No Evidence for Phase-Specific Effects of 40 Hz HD–tACS on Multiple Object Tracking

Abstract

Phase synchronization drives connectivity between neural oscillators, providing a flexible mechanism through which information can be effectively and selectively routed between task-relevant cortical areas. The ability to keep track of objects moving between the left and right visual hemifields, for example, requires the integration of information between the two cerebral hemispheres. Both animal and human studies have suggested that coherent (or phase-locked) gamma oscillations (30–80 Hz) might underlie this ability. While most human evidence has been strictly correlational, high-density transcranial alternating current stimulation (HD-tACS) has been used to manipulate ongoing interhemispheric gamma phase relationships. Previous research showed that 40 Hz tACS delivered bilaterally over human motion complex could bias the perception of a bistable ambiguous motion stimulus (Helfrich et al., 2014). Specifically, this work showed that in-phase (0° offset) stimulation boosted endogenous interhemispheric gamma coherence and biased perception toward the horizontal (whereby visual tokens moved between visual hemifields—requiring interhemispheric integration). By contrast, anti-phase (180° offset) stimulation decreased interhemispheric gamma coherence and biased perception toward the vertical (whereby tokens moved within separate visual hemifields). Here we devised a multiple object tracking arena comprised of four quadrants whereby discrete objects moved either entirely within the left and right visual hemifields, or could cross freely between visual hemifields, thus requiring interhemispheric integration. Using the same HD-tACS montages as Helfrich et al. (2014), we found no phase-specific effect of 40 Hz stimulation on overall tracking performance. While tracking performance was generally lower during between-hemifield trials (presumably reflecting a cost of integration), this difference was unchanged by in- vs. anti-phase stimulation. Our null results could be due to a failure to reliably modulate coherence in our study, or that our task does not rely as heavily on this network of coherent gamma oscillations as other visual integration paradigms.

Neural Oscillations and the Initiation of Voluntary Movement

“…The role of phase of brain oscillations has been extensively studied in perceptual and cognitive domains (Palva and Palva, 2007; Montemurro et al, 2008; VanRullen et al, 2011; Jensen et al, 2014; Bland et al, 2018). However, there are few studies investigating such phenomena in the motor domain, particularly in terms of lower-frequency oscillations and voluntary movement.…”

Effects of tACS-Like Electrical Stimulation on On-Center Retinal Ganglion Cells: Part I

“…For example, tACS over visual cortex affects the time course of visual perception, 13 and the ability to track visual objects. 14 Low gamma frequency tACS may enhance processing in dyslexia, 15 while theta frequency tACS affects visuospatial working memory. 16 Despite the use of various types of electrical stimulation in hundreds of human studies, little is known about its mechanism of action at the neural circuit level.…”

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

“…To display a balanced overview of the current insufficient knowledge on tACS, two major limitations should be presented: (1) some studies failed to show an effect in both healthy participants and patients, and (2) entrainment of brain oscillations is confounded by other proposed mechanisms of action. The individual functional as well as the structural variability of the brain, the wide range of stimulation parameters and technical difficulties could explain why tACS failed to alter the behavioural outcomes [ 67 , 129 , 186 , 214 , 215 ]. In this regard, strict modelling techniques and peri-stimulation EEG recordings could decipher the inability of tACS to induce electrophysiological or behavioural outcomes.…”