Detection of Transcranial Alternating Current Stimulation Aftereffects Is Improved by Considering the Individual Electric Field Strength and Self-Rated Sleepiness.

Non-invasive electrical stimulation methods, such as transcranial alternating current stimulation (tACS), are increasingly used in human neuroscience research and offer potential new avenues to treat neurological and psychiatric disorders. However, their often variable effects have also raised concerns in the scientific and clinical communities. This study aims to investigate the influence of subject-specific factors on the alpha tACS-induced aftereffect on the alpha amplitude (measured with electroencephalography, EEG) as well as on the connectivity strength between nodes of the default mode network (DMN) [measured with functional magnetic resonance imaging (fMRI)]. As subject-specific factors we considered the individual electrical field (EFIELD) strength at target regions in the brain, the frequency mismatch between applied stimulation and individual alpha frequency (IAF) and as a covariate, subject's changes in mental state, i.e., sleepiness. Eighteen subjects participated in a tACS and a sham session conducted on different days. Each session consisted of three runs (pre/stimulation/). tACS was applied during the second run at each subject's individual alpha frequency (IAF), applying 1 mA peak-to-peak intensity for 7 min, using an occipital bihemispheric montage. In every run, subjects watched a video designed to increase in-scanner compliance. To investigate the aftereffect of tACS on EEG alpha amplitude and on DMN connectivity strength, EEG data were recorded simultaneously with fMRI data. Self-rated sleepiness was documented using a questionnaire. Conventional statistics (ANOVA) did not show a significant aftereffect of tACS on the alpha amplitude compared to sham stimulation. Including individual EFIELD strengths and self-rated sleepiness scores in a multiple linear regression model, significant tACS-induced aftereffects were observed. However, the subject-wise mismatch between tACS frequency and IAF had no contribution to our model. Neither standard nor extended statistical methods confirmed a tACS-induced aftereffect on DMN functional connectivity. Our results show that it is possible and necessary to disentangle alpha amplitude changes due to intrinsic mechanisms and to external manipulation using tACS on the alpha amplitude that might otherwise be overlooked. Our results suggest that EFIELD is really the most significant factor that explains the alpha amplitude modulation during a tACS session. This knowledge helps to understand the variability of the tACS-induced aftereffects.

Stuttering severity relates to frontotemporal low-beta synchronization during pre-speech preparation.

OBJECTIVE: The neurophysiological dynamics of the occurrence of a stuttering event are largely unknown. This sensor-level EEG study investigated whether already the intention to speak alters the formation of the speech production network in stuttering. METHODS: We studied alpha (8-13 Hz), low beta (15-25 Hz) and high beta (25-30 Hz) power modulation in 19 adults with developmental stuttering (AWS) and 19 fluently speaking control participants during speech intention. RESULTS: Both groups show that the anticipation of overt reading coincides with broadband low-frequency suppression in posterior sensors, a common sign of network formation for speech production. Prior to fluent speech, frontotemporal alpha and low-beta power were weaker in AWS with mild stuttering but stronger in AWS with severe stuttering. These correlations were not significant prior stuttered speech. Further, post hoc comparisons confirmed the difference between AWS with mild and severe stuttering in low beta power. CONCLUSIONS: AWS with more severe stuttering seem to show stronger maintenance of the current cognitive or sensorimotor state, as stuttering severity was associated with increased beta power. Increased beta power levels may influence subsequent speech preparation and execution processes. SIGNIFICANCE: Upcoming breakdowns of the speech production network as evident in actual stuttering are related to beta power during the intention to speak.

Reduced alpha amplitudes predict perceptual suppression.

The amplitude of prestimulus alpha oscillations over parieto-occipital cortex has been shown to predict visual detection of masked and threshold-level stimuli. Whether alpha activity similarly predicts target visibility in perceptual suppression paradigms, another type of illusion commonly used to investigate visual awareness, is presently unclear. Here, we examined prestimulus alpha activity in the electroencephalogram (EEG) of healthy participants in the context of a generalized flash suppression (GFS) task during which salient target stimuli are rendered subjectively invisible in a subset of trials following the onset of a full-field motion stimulus. Unlike for masking or threshold paradigms, alpha (8-12 Hz) amplitude prior to motion onset was significantly higher when targets remained subjectively visible compared to trials during which the targets became perceptually suppressed. Furthermore, individual prestimulus alpha amplitudes strongly correlated with the individual trial-to-trial variability quenching following motion stimulus onset, indicating that variability quenching in visual cortex is closely linked to prestimulus alpha activity. We conclude that predictive correlates of conscious perception derived from perceptual suppression paradigms differ substantially from those obtained with "near threshold paradigms", possibly reflecting the effectiveness of the suppressor stimulus.

Extracting Robust Biomarkers From Multichannel EEG Time Series Using Nonlinear Dimensionality Reduction Applied to Ordinal Pattern Statistics and Spectral Quantities.

