10 Hz tACS Over Somatosensory Cortex Does Not Modulate Supra-Threshold Tactile Temporal Discrimination in Humans

Abstract

Perception of physical identical stimuli can differ over time depending on the brain state. One marker of this brain state can be neuronal oscillations in the alpha band (8–12 Hz). A previous study showed that the power of prestimulus alpha oscillations in the contralateral somatosensory area negatively correlate with the ability to temporally discriminate between two subsequent tactile suprathreshold stimuli. That is, with high alpha power subjects were impaired in discriminating two stimuli and more frequently reported to perceive only one stimulus. While this previous study found correlative evidence for a role of alpha oscillations on tactile temporal discrimination, here, we aimed to study the causal influence of alpha power on tactile temporal discrimination by using transcranial alternating current stimulation (tACS). We hypothesized that tACS in the alpha frequency should entrain alpha oscillations and thus modulate alpha power. This modulated alpha power should alter temporal discrimination ability compared to a control frequency or sham. To this end, 17 subjects received one or two electrical stimuli to their left index finger with different stimulus onset asynchronies (SOAs). They reported whether they perceived one or two stimuli. Subjects performed the paradigm before (pre), during (peri), and 25 min after tACS (post). tACS was applied to the contralateral somatosensory-parietal area with either 10, 5 Hz or sham on three different days. We found no significant difference in discrimination abilities between 10 Hz tACS and the control conditions, independent of SOAs. In addition to choosing all SOAs as the independent variable, we chose individually different SOAs, for which we expected the strongest effects of tACS. Again, we found no significant effects of 10 Hz tACS on temporal discrimination abilities. We discuss potential reasons for the inability to modulate tactile temporal discrimination abilities with tACS.

Investigating the effects of transcranial alternating current stimulation on primary somatosensory cortex

“…Our first result demonstrating that tACS does not modulate STDT regardless of stimulation frequency is in line with previous evidence showing no effect of 10 Hz tACS applied over S1 on STDT 23 . Our finding that tACS did not affect overall near-threshold tactile stimuli perception contrasts with that of Sliva et al 21 , who showed a decreased performance in near-threshold stimuli perception during alpha tACS.…”

“…In the present study we demonstrated that tACS failed to modify STDT and near-threshold tactile stimuli perception, not only when delivered at alpha frequency 22,23 , but even at beta and gamma frequencies. Moreover, our study showed that the phase of tACS stimulation over S1 did not affect near-threshold tactile stimuli perception.…”

Age-Dependent Effect of Transcranial Alternating Current Stimulation on Motor Skill Consolidation

“…In rabbits, motor behavioral effects (eyeblink) were induced at higher frequencies (30, 100 and 200 Hz) rather than 10 Hz (Márquez-Ruiz et al, 2016). Similarly, although tACS stimulation of the somatosensory cortex (SI) at alpha and high-gamma frequency was able to elicit tactile sensations (Feurra et al, 2011b), 10 Hz tACS did not modulate the ability to temporally discriminate between two subsequent tactile suprathreshold stimuli (Wittenberg et al, 2019). Overall, our data suggest that perturbations of neural oscillations can be beneficial for individuals with aberrant oscillations such as the elderly.…”

Perception of rhythmic speech is modulated by focal bilateral tACS

“…First, participants typically report sensations such as itching or tingling, but no rhythmic component (at least at common stimulation intensities, such as those applied here), and these sensations seem to diminish or disappear relatively quickly after stimulation onset (e.g., Antal et al, 2017;Kessler, Turkeltaub, Benson, & Hamilton, 2012). Second, participants do not seem to be able to reliably distinguish different tACS frequencies (Nakazono, Ogata, Kuroda, & Tobimatsu, 2016;Wittenberg, Morr, Schnitzler, & Lange, 2019), including those which produce stronger sensations (10 or 20 Hz) than the frequency applied in the current study (3.125 Hz). Kleinert, Szymanski, & Müller (2017) also reported that participants are unable to determine whether frontal and parietal regions were stimulated in-or out-of-phase from each other.…”