To See or Not to See: Prestimulus α Phase Predicts Visual Awareness

TLDR

Variability in conscious visual detection, despite unchanged stimuli, is likely due to fluctuations in brain activity. The authors aim to test a theory that increased α power constitutes pulsed inhibition of cortical activity, thereby influencing visual awareness. They propose that α power modulates cortical excitability, with each 100‑ms α cycle alternating between heightened and suppressed responsiveness. They find that the phase of posterior α rhythm predicts both detection and early cortical activation, with troughs suppressing activity and reducing detection, and that pre‑target ERPs also forecast target visibility.

Abstract

We often fail to see something that at other times is readily detectable. Because the visual stimulus itself is unchanged, this variability in conscious awareness is likely related to changes in the brain. Here we show that the phase of EEG α rhythm measured over posterior brain regions can reliably predict both subsequent visual detection and stimulus-elicited cortical activation levels in a metacontrast masking paradigm. When a visual target presentation coincides with the trough of an α wave, cortical activation is suppressed as early as 100 ms after stimulus onset, and observers are less likely to detect the target. Thus, during one α cycle lasting 100 ms, the human brain goes through a rapid oscillation in excitability, which directly influences the probability that an environmental stimulus will reach conscious awareness. Moreover, ERPs to the appearance of a fixation cross before the target predict its detection, further suggesting that cortical excitability level may mediate target detection. A novel theory of cortical inhibition is proposed in which increased α power represents a “pulsed inhibition” of cortical activity that affects visual awareness.