Neuling, T. | bioRxiv Science

bioRxiv2017

Local network-level integration mediates effects of transcranial Alternating Current Stimulation

Transcranial alternating current stimulation (tACS) has been proposed as a tool to draw causal inferences on the role of oscillatory activity in cognitive functioning and has the potential to induce long-term changes in cerebral networks. However, the mechanisms of action of tACS are not yet clear, though previous modeling works have suggested that variability may be mediated by local and network-level brain states. We used magnetoencephalography (MEG) to record brain activity from 17 healthy participants as they kept their eyes open (EO) or closed (EC) while being stimulated either with sham, weak, or strong alpha-tACS using a montage commonly assumed to target occipital areas. We reconstructed the activity of sources in all stimulation conditions by means of beamforming. The analysis of resting-state data revealed an interaction of the external stimulation with the endogenous alpha power difference between EO and EC in the posterior cingulate. This region is remote from occipital cortex, which showed strongest EC vs. EO alpha modulation, thus suggesting state-dependency long-range effects of tACS. In a follow-up analysis of this online-tACS effect, we find evidence that this dependency effect could be mediated by functional network changes: connection strength from the precuneus, a region adjusting for a measure of network integration in the two states (EC vs. EO during no-tACS), was significantly correlated with the state-dependency effect in the posterior cingulate (during tACS). No analogous correlation could be found for alpha power modulations in occipital cortex. Altogether, this is the first strong evidence to illustrate how functional network architectures can shape tACS effects.

neuroscience↗

bioRxiv2017

Envelope-tACS modulates intelligibility of speech in noise

Cortical entrainment of the auditory cortex to the broad-band temporal envelope of a speech signal is crucial for speech comprehension. This entrainment results in phases of high and low neural excitability which structure and decode the incoming speech signal. Entrainment to speech is strongest in the theta frequency range (4-8 Hz), the average frequency of the speech envelope. If a speech signal is degraded, for example masked by irrelevant information such as noise, entrainment to the speech envelope is weaker and speech intelligibility declines.\n\nBesides perceptually evoked cortical entrainment, transcranial alternating current stimulation (tACS) can entrain neural oscillations by applying an electric signal to the brain. Accordingly, tACS-induced entrainment in auditory cortex has been shown to improve auditory perception. The aim of the current study was to externally modulate speech intelligibility by means of tACS such that the electric current corresponds to the envelope of the presented speech stream.\n\nParticipants performed the Oldenburg sentence test with sentences presented in noise in combination with tACS. Critically, the time lag between sentence presentation and tACS was manipulated from 0 to 250 ms in 50-ms steps (auditory stimuli were simultaneous to or preceded tACS).\n\nFirst, we were able to show that envelope-tACS modulated sentence comprehension such that on average sentence comprehension at the time lag of the best performance was significantly better than sentence comprehension of the worst performance. Second, sentence comprehension across time lags was modulated sinusoidally.\n\nIn sum, envelope tACS modulates intelligibility of speech in noise presumably by enhancing (time lag with in-phase stimulation) and disrupting (time lag with out-of-phase stimulation) cortical entrainment to the speech envelope in auditory cortex.

neuroscience↗