their consistency with , where i denotes the trial number, N the total number of trials, and θ the phase. ITPC is a ratio between 0 and 1, in which 1 indicates perfect phase alignment. For both indices, each Morlet wavelet had 5 cycles49, and the frequency range was 3-50 Hz. With 5 cycles of Morlet wavelets, the spectral bandwidth and wavelet duration, characterized by full width at half maximum, were 4.7 Hz and 0.2 sec, respectively. Thus, evoked oscillatory amplitude and ITPC centered around 10 Hz characterized the alpha-band (8-12 Hz) activity. Additionally, pre-TMS α-phase was defined as the phase calculated from Morlet wavelets, 200 ms before the first TMS pulse (95 ms before the pre-cue onset).
We analyzed the evoked oscillation amplitude and ITPC in 7 planned time windows: W0 (5-105ms, before the first pulse), W1 (105-205ms, between the first and second pulses), W2 (205-305ms, between the second and third pulses), W3 (305-405ms, between the third and last pulses), W4 (405-505ms, first cycle after the last pulse), W5 (505-605ms, second cycle after the last pulse), and W6 (605-705ms, third cycle after the last pulse). Because these time windows, which are consistent with reported time intervals of entrainment effects by TMS44,49,51, were specified a priori, multiple comparison correction was not required. But within each time window topographic analyses (paired t-tests between conditions across all channels) were corrected for multiple comparison with cluster-based permutation tests86 (1000 permutations, one-tailed, alpha=.05, cluster alpha=.05). According to the trial structure of the visual discrimination task, stimuli were presented in the middle of W5 (530-580 ms). Online eye tracking ensured that the trials containing eye blinking were discarded so that the analyzed EEG time windows were free of such artifacts.
Several statistical tests were performed. (1) As entrainment should lead to localized elevation of evoked oscillation amplitude and ITPC near the stimulation site, t-tests were performed by averaging the neural signatures in channels O2 and PO4 in the 10-Hz band at all time windows. (2) To explore the temporal and spectral specificity of entrainment, t-tests of the two neural signatures were performed across all time-frequency bins averaged across channels O2 and PO4. (3) To explore the topography of neural activity before, during and after entrainment, t-tests were performed across all channels in the 10 Hz band at all time windows. (4) To examine whether ITPC enhancement following rhythmic TMS depended on pre-TMS alpha phase, trial-by-trial 10-Hz phases 200 ms before the first pulse (95 ms before the pre-cue onset) were assorted into 6 equidistant phase bins. ITPC across participants and time points in W2-W6 indexed by these 6 phase bins were analyzed via one-way repeated measures ANOVA and a regression analysis with a sine wave (y = a*sin(f*x*p/3+f)+c, where x is the bin number; a, f, f, and c were free parameters)49. (5) To examine the relation between IAF and ITPC, a linear regression was performed.
For the rhythmic-active condition, the arrhythmic-active condition is a more stringent control than the rhythmic-sham condition across all figures (Figs. 2-5, S6-S9) (see also 49). Across all time windows (Figs. 3-5, S6-S9), the contrasts between the rhythmic-active and arrhythmic-active conditions were significant whenever those between the rhythmic-active and rhythmic-sham condition were significant, except for evoked oscillation amplitude and ITPC in time window W0 (the 100 ms cycle before the first pulse) and evoked oscillation amplitude in time window W1 (the 100 ms cycle after the first pulse) (Figs. S6, S7A). Therefore, we report the statistical contrast between the rhythmic and arrhythmic-active stimulation conditions.