Directionality of the injected current targeting the P20/N20 source determines the efficacy of 140 Hz transcranial alternating current stimulation (tACS)-induced aftereffects in the somatosensory cortex

Abstract

Interindividual anatomical differences in the human cortex can lead to suboptimal current directions and may result in response variability of transcranial electrical stimulation methods. These differences in brain anatomy require individualized electrode stimulation montages to induce an optimal current density in the targeted area of each individual subject. We aimed to explore the possible modulatory effects of 140 Hz transcranial alternating current stimulation (tACS) on the somatosensory cortex using personalized multi-electrode stimulation montages. In two randomized experiments using either tactile finger or median nerve stimulation, we measured by evoked potentials the plasticity aftereffects and oscillatory power changes after 140 Hz tACS at 1.0 mA as compared to sham stimulation (n = 17, male = 9). We found a decrease in the power of oscillatory mu-rhythms during and immediately after tactile discrimination tasks, indicating an engagement of the somatosensory system during stimulus encoding. On a group level both the oscillatory power and the evoked potential amplitudes were not modulated by tACS neither after tactile finger stimulation nor after median nerve stimulation as compared to sham stimulation. On an individual level we could however demonstrate that lower angular difference (i.e., differences between the injected current vector in the target region and the source orientation vector) is associated with significantly higher changes in both P20/N20 and N30/P30 source activities. Our findings suggest that the higher the directionality of the injected current correlates to the dipole orientation the greater the tACS-induced aftereffects are.

Can individually targeted and optimized multi-channel tDCS outperform standard bipolar tDCS in stimulating the primary somatosensory cortex?

“…only individual target location, but especially individual target orientation plays an important role as also investigated and proposed by Refs. [16,18,26,28e30,32] and recently also found in a tACS study over the somatosensory cortex [33]. A total stimulation current of 2 mA might also be important, as it was earlier assumed that due to the depth of the central sulcus not enough currents reach the target area Brodmann area 3b with 1 mA [7,11].…”

“…Our individualized targeting and stimulation approach might thus help to better control tDCS experimental paradigms. Our approach is also interesting for tACS, where recent studies also showed that neuroimaging combined with electric field modeling is useful to explain inter-individual variability of stimulation effects [15,33].…”

Personalized alpha-tACS targeting left posterior parietal cortex modulates visuo-spatial attention and posterior evoked EEG activity

“…The copyright holder for this preprint this version posted January 4, 2023. ; https://doi.org/10.1101/2023.01.04.522700 doi: bioRxiv preprint the topology and orientation of the electric fields relative to a given stimulation target [47,48].…”

“…Only recently, the topology and magnitude of individual electric fields have been reported to correlate with the strength of tACS-modulations of neuronal activity [23,46]. Thus, by using algorithmic optimization of individual stimulation montages, personalized tACS has the potential to increase control over the topology and orientation of the electric fields relative to a given stimulation target [47,48]. In addition, this approach allows the post-hoc analysis of the estimated electric fields in conjunction with behavioral or neurophysiological outcome measures of tACS [23,45,cf.…”

“…In addition, this approach allows the post-hoc analysis of the estimated electric fields in conjunction with behavioral or neurophysiological outcome measures of tACS [23,45,cf. 46,48].…”