Query: tacs     ssvep     Stimulus intensity

title: Cumulative effects of single TMS pulses during beta tACS are stimulation intensity dependent
abstract: Single transcranial magnetic stimulation (TMS) pulses activate different components of the motor cortex neural circuitry in a stimulation intensity dependent way and may lead to a cumulative increase of corticospinal excitability (CSE) during the same stimulation session. Furthermore, transcranial alternating current stimulation (tACS) has been shown to increase in a frequency specific way the level of CSE probed by single pulse TMS. The interaction of these two phenomena, i.e. cumulative increases and baseline shifts of CSE, and the involved neural circuitry has not been studied yet.  The aim of this study was to investigate stimulation intensity specific online effects of simultaneous TMS and tACS on CSE.  pulse TMS was applied concurrent to 20 Hz tACS over the left primary motor cortex of thirteen healthy subjects to probe CSE indexed by motor evoked potentials (MEPs) recorded from the contralateral extensor carpi radialis muscle of the right hand during rest. Six different TMS intensities (90% 100% 110% 120% 130% and 140% of resting motor threshold, RMT) were studied in a randomized blocked design. In each block, 40 pulses were applied with an inter stimulus interval of 5 s and a jitter of 0.5 s, i.e. at a stimulation frequency of 0.2 0.25 Hz.  Beta tACS has a general facilitatory effect on CSE across the tested TMS intensities. The results of the block wise regression of the MEP amplitudes show a more specific effect. Combining tACS and TMS leads to a cumulative increase in CSE for the stimulation intensity of 120% RMT only (p 0.0004) CONCLUSION CSE increases due to beta tACS and cumulative TMS pulses may be mediated by different neuronal mechanisms.

title: Modulating Brain Rhythms of Pain using Transcranial Alternating Current Stimulation (tACS) A Sham controlled Study in Healthy Human Participants
abstract: Pain protects the body. However, pain can also occur for longer periods without serving protective functions. Such chronic pain conditions are difficult to treat. Thus, a better understanding of the underlying neural mechanisms and new approaches for the treatment of pain are urgently needed. Here, we investigated a causal role of oscillatory brain activity for pain and explored the potential of transcranial alternating current stimulation (tACS) as a new treatment approach for pain. To this end, we investigated whether tACS can modulate pain and pain related autonomic activity in 29 healthy human participants using a tonic heat pain paradigm as an experimental model of chronic pain. In 6 recording sessions, participants received tACS over prefrontal or somatosensory cortices at alpha or gamma frequencies or sham tACS. During tACS, pain ratings and autonomic responses were collected. TACS did not modulate pain intensity, the stability of pain ratings or the translation of the noxious stimulus into pain. Likewise, tACS did not change autonomic responses. Bayesian statistics further indicated a lack of tACS effects in most conditions. The only exception was alpha tACS over somatosensory cortex where evidence for tACS effects was inconclusive. Taken together, the present study did not find significant tACS effects on tonic experimental pain in healthy human participants. However, considering the conceptual plausibility of using tACS to modulate pain and the urgent need for novel pain treatments, further tACS studies are warranted. Based on the present findings, such studies might apply refined stimulation protocols targeting alpha oscillations in somatosensory cortices.

title: The Effect of the Viewing Distance of Stimulus on SSVEP Response for Use in Brain Computer Interfaces
abstract: Most visual stimulators used in steady state visual evoked potential (SSVEP) based brain computer interface (BCI) to elicit SSVEP response are set close to the BCI system limiting the portability and the applications of the system. Our aim is to develop a portable SSVEP based BCI adaptable to the change of the viewing distance of the stimulator which will allow users to operate the BCI without being restricted to a particular area. This study (1) investigates the impact of the distance between the user and the visual stimulator on the SSVEP response and (2) develops a portable visual stimulator that would adapt to change of distance. The finding suggests that a distance adaptable SSVEP based BCI is achievable by changing the intensity of visual stimulator to correspond to the change of the viewing distance. It is hoped that this work can help SSVEP BCI design and broaden its potential applications and users.

title: Spectrum based feature extraction using spectrum intensity ratio for SSVEP detection
abstract: Recent years, a Steady State Visual Evoked Potential (SSVEP) is used as a basis for Brain Computer Interface (BCI)[1] Various feature extraction and classification techniques are proposed to achieve BCI based on SSVEP. The feature extraction of SSVEP is developed in the frequency domain regardless of the limitation in flickering frequency of visual stimulus caused by hardware architecture. We introduce here the feature extraction using a spectrum intensity ratio. Results show that the detection ratio reaches 84% by using a spectrum intensity ratio with unsupervised classification. It also indicates the SSVEP is enhanced by proposed feature extraction with second harmonic.