In this study, ordinal pattern analysis and classical frequency-based EEG analysis methods are used to differentiate between EEGs of different age groups as well as individuals. As characteristic features, functional connectivity as well as single-channel measures in both the time and frequency domain are considered. We compare the separation power of each feature set after nonlinear dimensionality reduction using t-distributed stochastic neighbor embedding and demonstrate that ordinal pattern-based measures yield results comparable to frequency-based measures applied to preprocessed data, and outperform them if applied to raw data. Our analysis yields no significant differences in performance between single-channel features and functional connectivity features regarding the question of age group separation.

Stimulus dependence of contralateral dominance in human auditory cortex.

The auditory system is often considered to show little contralateral dominance but physiological reports on the contralateral dominance of activity evoked by monaural sound vary widely. Here, we show that part of this variation is stimulus-dependent: blood oxygen level dependent (BOLD) responses to 32 s of monaurally presented unmodulated noise (UN) showed activation in contralateral auditory cortex (AC) and deactivation in ipsilateral AC compared to nonstimulus baseline. Slow amplitude-modulated (AM) noise evoked strong contralateral activation and minimal ipsilateral activation. The contrast of AM-versus-UN was used to separate fMRI activity related to the slow amplitude modulation per se. This difference activation was bilateral although still stronger in contralateral AC. In magnetoencephalography (MEG), the response was dominated by the steady-state activity phase locked to the amplitude modulation. This MEG activity showed no consistent contralateral dominance across listeners. Subcortical BOLD activation was strongly contralateral subsequent to the superior olivary complex (SOC) and showed no significant difference between modulated and UN. An acallosal participant showed similar fMRI activation as the group, ruling transcallosal transmission an unlikely source of ipsilateral enhancement or ipsilateral deactivation. These results suggest that ascending activity subsequent to the SOC is strongly dominant contralateral to the stimulus ear. In contrast, the part of BOLD and MEG activity related to slow amplitude modulation is more bilateral and only observed in AC. Ipsilateral deactivation can potentially bias measures of contralateral BOLD dominance and should be considered in future studies.

Sustained BOLD and theta activity in auditory cortex are related to slow stimulus fluctuations rather than to pitch.

Human functional MRI (fMRI) and magnetoencephalography (MEG) studies indicate a pitch-specific area in lateral Heschl's gyrus. Single-cell recordings in monkey suggest that sustained-firing, pitch-specific neurons are located lateral to primary auditory cortex. We reevaluated whether pitch strength contrasts reveal sustained pitch-specific responses in human auditory cortex. Sustained BOLD activity in auditory cortex was found for iterated rippled noise (vs. noise or silence) but not for regular click trains (vs. jittered click trains or silence). In contrast, iterated rippled noise and click trains produced similar pitch responses in MEG. Subsequently performed time-frequency analysis of the MEG data suggested that the dissociation of cortical BOLD activity between iterated rippled noise and click trains is related to theta band activity. It appears that both sustained BOLD and theta activity are associated with slow non-pitch-specific stimulus fluctuations. BOLD activity in the inferior colliculus was sustained for both stimulus types and varied neither with pitch strength nor with the presence of slow stimulus fluctuations. These results suggest that BOLD activity in auditory cortex is much more sensitive to slow stimulus fluctuations than to constant pitch, compromising the accessibility of the latter. In contrast, pitch-related activity in MEG can easily be separated from theta band activity related to slow stimulus fluctuations.

Potential fMRI correlates of 40-Hz phase locking in primary auditory cortex, thalamus and midbrain.

MEG and BOLD fMRI are both related to post-synaptic activity, but not all components of the MEG appear to be equally reflected by the BOLD response. To evaluate potential BOLD correlates of the auditory 40-Hz steady-state response (SSR), 40-Hz amplitude-modulated (AM) tones and pure-tones with 1000-Hz tone frequency and 32-s tone duration were presented to 12 listeners in fMRI and MEG. The SSR evoked by AM-tones is readily separated from the onset, offset and sustained fields in MEG by a high-pass filter. For fMRI, a contrast of AM- versus pure-tones was used to estimate activity related more specifically to the 40-Hz SSR, but excluding other activity that is evoked by AM and pure-tones alike. This contrast showed sustained BOLD activation confined to the medial part of Heschl's gyrus, the location of the primary auditory cortex, as well as activity in the medial geniculate body (MGB) and the inferior colliculus (IC). Transient BOLD onset and offset responses were prominent throughout the auditory cortex. In contrast, sustained BOLD activity for pure-tones was weak and did not match well with the time course of the sustained field, suggesting that there is no reliable BOLD correlate of the sustained field in MEG. The sustained BOLD activity in primary auditory cortex is therefore more likely linked to the phase-locked SSR. Enhanced BOLD for AM compared to pure-tones in the IC and MGB may similarly be related to phase locking.