Abstract
Pre-stimulus oscillatory neural activity has been linked to the level of awareness of sensory stimuli. More specifically, the power of low-frequency oscillations (primarily in the alpha-band, i.e., 8-14Hz) prior to stimulus onset is inversely related to measures of subjective performance in visual tasks, such as confidence and visual awareness. Intriguingly, the same EEG signature does not seem to influence objective measures of task performance (i.e., accuracy). We here examined whether this dissociation holds when stringent accuracy measures are used. Previous EEG-studies have employed 2-alternative forced choice (2-AFC) discrimination tasks to link pre-stimulus oscillatory activity to correct/incorrect responses as an index of accuracy/objective performance at the single-trial level. However, 2-AFC tasks do not provide a good estimate of single-trial accuracy, as many of the responses classified as correct will be contaminated by guesses (with the chance correct response rate being 50%). Here instead, we employed a 19-AFC letter identification task to measure accuracy and the subjectively reported level of perceptual awareness on each trial. As the correct guess rate is negligible (~5%), this task provides a purer measure of accuracy. Our results replicate the inverse relationship between pre-stimulus alpha/beta-band power and perceptual awareness ratings in the absence of a link to discrimination accuracy. Pre-stimulus oscillatory phase did not predict either subjective awareness or accuracy. Our results hence confirm a dissociation of the pre-stimulus EEG power-task performance link for subjective versus objective measures of performance, and further substantiate pre-stimulus alpha power as a neural predictor of visual awareness.
Highlights
Discovering the neural mechanisms underlying perception remains a fundamental challenge for neuroscience
We implemented a letter discrimination task to examine the effects of pre-stimulus oscillatory activity on both discrimination accuracy and perceptual awareness ratings
Single-trial regression analyses revealed a negative correlation between pre-stimulus power (~9–1 2 Hz, Figure 3) and subjective awareness ratings, but no relationship between pre-stimulus power and discrimination accuracy
Summary
Discovering the neural mechanisms underlying perception remains a fundamental challenge for neuroscience. Iemi and colleagues (2017) proposed that if decreases in alpha power reflect an increase in global baseline excitability levels (see evidence from transcranial magnetic stimulation [TMS]-EEG studies; Dugué et al, 2011; Romei et al, 2008; Samaha et al, 2017), low alpha power may lead to a more liberal decision criterion, leaving perceptual sensitivity unaffected In line with this view, a number of recent studies have shown that pre-stimulus alpha power influences the decision criterion and covaries with subjective measures of task performance (i.e., confidence and perceptual awareness) but not objective measures such as accuracy (Lange et al, 2013; Limbach & Corballis, 2016; Craddock et al, 2017; Iemi et al, 2017; Iemi & Busch, 2017; Samaha et al, 2017; Benwell et al, 2018; Kloosterman et al, 2019; Wöstmann et al, 2019; Samaha et al, 2020; see Samaha et al, 2020, for a review). Our analyses aim to contribute to the understanding of the mechanisms by which baseline neural activity impacts visual perception
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