Does alpha phase modulate visual target detection? Three experiments with tACS phase-based stimulus presentation

Abstract

In recent years the influence of alpha (7-13 Hz) phase on visual processing has received a lot of attention. Magneto-/encephalography (M/EEG) studies showed that alpha phase indexes visual excitability and task performance. If occipital alpha phase is functionally relevant, the phase of occipital alpha-frequency transcranial alternating current stimulation (tACS) could modulate visual processing. Visual stimuli presented at different pre-determined, experimentally controlled, phases of the entraining tACS signal should then result in an oscillatory pattern of visual performance. We studied this in a series of experiments. In experiment one, we applied 10 Hz tACS to right occipital cortex (O2) and used independent psychophysical staircases to obtain contrast thresholds for detection of visual gratings in left or right hemifield, in six equidistant tACS phase conditions. In experiments two and three, tACS was at EEG-based individual peak alpha frequency. In experiment two, we measured detection rates for gratings with (pseudo-)fixed contrast levels. In experiment three, participants detected brief luminance changes in a custom-built LED device, at eight equidistant alpha phases. In none of the experiments did the primary outcome measure over phase conditions consistently reflect a one-cycle sinusoid as predicted. However, post-hoc analyses of reaction times (RT) suggested that tACS alpha phase did modulate RT in both experiments 1 and 2 (not measured in experiment 3). This observation is in line with the idea that alpha phase causally gates visual inputs through cortical excitability modulation.

tACS phase-specifically biases brightness perception of flickering light

“…Previous studies assessed the influence of tACS on ongoing or task-related activity in visual cortex [35,[45][46][47][48][49][50] with only low neural frequency-and phase-consistency of the targeted process, possibly hampering rigorous investigations of neural entrainment effects. In contrast, visual flicker stimulation enables the setting of the oscillatory phase in the visual cortex with high signal-to-noise ratio.…”

No effects of rhythmic visual stimulation on target discrimination: An online alpha entrainment experiment

“…Reaction times are less prone to these issues, were previously shown amenable to electrophysiological alpha‐phase modulation (Callaway & Yeager, 1960) and have generally proved a reliable outcome measure in attention tasks (Duecker et al, 2017; Schuhmann et al, 2019). Using transcranial alternating current stimulation (tACS), although post hoc, we recently found more promising alpha entrainment effects on reaction times than accuracy measures (de Graaf et al, 2020). Still, it cannot be excluded that our results would have been different had we used near‐threshold stimuli and assessed hit rates or d ′ scores.…”

Quantifying rhythmicity in perceptual reports

“…In recent years most of the rhythms reported in behavioral data were in the theta (Landau and Fries, 2012; Holcombe and Chen, 2013; Tomassini et al, 2015; Hogendoorn, 2016; Senoussi et al, 2019; Benedetto et al, 2021; Plöchl et al, 2021), alpha (Song et al, 2014; Benedetto and Morrone, 2019; Ho et al, 2019; de Graaf et al, 2020) and beta (Bell et al, 2020; Veniero et al, 2021) range. However these studies primarily employed ATC-based methods, which are suboptimal to detect rhythms of higher frequency.…”