Phase of beta-frequency tACS over primary motor cortex modulates corticospinal excitability

The assessment of corticospinal excitability by means of transcranial magnetic stimulation-induced motor evoked potentials is an established diagnostic tool in neurophysiology and a widely used procedure in fundamental brain research. However, concern about low reliability of these measures has grown recently. One possible cause of high variability of MEPs under identical acquisition conditions could be the influence of oscillatory neuronal activity on corticospinal excitability. Based on research showing that transcranial alternating current stimulation can entrain neuronal oscillations we here test whether alpha or beta frequency tACS can influence corticospinal excitability in a phase-dependent manner. We applied tACS at individually calibrated alpha- and beta-band oscillation frequencies, or we applied sham tACS. Simultaneous single TMS pulses time locked to eight equidistant phases of the ongoing tACS signal evoked MEPs. To evaluate offline effects of stimulation frequency, MEP amplitudes were measured before and after tACS. To evaluate whether tACS influences MEP amplitude, we fitted one-cycle sinusoids to the average MEPs elicited at the different phase conditions of each tACS frequency. We found no frequency-specific offline effects of tACS. However, beta-frequency tACS modulation of MEPs was phase-dependent. Post hoc analyses suggested that this effect was specific to participants with low (<19 Hz) intrinsic beta frequency. In conclusion, by showing that beta tACS influences MEP amplitude in a phase-dependent manner, our results support a potential role attributed to neuronal oscillations in regulating corticospinal excitability. Moreover, our findings may be useful for the development of TMS protocols that improve the reliability of MEPs as a meaningful tool for research applications or for clinical monitoring and diagnosis

Piecing together the puzzle of emotional consciousness

The search for neural correlates of emotional consciousness has gained momentum in the last decades. Nonetheless, disagreements concerning the mechanisms that determine the experiential qualities of emotional consciousness – the what is it like to feel an emotion - as well as on their neural correlates have far reaching consequence on how researchers study and measure emotion, sometimes leading to seemingly irresolvable impasses. The current paper lays out in a balanced way the viewpoint of both cognitive and precognitive approaches to emotional consciousness, on the basis of commonalities and differences between the claims of some relevant theories of emotions. We examine the sufficiency of the existing evidence in support of the proposed theories of emotional consciousness, by going through the methodological specificity of the study of emotional consciousness and its unique challenges, highlighting what can and cannot be imported by advances on research on perceptual consciousness. We propose that there are three key experimental contrasts which are each equally necessary in the search for the neural correlates of emotional consciousness, each contrast alone coming with its own limitations. We conclude by acknowledging some of the most promising avenues in the field which may help go beyond current limitations and collaboratively piece together the puzzle of emotional consciousness

Influence of continuous flash suppression mask frequency on stimulus visibility

The continuous flash suppression (CFS) paradigm is increasingly used in consciousness research, but its mechanisms are still not fully understood. To better understand its temporal properties, we presented the CFS masks at 9 frequencies, and examined their influence on stimuli visibility, while taking into account the inter-individual variability and the change of CFS suppression as the experiment progressed. The frequencies consisted of fundamental frequencies of 3, 4 and 5Hz, and their 2nd and 3rd harmonics, which included the 10Hz frequency typically used in most of the CFS studies. We found that the suppression of stimulus awareness was stronger under 4, 6 and 8Hz than 10Hz. After controlling for inter-individual variability with mixed-effects analysis, we found that the number of seen trials was lower for the 4 Hz-basis frequencies than the 5Hz ones, and was lower for the 2nd than 3rd harmonic. We propose that this may be caused by an interaction between the CFS masks and the ongoing sampling of the attentional mechanism. Examining individual data, we also found a habituation effect that the participants saw significantly more stimuli as the experiment progressed. Our results suggest that these factors need to be taken care of in future CFS studies in order to achieve optimal visual awareness suppression and ensure the generalizability of results

Dynamic Interactions between Emotion Perception and Action Preparation for Reacting to Social Threat:A Combined cTBS-fMRI Study

Expressions of emotion are powerful triggers for situation-appropriate responses by the observer. Of particular interest regarding the preparation of such adaptive actions are parietal and premotor cortices, given their potential for interaction with the amygdala (AMG), which is known to play a crucial role in the processing of affective information and in motor response. We set out to disentangle the respective roles of the inferior parietal lobule (IPL) and ventral premotor cortex (PMv) in humans in the processing of emotional body expressions by assessing remote effects of continuous theta burst stimulation (cTBS) in the action network and in AMG. Participants were presented with blocks of short videos showing either angry or neutral whole-body actions. The experiment consisted of three fMRI sessions: two sessions were preceded by stimulation of either right IPL (rIPL) or right PMv (rPMv); and a third session assessed baseline activity. Interestingly, whereas at baseline the left AMG did not differentiate between neutral and angry body postures, a significant difference between these conditions emerged after stimulation of either rIPL or rPMv, with much larger responses to angry than to neutral stimuli. In addition, the effects of cTBS stimulation and emotion were also observed in two other action-relevant areas, the supplementary motor area and the superior parietal cortex. Together, these results show how areas involved in action and emotion perception and in action preparation interact dynamically

The Influence of Conscious and Unconscious Body Threat Expressions on Motor Evoked Potentials Studied With Continuous Flash Suppression

The observation of threatening expression in others is a strong cue for triggering an action response. One method of capturing such action responses is by measuring the amplitude of motor evoked potentials (MEPs) elicited with single pulse TMS over the primary motor cortex. Indeed, it has been shown that viewing whole body expressions of threat modulate the size of MEP amplitude. Furthermore, emotional cues have been shown to act on certain brain areas even outside of conscious awareness. In the current study, we explored if the influence of viewing whole body expressions of threat extends to stimuli that are presented outside of conscious awareness in healthy participants. To accomplish this, we combined the measurement of MEPs with a continuous flash suppression task. In experiment 1, participants were presented with images of neutral bodies, fearful bodies, or objects that were either perceived consciously or unconsciously, while single pulses of TMS were applied at different times after stimulus onset (200, 500, or 700 ms). In experiment 2 stimuli consisted of neutral bodies, angry bodies or objects, and pulses were applied at either 200 or 400 ms post stimulus onset. In experiment 1, there was a general effect of the time of stimulation, but no condition specific effects were evident. In experiment 2 there were no significant main effects, nor any significant interactions. Future studies need to look into earlier effects of MEP modulation by emotion body stimuli, specifically when presented outside of conscious awareness, as well as an exploration of other outcome measures such as intracortical facilitation