Testing the effect of tACS over parietal cortex in modulating endogenous alpha rhythm and temporal integration windows in visual perception

Abstract

Neural oscillations in the alpha band (8-12 Hz) have been proposed as a key mechanism for the temporal resolution of visual perception. Higher alpha frequencies have been related to improved segregation of visual events over time, whereas lower alpha frequencies have been related to improved temporal integration. Similarly, also the phase of ongoing alpha has been shown to correlate with temporal integration/segregation. To test a causal relationship between alpha oscillations and perception, we here employed multi-channel transcranial alternating current stimulation (mc-tACS) over the right parietal cortex, whereas participants performed a visual temporal integration/ segregation task that used identical stimuli with different instructions. Before and after mc-tACS we recorded the resting-state electroencephalogram (EEG) to extract the individual alpha frequency (IAF) and delivered electrical stimulation at slightly slower and faster frequencies (IAF±2 Hz). We hypothesized that this would not only drive endogenous alpha rhythms, but also affect temporal integration and segregation in an opposite way. However, the mc-tACS protocol used here did not consistently increase or decrease the IAF after the stimulation and did not affect temporal integration/segregation accuracy as expected. Although we found some preliminary evidence for an influence of tACS phase on temporal integration accuracy, the ongoing phase of mc-tACS oscillations did not reliably modulate temporal integration/segregation accuracy in a sinusoidal way as would have been predicted by an effective entrainment of brain oscillations. These findings may guide future studies using different stimulation montages for investigating the role of cortical alpha oscillations for human vision.

Binding Mechanisms in Visual Perception and Their Link With Neural Oscillations: A Review of Evidence From tACS

“…By comparing performance across these phase bins, they were able to explore whether a phase-specific modulation of performance emerged from their data. Even if they found preliminary evidence of a relation between integration accuracy and tACS phase, a more stringent permutation test did not confirm the strength of the effect ( Figure 4D) (Ronconi et al, 2020a). In sum, even if tACS was previously shown to shape detection accuracy in a sinusoidal fashion (Helfrich et al, 2014b;de Graaf et al, 2020), this study provides only partial evidence on whether tACS is able to effectively modulate segregation/integration performance in a phase-dependent manner.…”

“…In line with the electrophysiological literature (for reviews see VanRullen, 2016;White, 2018), some studies also showed a phase-dependent modulation of accuracy and reaction times induced by occipital tACS in detection tasks (Helfrich et al, 2014b;de Graaf et al, 2020). Nonetheless, it should be noted that other studies using parietal tACS did not convincingly find consistent evidence in a task that directly measured both integration and segregation performance (Ronconi et al, 2020a). This study addressed possible issues concerning tACS effectiveness such as the choice of the stimulation montage and current distribution modeling that should be investigated more deeply in future studies to increase tACS efficacy in the exploration of visual temporal binding.…”

“…However, other studies did not show such modulation. Ronconi et al (2020a) employed mc-tACS over the right parietal cortex (electrode position P4) ( Figure 4A) at individually tailored alpha frequencies (IAF ± 2 Hz) to test visual temporal segregation and integration. Indeed, as previously reported, a better segregation performance is expected with higher alpha frequencies (e.g., IAF + 2 Hz), while an improved integration should be associated with lower alpha frequencies (e.g., IAF -2 Hz) Ronconi et al, 2018).…”

“…Support for this view comes from studies that showed a phase-dependent modulation of occipital alpha tACS in target detection accuracy (Helfrich et al, 2014b) and reaction times in three visual tasks (de Graaf et al, 2020). To explore a possible phase-dependent tACS modulation, Ronconi et al (2020a) computed the exact phase point at the onset of each trial and divided the tACS alpha cycle into eight bins. By comparing performance across these phase bins, they were able to explore whether a phase-specific modulation of performance emerged from their data.…”

“…In principle, the same logic applied to alpha frequency in previous studies may be applied to theta frequency as well. However, even if several studies investigated the role of theta activity in other cognitive domains with tACS (Polanía et al, 2012;Pahor and Jaušovec, 2014;Vosskuhl et al, 2015; FIGURE 4 | Illustrative representation of the study and results by Ronconi et al (2020a). (A) mc-tACS montage: the stimulation electrode (red) was placed on P4, the return electrodes (blue) were placed on C4, Pz, O2, and P8.…”