Low pre‐stimulus EEG alpha power amplifies visual awareness but not visual sensitivity

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., 8–14 Hz) 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.

Spontaneous alpha-band oscillations bias subjective contrast perception

“…Our regression analysis showed, in addition to the effect of lateralization on contrast appearance, and the effect of bilateral pre-stimulus alpha amplitude on objective accuracy in the contrast judgment task: stronger amplitude was related to worse performance. This result was unexpected because it conflicts with recent studies showing that pre-stimulus alpha amplitude affects criterion, not accuracy in visual detection tasks (Limbach and Corballis, 2016; Iemi et al, 2017; Iemi and Busch, 2018), and does not affect performance in discrimination tasks (Samaha et al, 2017; Benwell et al, 2017; Samaha et al, 2020b; Benwell et al, 2021). Given that states of high alpha amplitude indicate low arousal (Johnston et al, 2020), diminishing the quality of stimulus representations (Zhou et al, 2021), one possibility is that the effect we observed was merely a by-product of fatigue, which in turn caused the effect on accuracy.…”

Periodic attention operates faster during more complex visual search

“…Here, we recorded electro-encephalography (EEG) during visual search tasks with varying attentional load to test the link between brain oscillations and task demands, and further characterize periodic attentional sampling. We first replicated previous results showing that prestimulus phase [20,[31][32][33][34][35][36][37][38][39][40] and post-stimulus phase-locking across trials [20,41] of low-frequency oscillations predict successful task performance (same task as in [20]). Specifically, the phase of spontaneous oscillations (before stimuli onset) predicts trial outcome; and post-stimulus phase alignment (phase-locking) aids performance, which we call phase reset here to refer, strictly, to the mathematical description of the effect [42].…”

From Shorter to Longer Timescales: Converging Integrated Information Theory (IIT) with the Temporo-Spatial Theory of Consciousness (TTC)

“…The assumption of additive pre-post-stimulus interaction stands counter to various fMRI studies [ 69 , 70 , 71 ] and EEG/MEG [ 55 , 72 , 73 , 74 , 75 , 76 ] that show the importance of pre-stimulus activity changes for conscious contents (see also [ 77 ]).…”