Neurocognitive, physiological, and biophysical effects of transcranial alternating current stimulation

Brain oscillations are regarded as important for perception as they open and close time windows for neural spiking to enable the effective communication within and across brain regions. In the past, studies on perception primarily relied on the use of electrophysiological techniques for probing a correlative link between brain oscillations and perception. The emergence of noninvasive brain stimulation techniques such as transcranial alternating current stimulation (tACS) provides the possibility to study the causal contribution of specific oscillatory frequencies to perception. Here, we review the studies on visual, auditory, and somatosensory perception that employed tACS to probe the causality of brain oscillations for perception. The current literature is consistent with a causal role of alpha and gamma oscillations in parieto-occipital regions for visual perception and theta and gamma oscillations in auditory cortices for auditory perception. In addition, the sensory gating by alpha oscillations applies not only to the visual but also to the somatosensory domain. We conclude that albeit more refined perceptual paradigms and individualized stimulation practices remain to be systematically adopted, tACS is a promising tool for establishing a causal link between neural oscillations and perception.

https://journals.sagepub.com/doi/pdf/10.1177/1073858419828646

Probing the Link Between Perception and Oscillations: Lessons from Transcranial Alternating Current Stimulation.

tACS phase-specifically biases brightness perception of flickering light

Sensory perception can be modulated by the phase of neural oscillations, especially in the theta and alpha ranges. Oscillatory activity in the visual cortex can be entrained by transcranial alternating current stimulation (tACS) as well as periodic visual stimulation (i.e., flicker). Combined tACS and visual flicker stimulation modulates BOLD response, and concurrent 4-Hz auditory click train, and tACS modulate auditory perception in a phase-dependent way. In this study, we investigated whether phase synchrony between concurrent tACS and periodic visual stimulation (i.e., flicker) can modulate performance on a visual matching task. Participants completed a visual matching task on a flickering visual stimulus while receiving either in-phase (0°) or asynchronous (180°, 90°, or 270°) tACS at alpha or theta frequency. Stimulation was applied over either occipital cortex or dorsolateral pFC. Visual performance was significantly better during theta frequency tACS over the visual cortex when it was in-phase (0°) with visual stimulus flicker, compared with antiphase (180°). This effect did not appear with alpha frequency flicker or with dorsolateral pFC stimulation. Furthermore, a control sham group showed no effect. There were no significant performance differences among the asynchronous (180°, 90°, and 270°) phase conditions. Extending previous studies on visual and auditory perception, our results support a crucial role of oscillatory phase in sensory perception and demonstrate a behaviorally relevant combination of visual flicker and tACS. The spatial and frequency specificity of our results have implications for research on the functional organization of perception.

Theta Phase-dependent Modulation of Perception by Concurrent Transcranial Alternating Current Stimulation and Periodic Visual Stimulation.

Effectiveness assessment of repetitive transcranial alternating current stimulation with concurrent EEG and fNIRS measurement

Transcranial alternating current stimulation (tACS) has emerged as a promising tool for modulating cortical oscillations In previous electroencephalogram (EEG) studies, tACS has been found to modulate brain oscillatory activity in a frequency-specific manner However, the spatial distribution and hemodynamic response for this modulation remains poorly understood Functional magnetic resonance imaging (fMRI) has the advantage of measuring neuronal activity in regions not only below the tACS electrodes but also across the whole brain with high spatial resolution Here, we measured fMRI signal while applying tACS to modulate rhythmic visual activity During fMRI acquisition, tACS at different frequencies (4, 8, 16, and 32 Hz) was applied along with visual flicker stimulation at 8 and 16 Hz We analyzed the blood-oxygen-level-dependent (BOLD) signal difference between tACS-ON vs tACS-OFF, and different frequency combinations (eg, 4 Hz tACS, 8 Hz flicker vs 8 Hz tACS, 8 Hz flicker) We observed significant tACS modulation effects on BOLD responses when the tACS frequency matched the visual flicker frequency or the second harmonic frequency The main effects were predominantly seen in regions that were activated by the visual task and targeted by the tACS current distribution These findings bridge different scientific domains of tACS research and demonstrate that fMRI could localize the tACS effect on stimulus-induced brain rhythms, which could lead to a new approach for understanding the high-level cognitive process shaped by the ongoing oscillatory signal

Frequency-dependent tACS modulation of BOLD signal during rhythmic visual stimulation

The invention discloses a magnetic resonance imaging method used for oscillatory magnetic field detection and based on a spin-locked technology and application, and names as Spin-Locked Oscillatory Excitation, SLOE. SLOE comprises a spin-locked preparation sequence insensitive to main magnetic field and radio frequency field inhomogeneity and spin planar echo acquisition. According to the method, after the spin-locked preparation sequence, any additional excitation pulse is no longer applied, and all acquired signals are magnetization vector signals deflected by oscillatory magnetic field excitation. In a data processing process, due to the fact that the initial phase of oscillatory current scanned every time is random, the fluctuation of a check signal time sequence is statistically used to detect an activation signal. According to the invention, defects of conventional neuronal discharge sequence detection are solved; the sensitivity of tiny oscillatory magnetic field detection is greatly improved; neuronal discharge can be successfully detected through magnetic resonance detection in vivo; and technological innovation in the field can be promoted.

Magnetic resonance imaging method used for oscillatory magnetic field detection and based on spin locking technology and application

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Frequency-dependent tACS modulation of BOLD signal during rhythmic visual stimulation

Magnetic resonance imaging method used for oscillatory magnetic field detection and based on spin locking technology and application