Intrinsic 40Hz-phase asymmetries predict tACS effects during conscious auditory perception

Abstract

Synchronized oscillatory gamma-band activity (30-100Hz) has been suggested to constitute a key mechanism to dynamically orchestrate sensory information integration across multiple spatio-temporal scales. We here tested whether interhemispheric functional connectivity and ensuing auditory perception can selectively be modulated by high-density transcranial alternating current stimulation (HD-tACS). For this purpose, we applied multi-site HD-tACS at 40Hz bilaterally with a phase lag of 180° and recorded a 64-channel EEG to study the oscillatory phase dynamics at the source-space level during a dichotic listening (DL) task in twenty-six healthy participants. In this study, we revealed an oscillatory phase signature at 40Hz which reflects different temporal profiles of the phase asymmetries during left and right ear percept. Here we report that 180°-tACS did not affect the right ear advantage during DL at group level. However, a follow-up analysis revealed that the intrinsic phase asymmetries during sham-tACS determined the directionality of the behavioral modulations: While a shift to left ear percept was associated with augmented interhemispheric asymmetry (closer to 180°), a shift to right ear processing was elicited in subjects with lower asymmetry (closer to 0°). Crucially, the modulation of the interhemispheric network dynamics depended on the deviation of the tACS-induced phase-lag from the intrinsic phase asymmetry. Our characterization of the oscillatory network trends is giving rise to the importance of phase-specific gamma-band coupling during ambiguous auditory perception, and emphasizes the necessity to address the inter-individual variability of phase asymmetries in future studies by tailored stimulation protocols.

The Modulation of Cognitive Performance with Transcranial Alternating Current Stimulation: A Systematic Review of Frequency-Specific Effects

“…In addition, gamma-tACS improved the detection of right-attend stimuli [36] as well as endogenous selective visual attention [30,35,38]. In contrast, alpha-tACS decreased selective visual [30,35,38] and auditory attention [31,37]. Theta-and gamma-tACS showed no effect on selective auditory attention [31,37].…”

“…In contrast, alpha-tACS decreased selective visual [30,35,38] and auditory attention [31,37]. Theta-and gamma-tACS showed no effect on selective auditory attention [31,37]. Only one study aimed for phase coherence effects: anti-phase beta-tACS reduced the attentional blink [33].…”

“…They found disrupted selective attention with alpha-tACS but no effect of theta-tACS. Meier et al [37] used a multi-site bilateral application of gamma-tACS in another selective attention task (i.e., dichotic listening). While they found no benefit for accuracy, they revealed an advantage for left ear perception with gamma-tACS.…”

Transcranial alternating current stimulation (tACS): from basic mechanisms towards first applications in psychiatry

“…In the light of these discoveries, tACS was able to manipulate auditory perception in healthy participants by decoupling this interhemispheric connectivity. A remarkable finding was that the individual indigenous brain oscillations prior to stimulation dictated the resulting effects of tACS, favouring the use of individually tailored stimulation paradigms [ 124 ].…”

Effects of Transcranial Electrical Stimulation on Human Auditory Processing and Behavior—A Review

“…Mediated by cortico-cortical callosal fibers [ 90 ], the perception of the left ear was related to the increased functional connectivity of the functional gamma-band [ 93 ]. Therefore, Jan et al studied the effect of HD-tACS on selectively regulating the functional connection between cerebral hemispheres and tested the changes in auditory perception of the left and right ears [ 94 ]. In brief, subjects received multi-site HD-tACS at 40 Hz bilaterally with a phase lag of 180° and sham stimulation within 1–12 days apart.…”