title: Optimizing Spatial Contrast of a New Checkerboard Stimulus for Eliciting Robust SSVEPs
abstract: Most existing steady state visual evoked potential (SSVEP) based brain computer interfaces (BCIs) adopted low or medium frequency stimuli towards high information transfer rates (ITRs) However, the intense flashes will deteriorate the user experience. With the increase of stimulation frequency, the discomfort caused by flickering stimuli can be significantly reduced, but the amplitude of SSVEP is also decreased, which greatly increases the difficulty of SSVEP detection. To cope with the tradeoff between SSVEP intensity and user experience, this study proposed a new checkerboard stimulus scheme with spatially alternated flickering and background squares. This study compared black white, black, and white background checkerboard stimuli, together with a traditional single flickering stimulus in eliciting SSVEPs at different frequencies. The data analysis results showed that the signal to noise ratio (SNR) of SSVEP for the black background checkerboard stimulus was comparable to the single flickering stimulus, both of which were significantly higher than the other two checkerboard stimuli in the high frequency range (40 Hz and 60 Hz) Additionally, with a consensus of reduction in subjective visual irritation, the black background checkerboard stimulus can lead to improved user experience of the SSVEP based BCIs.

title: Shift in lateralization during illusory self motion: EEG responses to visual flicker at 10 Hz and frequency specific modulation by tACS
abstract: Self motion perception is a key aspect of higher vestibular processing, suggested to rely upon hemispheric lateralization and alpha band oscillations. The first aim of this study was to test for any lateralization in the EEG alpha band during the illusory sense of self movement (vection) induced by large optic flow stimuli. Visual stimuli flickered at alpha frequency (approx. 10 Hz) in order to produce steady state visually evoked potentials (SSVEPs) a robust EEG measure which allows probing the frequency specific response of the cortex. The first main result was that differential lateralization of the alpha SSVEP response was found during vection compared with a matched random motion control condition, supporting the idea of lateralization of visual vestibular function. Additionally, this effect was frequency specific, not evident with lower frequency SSVEPs. The second aim of this study was to test for a causal role of the right hemisphere in producing this lateralization effect and to explore the possibility of selectively modulating the SSVEP response. Transcranial alternating current stimulation (tACS) was applied over the right hemisphere simultaneously with SSVEP recording, using a novel artefact removal strategy for combined tACS EEG. The second main result was that tACS enhanced SSVEP amplitudes, and the effect of tACS was not confined to the right hemisphere. Subsequent control experiments showed the effect of tACS requires the flicker frequency and tACS frequency to be closely matched and tACS to be of sufficient intensity. Combined tACS SSVEPs are a promising method for future investigation into the role of neural oscillations and for optimizing tACS.

title: Transcranial Alternating Current Stimulation at Alpha Frequency Reduces Pain When the Intensity of Pain is Uncertain.
abstract: Alpha activity directly before pain onset has been implicated in pain experience with higher prestimulus alpha associated with lower reported pain. However, expectations about pain intensity also seem to affect prestimulus alpha activity. To date, evidence for a relationship between alpha activity and pain experience has been largely correlational. Transcranial alternating current stimulation at alpha frequency (alpha tACS) permits direct manipulation of alpha activity and therefore an examination of the potential causal relationship between alpha activity and pain. We investigated whether somatosensory alpha tACS could reduce pain experience and whether this was influenced by uncertainty about pain intensity. In a within subjects design, perceived pain intensity and unpleasantness were assessed in 23 participants during alpha tACS and sham stimulation. Visual cues preceding the pain stimulus were used to manipulate uncertainty. A significant tACS x Uncertainty x Stimulus intensity interaction was found for reported pain intensity (F2,44 4.50, P .017, partial e2 .17) and unpleasantness (F1,22 4.78, P .040, partial e2 .18) Pain experience during the application of somatosensory alpha tACS was significantly lowered compared with sham stimulation, but only when the intensity of an upcoming stimulus was uncertain.  To our knowledge, this is the first study to suggest that somatosensory alpha tACS might lead to a reduction in pain. Interventions targeting alpha activity may have the potential to alleviate chronic pain. However, a patient's expectation about the intensity of upcoming pain must also be taken into account.

