Image_1_Absence of Alpha-tACS Aftereffects in Darkness Reveals Importance of Taking Derivations of Stimulation Frequency and Individual Alpha Variability Into Account.pdf

<p>Transcranial alternating current stimulation (tACS) has found widespread use as a basic tool in the exploration of the role of brain oscillations. Many studies have shown that frequency-specific tACS is able to not only alter cognitive processes during stimulation, but also cause specific physiological aftereffects visible in the electroencephalogram (EEG). The relationship between the emergence of these aftereffects and the necessary duration of stimulation is inconclusive. Our goal in this study was to narrow down the crucial length of tACS-blocks, by which aftereffects can be elicited. We stimulated participants with α-tACS in four blocks of 1-, 3-, 5-, and 10-min length, once in increasing and once in decreasing order. After each block, we measured the resting EEG for 10 min during a visual vigilance task. We could not find lasting enhancement of α-power following any stimulation block, when comparing the stimulated groups to the sham group. These findings offer no information regarding the crucial stimulation duration. In addition, this conflicts with previous findings, showing a power increase following 10 min of tACS in the alpha range. We performed additional explorative analyses, based on known confounds of (1) mismatches between stimulation frequency and individual alpha frequency and (2) abnormalities in baseline α-activity. The results of an ANCOVA suggested that both factor explain variance, but could not resolve how exactly both factors interfere with the stimulation effect. Employing a linear mixed model, we found a significant effect of stimulation following 10 min of α-tACS in the increasing sequence and a significant effect of the mismatch between stimulated frequency and individual alpha frequency. The implications of these findings for future research are discussed.</p

EEG-informed fMRI Results.

<p>Results from time-frequency EEG-informed fMRI analyses reveal overlapping effects across frequency bands for response inhibition, especially within the left MFG and to a lesser extent the right inferior frontal region and cingulate gyrus; however, theta band parameterization additionally reveals larger activation within the superior frontal gyrus. T-values are reported in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096159#pone-0096159-t003" target="_blank">Tables 3</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096159#pone-0096159-t005" target="_blank">5</a>, along with voxel data and MNI co-ordinates.</p

fMRI Results.

<p>fMRI Results.</p

Time-frequency plots.

<p>The time-frequency plots for Go, Stim1 and Stim2 for context1 from electrode FCz are displayed in power (dB) from 400 ms prior to stimulus onset (0) to 800 ms post-stimulus onset. The temporal windows and associated time-frequency bands δ = delta, θ = theta, β1 = low beta, β2 =  high beta) are illustrated directly on the plots.</p

Multimodal Results for fMRI Parameterization with Frontocentral (FC) Theta for contrast Stim1> Go1.

<p>Multimodal Results for fMRI Parameterization with Frontocentral (FC) Theta for contrast Stim1> Go1.</p

Stimulus-Response Mappings Shape Inhibition Processes: A Combined EEG-fMRI Study of Contextual Stopping

<div><p>Humans are rarely faced with one simple task, but are typically confronted with complex stimulus constellations and varying stimulus-relevance in a given situation. Through modifying the prototypical stop-signal task and by combined recording and analysis of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), we studied the effects of stimulus relevance for the generation of a response or its inhibition. Stimulus response mappings were modified by contextual cues, indicating which of two different stimuli following a go stimulus was relevant for stopping. Overall, response inhibition, that is comparing successful stopping to a stop-signal against go-signal related processes, was associated with increased activity in right inferior and left midfrontal regions, as well as increased EEG delta and theta power; however, stimulus-response conditions in which the most infrequent stop-signal was relevant for inhibition, were associated with decreased activity in regions typically involved in response inhibition, as well as decreased activity in the delta and theta bands as compared to conditions wherein the relevant stop-signal frequency was higher. Behaviorally, this (aforementioned) condition, which demanded inhibition only from the most infrequent stimulus, was also associated with reduced reaction times and lower error rates. This pattern of results does not align with typical stimulus frequency-driven findings and suggests interplay between task relevance and stimulus frequency of the stop-signal. Moreover, with a multimodal EEG-fMRI analysis, we demonstrated significant parameterization for response inhibition with delta, theta and beta time-frequency values, which may be interpreted as reflecting conflict monitoring, evaluative and/or motor processes as suggested by previous work (Huster et al., 2013; Aron, 2011). Further multimodal results suggest a possible neurophysiological and behavioral benefit under conditions whereby the most infrequent stimulus demanded inhibition, indicating that the frequency of the stop-signal interacts with the current stimulus-response contingency. These results demonstrate that response inhibition is prone to influence from other cognitive functions, making it difficult to dissociate real inhibitory capabilities from the influence of moderating mechanisms.</p></div

Experimental design.

<p>Depicted is the experimental design, outlining the three contexts and illustrating the response selection for the go signal (triangle) and two infrequently presented stimuli (square and circle). The current context is always presented in the four corners of the presentation screen. The presentation of Stim1 (13%) is more infrequent than the presentation of Stim2 (26%), whereas, the majority of trials are go-trials (61%). For context 1, a button press for Stim1 (blue square) and Stim2 (violet circle) is considered failed inhibition; whereas, a button press for either Stim1 (blue square) in context 2 or for Stim2 (violet circle) in context 3 is failed inhibition. That is, subjects are required to withhold their response to both Stim1 and Stim2 during context 1; however, in context 2 and context 3, stimulus-response mappings change such that the stopping demands are different (stop Stim1 in context 2 and Stim2 in context 3).</p

Unimodal fMRI Results.

<p>Results from the unimodal fMRI analysis reveal increased activity in the rIFG and left MFG during response inhibition (Stim1> Go) and decreased activity in the rIFG under stimulus-response scenarios in which the most infrequent stimuli demanded inhibition (Stim1-Context1> Stim1-Context2). T-values are reported in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096159#pone-0096159-t002" target="_blank">Table 2</a>, along with voxel data and MNI co-ordinates.</p

Behavioral Data.

<p>Behavioral Data.</p

Multimodal Results for fMRI Parameterization with Frontocentral (FC) Delta for contrast Stim1> Go1.

<p>Multimodal Results for fMRI Parameterization with Frontocentral (FC) Delta for contrast Stim1> Go1.</p