title: Predictive Model for SSVEP Magnitude Variation: Applications to Continuous Control in Brain Computer Interfaces
abstract: The steady state visual evoked potential based brain computer interface (SSVEP BCI) is a typically recognized visual stimulus frequency from brain responses. Each frequency represents one command to control a machine. For example, multiple target stimuli with different frequencies can be used to control the moving speeds of a robot. Each target stimulus frequency corresponds to a speed level. Such a conventional SSVEP BCI is choice selection paradigm with discrete information, allowing users to discretely control the speed of a movable object. This can result in non smooth object movement. To overcome the problem, in this study, a conceptual design of a SSVEP BCI with continuous information for continuous control is proposed to allow users to control the moving speed of an object smoothly. A predictive model for SSVEP magnitude variation plays an important role in the proposed design. Thus, this study mainly focuses on a feasibility study concerning the use of SSVEP magnitude prediction for BCI. A basic experiment is therefore conducted to gather SSVEP responses from varying stimulus intensity using times with a fixed frequency. Random Forest Regression (RF) is outperformed by simple regression and neural networks in these predictive tasks. Finally, the advantages of the proposed SSVEP BCI is demonstrated by streaming real SSVEP responses from ten healthy subjects into a brain controlled robotic simulator. The results from this study show that the proposed SSVEP BCI containing both frequency recognition and magnitude prediction is a promising approach for future continuous control applications.

title: P207 Affecting declarative long term memory with transcranial alternating current stimulation (tACS)
abstract: Previous studies imply that tACS can entrain the ongoing physiological neuronal oscillations depending on the targeted function and the parameters (frequency, intensity and phase) of the stimulation. The aim of the present study was to clarify if tACScould influence the performance of healthy subjects on a verbal paired associate learning paradigm.  For that, we conducted three different crossover, randomized, single blind, placebo controlled studies, using a battery driven Starstim stimulator system (Neuroelectrics, Barcelona, Spain) In the first study n 6) we applied 100 Hz tACS with 1.5 mA (active electrode T7, return Fpz and T8) in the second one n 6) 1.0 mA, 6 Hz (3 s) followed by 80 Hz (5 s) tACS during ISI and Stimulus presentation using the same electrodes configuration; in the third one n 15) we combined a temporal and pre frontal stimulation using 7 electrodes. Here the stimulation intensity was 1 mA at 6 Hz (temporal) and 80 Hz (pre frontal) combined stimulation. For all the studies, we used 8 cm 2 round rubber electrodes inside a saline soaked sponge pocket, applying 10 min stimulation during the word pairs learning phase. After the learning phase, the subjects had a 10 min pause, followed by a test phase with a cued recall. The subjects returned after 24 h for a second cued recall test. All subjects receivedactive and sham tACS in a counterbalanced, randomized manner.  In the first two studies, no significant difference were found in the accuracy performance between days across conditions, as well as between days within conditions; in the third study, we found no significant difference between days across conditions, but significant difference between days in the active t 2.98, p 0.010) and sham t 2.52, p 0.024) stimulation.  The stimulation parameters tested could not influence the memory performance assessed by this memory task with the present number of subjects in the first two studies. Further variations of waveforms, intensity and stimulation parameters should be tested with the same task in order to probe the possible influence of tACS on episodic long term memory. The final results will be presented in the conference.

title: P189 Transcranial modulation of visually induced gamma power: a concurrent TACS MEG study
abstract: Alpha oscillations (8 12 Hz) are proposed to mediate 'attentional gating' by suppressing neuronal processing in task irrelevant sensory regions, reflected by 'pulsed inhibition' of stimulus induced gamma band activity (40 100 Hz) Concurrent TACS MEG has the potential to reveal TACS entrainment of alpha oscillations at the stimulation site and thus their causal impact on neuronal activity representing stimulus processing.  We aimed to provide evidence that TACS can entrain the phase of alpha oscillations in the visual cortex by assessing its modulatory impact on the power of visual stimulus induced gamma oscillations via simultaneous MEG recordings.  TACS was applied during MEG in intermingled 5.4s trials either in Oz Cz or FPz Cz montage. Montage and stimulation frequency were randomized across trials, applying either individual alpha frequency (IAF) IAF 4 Hz, IAF+4 Hz, or no stimulation, resulting in a 2 (Montage) x 3 (Frequency) design plus a 'NoStim' condition. Stimulation intensity for both montages was adjusted separately to 95% retinal phosphene threshold. Participants performed a simple visual detection task, attending an inward moving grating, known to induce strong occipital gamma responses. TACS artifacts were reduced via regression methods, LCMV beamformer time course extraction from the visual cortex, and high pass filtering. Importantly, the gamma frequency of interest ~50 60 Hz) was largely unaffected by TACS artifacts. We compared TACS to gamma phase power coupling (PAC) for the respective experimental conditions.  We found a rhythmic modulation of stimulus induced gamma power by TACS at IAF (i.e. TACS gamma phase amplitude coupling) for the Oz Cz montage, but not for the FPz Cz montage which was introduced to control for subthreshold retinal entrainment effects.  Our findings (i) demonstrate that concurrent TACS MEG can principally be used to study cross frequency coupling between transcranially entrained slower and spontaneous or stimulus related faster oscillations, (ii) cannot merely result from subthreshold retinal entrainment, and (iii) suggest that alpha oscillations are indeed a possible means for top down suppression of bottom up sensory processing.